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High Spatial Resolution Galactic 3D Extinction Mapping with IPHAS
Abstract
We present an algorithm (MEAD, for ‘Mapping Extinction Against Distance’) which will determine intrinsic (r′−i′) colour, extinction, and distance for early-A to K4 stars extracted from the IPHAS r′/i′/Hα photometric data base. These data can be binned up to map extinction in three dimensions across the northern Galactic plane. The large size of the IPHAS data base (∼200 million unique objects), the accuracy of the digital photometry it contains and its faint limiting magnitude (r′∼ 20) allow extinction to be mapped with fine angular (∼10 arcmin) and distance (∼ 0.1 kpc) resolution to distances of up to 10 kpc, outside the solar circle. High reddening within the solar circle on occasion brings this range down to ∼2 kpc. The resolution achieved, both in angle and depth, greatly exceeds that of previous empirical 3D extinction maps, enabling the structure of the Galactic Plane to be studied in increased detail. MEAD accounts for the effect of the survey magnitude limits, photometric errors, unresolved interstellar medium (ISM) substructure and binarity. The impact of metallicity variations, within the range typical of the Galactic disc is small. The accuracy and reliability of MEAD are tested through the use of simulated photometry created with Monte Carlo sampling techniques. The success of this algorithm is demonstrated on a selection of fields and the results are compared to the literature.
... In recent years a number of deep optical and infrared imaging surveys have enabled the production of extensive 3D reddening maps of the Galactic plane, allowing distances to be estimated to stars and nebulae by plotting reddening versus distance for their sightlines (e.g. Marshall et al. 2006;Sale et al. 2009Sale et al. , 2014Sale 2012;Lallement et al. 2014Lallement et al. , 2019Green et al. 2015Green et al. , 2019. ...
... From the initial IPHAS data release (González-Solares et al. 2008) to the second data release (Barentsen et al. 2014) the applications of IPHAS data have expanded. Sale et al. (2009), Sale (2012), and Sale et al. (2014) have demonstrated how the information from colours using the r , i , and (in particular) H α filters enables spectral types and interstellar dust extinctions to be inferred for objects in the IPHAS survey, enabling 3D extinction mapping to be carried out for sightlines covered by the survey. For example, Giammanco et al. (2011) applied the IPHAS-based extinction mapping technique MEAD (Mapping Extinction Against Distance; Sale et al. 2009) to planetary nebulae, using spectroscopic Balmer-line ratios to estimate reddenings and distances for 70 planetary nebulae. ...
... Sale et al. (2009), Sale (2012), and Sale et al. (2014) have demonstrated how the information from colours using the r , i , and (in particular) H α filters enables spectral types and interstellar dust extinctions to be inferred for objects in the IPHAS survey, enabling 3D extinction mapping to be carried out for sightlines covered by the survey. For example, Giammanco et al. (2011) applied the IPHAS-based extinction mapping technique MEAD (Mapping Extinction Against Distance; Sale et al. 2009) to planetary nebulae, using spectroscopic Balmer-line ratios to estimate reddenings and distances for 70 planetary nebulae. ...
We report Hα filter photometry for 197 northern hemisphere planetary nebulae (PNe) obtained using imaging data from the IPHAS survey. Hα+[N ii] fluxes were measured for 46 confirmed or possible PNe discovered by the IPHAS survey and for 151 previously catalogued PNe that fell within the area of the northern Galactic Plane surveyed by IPHAS. After correcting for [N ii] emission admitted by the IPHAS Hα filter, the resulting Hα fluxes were combined with published radio free-free fluxes and Hβ fluxes, in order to estimate mean optical extinctions to 143 PNe using ratios involving their integrated Balmer line fluxes and their extinction-free radio fluxes. Distances to the PNe were then estimated using three different 3D interstellar dust extinction mapping methods, including the IPHAS-based H-MEAD algorithm of Sale (2014). These methods were used to plot dust extinction versus distance relationships for the lines of sight to the PNe; the intercepts with the derived dust optical extinctions allowed distances to the PNe to be inferred. For 17 of the PNe in our sample reliable Gaia DR2 distances were available and these have been compared with the distances derived using three different extinction mapping algorithms as well as with distances from the nebular radius vs. Hα surface brightness relation of Frew et al. (2016). That relation and the H-MEAD extinction mapping algorithm yielded the closest agreement with the Gaia DR2 distances.
... Despite the proximity of Cyg OB2 compared with other massive clusters, it is affected by a large visual extinction due to both the dust associated with the Great Cygnus Rift in the foreground and its parental cloud. The most recent estimates Covey et al. (2007;dotted line), and those predicted by the ZAMS of Siess et al. (2000), with a different transformation from the UBVRI to the ugriz photometric system applied. of the visual extinction in the direction of Cyg OB2 are those of Sale et al. (2009), who studied how A V varies with distance in the direction of the association and found an increase of A V from 2 m to 5 m in the region between 1 kpc and 2 kpc away; Drew et al. (2008), who found that the cluster locus in the r − Hα versus r − i exhibits an extinction between A V = 2.5 m and A V = 7 m , with a larger extinction in the area surrounding the association; and Wright et al. (2010), who found a median A V = 7.5 m in the central cluster and A V = 5.5 m in the northwest field observed by Chandra. ...
... The value of distance at which we obtained the best fit is 850±25 pc, with A V = 1 m . This value is only slightly smaller than the distance found by Sale et al. (2009), who estimated that the extinction along this line of sight starts to increase from A V = 2 m (∼1 kpc) to A V = 5 m (∼2 kpc). Our estimate also confirms that the nebulosity responsible for the high visual extinction is not associated with Cyg OB2, but is in the foreground. ...
... Sources in the southern Chandra field (centered on Cyg OB2) have a median extinction equal to 4.33 m . This median extinction is smaller than the value found by Wright et al. (2010) with an isochrone-fitting technique, but it is within the range of extinction between 1 kpc and 2 kpc found by Sale et al. (2009; 2 < A V < 5) and Drew et al. (2008;2.5 < A V < 7). In the histogram (middle panel) the large differential reddening affecting the central cluster is also evident, since the distribution does not show an evident peak and the bins from A V = 3.0 m to A V = 5.0 m are almost equally populated. ...
In order to fully understand the gravitational collapse of molecular clouds, the star formation process, and the evolution of circumstellar disks, these phenomena must be studied in different Galactic environments with a range of stellar contents and positions in the Galaxy. The young massive association Cygnus OB2, in the Cygnus-X region, is a unique target to study how star formation and the evolution of circumstellar disks proceed in the presence of a large number of massive stars. We present a catalog obtained with recent optical observations in the r, i, z filters with OSIRIS, mounted on the 10.4 m Gran Telescopio CANARIAS telescope, which is the deepest optical catalog of Cyg OB2 to date. The catalog consists of 64,157 sources down to M = 0.15 M
☉ at the adopted distance and age of Cyg OB2. A total of 38,300 sources have good photometry in all three bands. We combined the optical catalog with existing X-ray data of this region, in order to define the cluster locus in the optical diagrams. The cluster locus in the r – i versus i – z diagram is compatible with an extinction of the optically selected cluster members in the 2.64m
< AV < 5.57m
range. We derive an extinction map of the region, finding a median value of AV = 4.33m
in the center of the association, decreasing toward the northwest. In the color-magnitude diagrams, the shape of the distribution of main-sequence stars is compatible with the presence of an obscuring cloud in the foreground ~850 ± 25 pc from the Sun.
... Drew et al. (2008) using data from the INT/WFC Photometric Hα Survey (IPHAS) found clustered A-type stars at distance of 20 arcmin south of the centre of Cyg OB2 and estimated extinction for these stars 4.5 < AV < 7 mag. IPHAS data also were used by Sale et al. (2009) for constructing a 3D map of Galactic extinction. They show that towards Cyg OB2 at distance of 1 kpc AV reaches about 2 mag and then rapidly increases up to 6 mag at 2 kpc. ...
... But such high interstellar extinction is not typical for the stars in solar neighborhood. According to the study of Sale et al. (2009), for the stars in the field of Cyg OB2 interstellar extinction reaches 5 mag at a distance of more than 2 kpc. Therefore, we assume that J203247.17+411501.0 is a giant and it is located behind the association at about 2.5 kpc. ...
To explain the nature of the high reddening (AV ≃ 10 mag) towards one of the most luminous stars in the Galaxy – Cyg OB2 #12 (B5 Ia-0), also known as MT304, we carried out
spectrophotometric observations of 24 stars located in its vicinity. We included five new B-stars among the members of Cygnus
OB2, and for five more photometrically selected stars we spectroscopically confirmed their membership. We constructed the
map of interstellar extinction within 2.5 arcmin radius and found that interstellar extinction increases towards MT304. According
to our results the most reddened OB-stars in the association after MT304 are J203240.35+411420.1 and J203239.90+411436.2,
located about 15 arcsec away from it. Interstellar extinction towards these stars is about 9 mag. The increase of reddening
towards MT304 suggests that the reddening excess may be caused by the circumstellar shell ejected by the star during its evolution.
This shell absorbs 1 mag, but its chemical composition and temperature are unclear. We also report the detection of a second
component of MT304, and discovery of an even fainter third component, based on data of speckle interferometric observations
taken with the Russian 6-m telescope.
... As this relation depends slightly on the source (e.g.,Sale et al. 2009), we ...
V1741 Sgr (= SPICY 71482/Gaia22dtk) is a Classical T Tauri star on the outskirts of the Lagoon Nebula. After at least a decade of stability, in mid-2022, the optical source brightened by ∼3 mag over two months, remained bright until early 2023, then dimmed erratically over the next four months. This event was monitored with optical and infrared spectroscopy and photometry. Spectra from the peak (October 2022) indicate an EX Lup-type (EXor) accretion outburst, with strong emission from H i, He i, and Ca ii lines and CO bands. At this stage, spectroscopic absorption features indicated a temperature of T ∼ 4750 K with low-gravity lines (e.g. Ba ii and Sr ii). By April 2023, with the outburst beginning to dim, strong TiO absorption appeared, indicating a cooler T ∼ 3600 K temperature. However, once the source had returned to its pre-outburst flux in August 2023, the TiO absorption and the CO emission disappeared. When the star went into outburst, the source’s spectral energy distribution became flatter, leading to bluer colours at wavelengths shorter than ∼1.6 μm and redder colours at longer wavelengths. The brightening requires a continuum emitting area larger than the stellar surface, likely from optically thick circumstellar gas with cooler surface layers producing the absorption features. Additional contributions to the outburst spectrum may include blue excess from hotspots on the stellar surface, emission lines from diffuse gas, and reprocessed emission from the dust disc. Cooling of the circumstellar gas would explain the appearance of TiO, which subsequently disappeared once this gas had faded and the stellar spectrum reemerged.
... The circumstellar contribution is, however, unlikely to dominate for Cyg OB2 members, since stars with and without evidence for circumstellar disks share similarly large extinctions. The molecular cloud is, moreover, also unlikely to dominate the total extinction, since previous studies point toward a large contribution from the Great Cygnus Rift, in the foreground (Sale et al. 2009;Gottschalk et al. 2012). We discuss results obtained adopting A V values derived by Guarcello et al. (2023b) using both the Fukugita et al. (1996) and Fitzpatrick & Massa (2007) optical extinction laws. ...
We analyze the X-ray spectra of the ∼8000 sources detected in the Cygnus OB2 Chandra Legacy Survey (this focus issue), with the goals of characterizing the coronal plasma of the young low-mass stars in the region and estimating their intrinsic X-ray luminosities. We adopt two different strategies for X-ray sources for which more or less than 20 photons were detected. For the brighter sample we fit the spectra with absorbed isothermal models. In order to limit uncertainties, for most of the fainter Cygnus OB2 members in this sample we constrain the spectral parameters to characteristic ranges defined from the brightest stars. For X-ray sources with <20 net photons we adopt a conversion factor from detected photon flux to intrinsic flux. This was defined, building on the results for the previous sample, as a function of the 20% quantile of the detected photon energy distributions, which we prove to also correlate well with extinction. We then use the X-ray extinction from the spectral fits to constrain the ratio between optical and X-ray extinction toward Cyg OB2, finding it consistent with standard “Galactic” values, when properly accounting for systematics. Finally, we exploit the large number of sources to constrain the average coronal abundances of several elements, through two different ensemble analyses of the X-ray spectra of low-mass Cyg OB2 members. We find the pattern of abundances to be largely consistent with that derived for the young stellar coronae in the Orion Nebula Cluster.
... spectral type) that means reddened OB stars are extremely challenging to separate out from cool less-reddened stars by means of spectral energy distribution analysis, such as that performed on Gaia prism data. There is a discussion of this problem in the extinction mapping work of Sale et al. ( 2009 ). The low Gaia -assigned effective temperatures in Fig. B1 are accompanied by extinctions that are typically ∼25 per cent lower than those of MS17. ...
Massive O and early B stars are important markers of recent star formation and exert a significant influence on their environments during their short lives via photoionization and winds and when they explode as supernovae. In the Milky Way they can be detected at great distances but often lie behind large dust columns, making detection at short wavelengths difficult. In this study the use of the less extinguished far-red spectrum (8400 – 8800 Å) for radial velocity measurement is examined. Results are reported for a sample of 164 confirmed OB stars within a 2-degree field positioned on the Carina Arm. Most stars are at distances between 3 and 6 kpc, and Westerlund 2 is at the field edge. The measured radial velocities have errors concentrated in the 3–10 km s−1 range, with a systematic uncertainty of 2–3 km s−1. These are combined with Gaia-mission astrometry to allow full space motions to be constructed. Up to 22 stars are likely to be runaways although 8 of them are as likely to be interloping (so far undetected) binaries. The mean azimuthal motion of the sample fits in with recent measurements of Galactic disk rotation. In the Galactocentric radial direction the mean motion indicates modest infall at a speed of ∼ 10 km s−1. This experiment shows that weak Paschen lines in the far-red can yield credible radial velocity determination, offering the prospect of exploring OB-star kinematics over much more of the Galactic disk than has hitherto been possible.
... spectral type) that means reddened OB stars are extremely challenging to separate out from cool less-reddened stars by means of spectral energy distribution analysis, such as that performed on Gaia prism data. There is a discussion of this problem in the extinction mapping work of Sale et al. (2009). The low Gaia-assigned effective temperatures in Fig. B1 are accompanied by extinctions that are typically ∼25% lower than those of MS17. ...
Massive O and early B stars are important markers of recent star formation and exert a significant influence on their environments during their short lives via photoionization and winds and when they explode as supernovae. In the Milky Way they can be detected at great distances but often lie behind large dust columns, making detection at short wavelengths difficult. In this study the use of the less extinguished far-red spectrum (8400 -- 8800 \AA ) for radial velocity measurement is examined. Results are reported for a sample of 164 confirmed OB stars within a 2-degree field positioned on the Carina Arm. Most stars are at distances between 3 and 6 kpc, and Westerlund 2 is at the field edge. The measured radial velocities have errors concentrated in the 3--10 km s$^{-1}$ range, with a systematic uncertainty of 2--3 km s$^{-1}$. These are combined with Gaia-mission astrometry to allow full space motions to be constructed. Up to 22 stars are likely to be runaways although 8 of them are as likely to be interloping (so far undetected) binaries. The mean azimuthal motion of the sample fits in with recent measurements of Galactic disk rotation. In the Galactocentric radial direction the mean motion indicates modest infall at a speed of $\sim$ 10 km s$^{-1}$. This experiment shows that weak Paschen lines in the far-red can yield credible radial velocity determination, offering the prospect of exploring OB-star kinematics over much more of the Galactic disk than has hitherto been possible.
... Interestingly, the Galactic disc scale length h R of about 2-2.5 kpc (for both the thin and thick discs according to different estimates, Bland-Hawthorn & Gerhard 2016) seems to be too small if we take into account the rather large radius of the galaxy (no strong evidence for a cut-off at R 15−30 kpc has been found, López-Corredoira et al. 2002;Sale et al. 2009;Carraro et al. 2010;López-Corredoira & Molgó 2014;Chrobáková et al. 2020). This is particularly strange as the majority of the galactic discs are truncated at 3.5-4 h R (see e.g. ...
We present a detailed analysis of the Galaxy structure using an unWISE wide-field image at $3.4\mu$m. We perform a 3D photometric decomposition of the Milky Way taking into account (i) the projection of the Galaxy on the celestial sphere and (ii) that the observer is located within the Galaxy at the solar radius. We consider a large set of photometric models starting with a pure disc model and ending with a complex model which consists of thin and thick discs plus a boxy-peanut-shaped bulge. In our final model, we incorporate many observed features of the Milky Way, such as the disc flaring and warping, several over-densities in the plane, and the dust extinction. The model of the bulge with the corresponding X-shape structure is obtained from N-body simulations of a Milky Way-like galaxy. This allows us to retrieve the parameters of the aforementioned stellar components, estimate their contribution to the total Galaxy luminosity, and constrain the position angle of the bar. The mass of the thick disc in our models is estimated to be 0.4-1.3 of that for the thin disc. The results of our decomposition can be directly compared to those obtained for external galaxies via multicomponent photometric decomposition.
... Interestingly, the Galactic disc scale length hR of about 2-2.5 kpc (for both the thin and thick discs according to different estimates, Bland-Hawthorn & Gerhard 2016), seems to be too small if we take into account the rather large radius of the galaxy (no strong evidence for a cut-off at R 15−30 kpc has been found, López-Corredoira et al. 2002;Sale et al. 2009;Carraro et al. 2010;López-Corredoira & Molgó 2014;Chrobáková et al. 2020). This is particularly strange as the majority of the galactic discs are truncated at 3.5 − 4 hR (see e.g. ...
We present a detailed analysis of the Galaxy structure using an unWISE wide-field image at $3.4\mu$m. We perform a 3D photometric decomposition of the Milky Way taking into account i) the projection of the Galaxy on the celestial sphere and ii) that the observer is located within the Galaxy at the solar radius. We consider a large set of photometric models starting with a pure disc model and ending with a complex model which consists of thin and thick discs plus a boxy-peanut-shaped bulge. In our final model, we incorporate many observed features of the Milky Way, such as the disc flaring and warping, several over-densities in the plane, and the dust extinction. The model of the bulge with the corresponding X-shape structure is obtained from N-body simulations of a Milky Way-like galaxy. This allows us to retrieve the parameters of the aforementioned stellar components, estimate their contribution to the total Galaxy luminosity, and constrain the position angle of the bar. The mass of the thick disc in our models is estimated to be 0.4-1.3 of that for the thin disc. The results of our decomposition can be directly compared to those obtained for external galaxies via multicomponent photometric decomposition.
... From a colour excess -distance relation ( ( − ) vs. ), an absorption ( V ) and colour excess relation, an absorption -hydrogen column density ( H ) relation and a hydrogen column density -dispersion measure relation it is possible to link dispersion measure to distance. The first step is to use 3D dust mapping as in Green et al. (2019), Sale et al. (2009), Sale et al. (2014. Here we use the results from Green et al. (2019) to link the detected colour excess ( ( − )) to distance. ...
Magnetars are a promising candidate for the origin of Fast Radio Bursts (FRBs). The detection of an extremely luminous radio burst from the Galactic magnetar SGR J1935+2154 on 2020 April 28 added credence to this hypothesis. We report on simultaneous and non-simultaneous observing campaigns using the Arecibo, Effelsberg, LOFAR, MeerKAT, MK2 and Northern Cross radio telescopes and the MeerLICHT optical telescope in the days and months after the April 28 event. We did not detect any significant single radio pulses down to fluence limits between 25 mJy ms and 18 Jy ms. Some observing epochs overlapped with times when X-ray bursts were detected. Radio images made on four days using the MeerKAT telescope revealed no point-like persistent or transient emission at the location of the magnetar. No transient or persistent optical emission was detected over seven days. Using the multi-colour MeerLICHT images combined with relations between DM, NH and reddening we constrain the distance to SGR J1935+2154, to be between 1.5 and 6.5 kpc. The upper limit is consistent with some other distance indicators and suggests that the April 28 burst is closer to two orders of magnitude less energetic than the least energetic FRBs. The lack of single-pulse radio detections shows that the single pulses detected over a range of fluences are either rare, or highly clustered, or both. It may also indicate that the magnetar lies somewhere between being radio-quiet and radio-loud in terms of its ability to produce radio emission efficiently.
... From a colour excess -distance relation ( ( − ) vs. ), an absorption ( V ) and colour excess relation, an absorption -hydrogen column density ( H ) relation and a hydrogen column density -dispersion measure relation it is possible to link dispersion measure to distance. The first step is to use 3D dust mapping as in Green et al. (2019), Sale et al. (2009), Sale et al. (2014. Here we use the results from Green et al. (2019) to link the detected colour excess ( ( − )) to distance. ...
Magnetars are a promising candidate for the origin of Fast Radio Bursts (FRBs). The detection of an extremely luminous radio burst from the Galactic magnetar SGR J1935+2154 on 2020 April 28 added credence to this hypothesis. We report on simultaneous and non-simultaneous observing campaigns using the Arecibo, Effelsberg, LOFAR, MeerKAT, MK2 and Northern Cross radio telescopes and the MeerLICHT optical telescope in the days and months after the April 28 event. We did not detect any significant single radio pulses down to fluence limits between 25 mJy ms and 18 Jy ms. Some observing epochs overlapped with times when X-ray bursts were detected. Radio images made on four days using the MeerKAT telescope revealed no point-like persistent or transient emission at the location of the magnetar. No transient or persistent optical emission was detected over seven days. Using the multi-colour MeerLICHT images combined with relations between DM, NH and reddening we constrain the distance to SGR J1935+2154, to be between 1.5 and 6.5 kpc. The upper limit is consistent with some other distance indicators and suggests that the April 28 burst is closer to two orders of magnitude less energetic than the least energetic FRBs. The lack of single-pulse radio detections shows that the single pulses detected over a range of fluences are either rare, or highly clustered, or both. It may also indicate that the magnetar lies somewhere between being radio-quiet and radio-loud in terms of its ability to produce radio emission efficiently.
... L'infrarouge n'est pas le seul domaine de longueurs d'ondes qui permette l'étude de l'ex tinction interstellaire. Sale et al. (2009) 9). La très grande quantité d'objets contenus dans le catalogue IPHAS (~200 millions) permet une cartographie 3D précise jusqu'à 10 kpc de la moitié nord du plan Galactique. ...
Notre Galaxie est de type spirale barrée. Cependant, sa structure reste très mal connue à ce jour et le nombre et la forme de ses bras ainsi que la longueur et l'orientation de la barre sont encore l'objet de vifs débats. Une des techniques à notre disposition pour étudier notre Galaxie est la photométrie. Mais elle est altérée par le milieu interstellaire traversé entre le point d'émission et le point d'observation : c'est l'extinction interstellaire. Elle varie en fonction des propriétés du milieu traversé, c'est pourquoi il est essentiel de disposer de cartographies détaillées de l'extinction interstellaire. Peu de cartographies 3D (position / valeur non-cumulative) existent à ce jour et elles reposent pour la plupart sur un modèle de population stellaire Galactique. Le but de ma thèse a été d'établir de nouvelles techniques de cartographie 3D de l'extinction, indépendantes de toute modélisation de la Galaxie. Pour cela, j'ai développé un algorithme permettant de retrouver l'extinction interstellaire dans une direction d'observation en exploitant les propriétés des étoiles du Red Clump. En présence d'extinction, ces étoiles forment une traînée dans les CMD. En arrivant à détecter la position de cette traînée, on peut déduire la relation distance-extinction dans le champ étudié. En appliquant massivement ce procédé, j'ai établi une carte 3D de l'extinction dans le premier quadrant. Dans un deuxième temps, j'ai utilisé une déconvolution bayésienne pour mettre en place une méthode permettant d'établir une cartographie de l'extinction interstellaire qui sera par la suite adaptable pour obtenir une cartographie 3D de la densité d'étoiles.
... One of the biggest strengths of IPHAS (as well as KIS) is the inclusion of the narrow H α photometric band. This has two uses: (i) the (r − Hα) colour, combined with a broad-band colour such as (r − i) or (g − i), enables a fix on intrinsic colour and extinction for the majority of detected sources (see the discussion of this and its exploitation for extinction mapping initiated by Sale et al. 2009), (ii) large numbers of emission line stars are made evident when the (r − Hα) colour is strong enough to represent an excess relative to the main stellar locus -removing the need for large-scale spectroscopic surveys (see e.g. Witham et al. 2006Witham et al. , 2007Witham et al. , 2008Raddi et al. 2013;Scaringi et al. 2013;Gkouvelis et al. 2016). ...
We present a sub-arcsecond crossmatch of Gaia DR2 against the INT Photometric Ha Survey of the Northern Galactic Plane Data Release 2 (IPHAS DR2) and the Kepler-INT Survey (KIS). The resulting value-added catalogues (VACs) provide additional precise photometry to the Gaia photometry (r, i, and Ha for IPHAS, with additional U and g for KIS). In building the catalogue, proper motions given in Gaia DR2 are wound back to match the epochs of IPHAS DR2, thus ensuring high proper motion objects are appropriately crossmatched. The catalogues contain 7927 224 and 791 071 sources for IPHAS and KIS, respectively. The requirement of > 5σ parallax detection for every included source means that distances out to 1-1.5 kpc are well covered. We define two additional parameters for each catalogued object: (i) fc, a magnitude-dependent tracer of the quality of the Gaia astrometric fit; (ii) fFP, the false-positive rate for parallax measurements determined from astrometric fits of a given quality at a given magnitude. Selection cuts based on these parameters can be used to clean colour-magnitude and colour-colour diagrams in a controlled and justified manner.We provide both full and light versions of the VAC, with VAC-light containing only objects that represent our recommended trade-off between purity and completeness. Uses of the catalogues include the identification of new variable stars in the matched data sets, and more complete identification of Ha-excess emission objects due to separation of high-luminosity stars from the main sequence. © 2018 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society.
... One of the biggest strengths of IPHAS (as well as KIS) is the inclusion of the narrow Hα photometric band. This has two uses: (i) the (r − Hα) colour, combined with a broadband colour such as (r − i) or (g − i), enables a fix on intrinsic colour and extinction for the majority of detected sources (see the discussion of this and its exploitation for extinction mapping initiated by Sale et al. 2009), (ii) large numbers of emission line stars are made evident when the (r − Hα) colour is strong enough to represent an excess relative to the main stellar locus -removing the need for large-scale spectroscopic surveys (see e.g. Witham et al. 2006Witham et al. , 2007Witham et al. , 2008Raddi et al. 2013;Scaringi et al. 2013;Gkouvelis et al. 2016). ...
We present a sub-arcsecond cross-match of Gaia DR2 against the INT Photometric H-alpha Survey of the Northern Galactic Plane Data Release 2 (IPHAS DR2) and the Kepler-INT Survey (KIS). The resulting value-added catalogues (VACs) provide additional precise photometry to the Gaia photometry (r, i and H-alpha for IPHAS, with additional U and g for KIS). In building the catalogue, proper motions given in \gaia\ DR2 are wound back to match the epochs of IPHAS DR2, thus ensuring high proper motion objects are appropriately cross-matched. The catalogues contain 7,927,224 and 791,071 sources for IPHAS and KIS, respectively. The requirement of >5-sigma parallax detection for every included source means that distances out to 1--1.5 kpc are well covered. We define two additional parameters for each catalogued object: (i) $f_c$, a magnitude-dependent tracer of the quality of the Gaia astrometric fit; (ii) $f_{FP}$, the false-positive rate for parallax measurements determined from astrometric fits of a given quality at a given magnitude. Selection cuts based on these parameters can be used to clean colour-magnitude and colour-colour diagrams in a controlled and justified manner. We provide both full and light versions of the VAC, with VAC-light containing only objects that represent our recommended trade-off between purity and completeness. Uses of the catalogues include the identification of new variable stars in the matched data sets, and more complete identification of H-alpha-excess emission objects thanks to separation of high-luminosity stars from the main sequence.
... Last but not least, it will be very informative to directly compare the results presented here with those obtained by other methods, such as near-infrared color excess method (Lombardi & Alves 2001;Lombardi et al. 2011;Majewski et al. 2011), Hα-based method (Sale et al. 2009), and Wolf method that is sensitive to gray dust (Yasuda et al. 2007;Gorbikov & Brosch 2010), to uncover and quantify various systematic errors that are likely to exist in all methods. ...
We use Sloan Digital Sky Survey (SDSS) photometry of 73 million stars to simultaneously constrain best-fit main-sequence stellar spectral energy distribution (SED) and amount of dust extinction along the line of sight toward each star. Using a subsample of 23 million stars with Two Micron All Sky Survey (2MASS) photometry, whose addition enables more robust results, we show that SDSS photometry alone is sufficient to break degeneracies between intrinsic stellar color and dust amount when the shape of extinction curve is fixed. When using both SDSS and 2MASS photometry, the ratio of the total to selective absorption, R{sub V} , can be determined with an uncertainty of about 0.1 for most stars in high-extinction regions. These fits enable detailed studies of the dust properties and its spatial distribution, and of the stellar spatial distribution at low Galactic latitudes (|b| < 30 Degree-Sign ). Our results are in good agreement with the extinction normalization given by the Schlegel et al. (SFD) dust maps at high northern Galactic latitudes, but indicate that the SFD extinction map appears to be consistently overestimated by about 20% in the southern sky, in agreement with recent study by Schlafly et al. The constraints on the shape of the dust extinction curve across the SDSS and 2MASS bandpasses disfavor the reddening law of O'Donnell, but support the models by Fitzpatrick and Cardelli et al. For the latter, we find a ratio of the total to selective absorption to be R{sub V} = 3.0 {+-} 0.1(random){+-}0.1 (systematic) over most of the high-latitude sky. At low Galactic latitudes (|b| < 5 Degree-Sign ), we demonstrate that the SFD map cannot be reliably used to correct for extinction because most stars are embedded in dust, rather than behind it, as is the case at high Galactic latitudes. We analyze three-dimensional maps of the best-fit R{sub V} and find that R{sub V} = 3.1 cannot be ruled out in any of the 10 SEGUE stripes at a precision level of {approx}0.1-0.2. Our best estimate for the intrinsic scatter of R{sub V} in the regions probed by SEGUE stripes is {approx}0.2. We introduce a method for efficient selection of candidate red giant stars in the disk, dubbed 'dusty parallax relation', which utilizes a correlation between distance and the extinction along the line of sight. We make these best-fit parameters, as well as all the input SDSS and 2MASS data, publicly available in a user-friendly format. These data can be used for studies of stellar number density distribution, the distribution of dust properties, for selecting sources whose SED differs from SEDs for high-latitude main-sequence stars, and for estimating distances to dust clouds and, in turn, to molecular gas clouds.
... Because infrared extinction curves are assumed to be independent of R V , this approach is more robust to R V variation than those using B − V color excesses. Similarly, Sale et al. (2009) created an algorithm capable of determining extinction in A to early K-type stars by comparing intrinsic to observed r − i colors (wavelengths with low R V -dependence) from the INT/WFC Photometric Hα Survey (Drew et al., 2005), which they used to map extinction in high resolution at low Galactic latitudes. ...
We use stellar spectra from the Sloan Digital Sky Survey (SDSS) to create a high- latitude extinction map of the local Galaxy. Our technique compares spectra from SDSS stars in low-extinction lines of sight, as determined by Schlegel, Finkbeiner, & Davis, to other SDSS spectra in order to derive improved distance estimates and accurate line-of-sight extinctions. Unlike most previous studies, which have used a two-color method to determine extinction, we fit extinction curves to fluxes across a large spectral range. We apply this method to SDSS K dwarfs, nearby L dwarfs, and stars in the DR7 M dwarf sample. Our result is an extinction map that extends from tens of pc to several kpc away from the sun. We also use a similar technique to create a map of RV values within approximately 1 kpc of the sun, and find they are consistent with the widely accepted diffuse interstellar medium value of 3.1. Using our extinction data, we derive a dust scale height for the local galaxy of 119 ± 15 parsecs and find evidence for a local dust cavity.
We have devised a predominantly Naive Bayes−based method to classify X-ray sources detected by Chandra in the Cygnus OB2 association into members, foreground objects, and background objects. We employ a variety of X-ray, optical, and infrared characteristics to construct likelihoods using training sets defined by well-measured sources. Combinations of optical photometry from the Sloan Digital Sky Survey ( riz ) and Isaac Newton Telescope Photometric H α Survey ( r I i I H α ), infrared magnitudes from United Kingdom Infrared Telescope Deep Sky Survey and Two-Micron All Sky Survey ( JHK ), X-ray quantiles and hardness ratios, and estimates of extinction A v are used to compute the relative probabilities that a given source belongs to one of the classes. Principal component analysis is used to isolate the best axes for separating the classes for the photometric data, and Gaussian component separation is used for X-ray hardness and extinction. Errors in the measurements are accounted for by modeling as Gaussians and integrating over likelihoods approximated as quartic polynomials. We evaluate the accuracy of the classification by inspection and reclassify a number of sources based on infrared magnitudes, the presence of disks, and spectral hardness induced by flaring. We also consider systematic errors due to extinction. Of the 7924 X-ray detections, 5501 have a total of 5597 optical/infrared matches, including 78 with multiple counterparts. We find that ≈6100 objects are likely association members, ≈1400 are background objects, and ≈500 are foreground objects, with an accuracy of 96%, 93%, and 80%, respectively, with an overall classification accuracy of approximately 95%.
Until recently, black holes (BHs) could be discovered only through accretion from other stars in X-ray binaries, or in merging double compact objects. Improvements in astrometric and spectroscopic measurements have made it possible to detect BHs also in noninteracting BH binaries (nBHBs) through a precise analysis of the companion’s motion. In this study, using an updated version of the StarTrack binary-star population modeling code and a detailed model of the Milky Way (MW) galaxy, we calculate the expected number of detections for Gaia and LAMOST surveys. We develop a formalism to convolve the binary population synthesis output with a realistic stellar density distribution, star formation history (SFH), and chemical evolution for the MW, which produces a probability distribution function of the predicted compact-binary population over the MW. This avoids the additional statistical uncertainty that is introduced by methods that Monte Carlo sample from binary population synthesis output to produce one potential specific realization of the MW compact-binary distribution, and our method is also comparatively fast to such Monte Carlo realizations. Specifically, we predict ∼41–340 nBHBs to be observed by Gaia, although the numbers may drop to ∼10–70 if the recent (≲100 Myr) star formation is low (∼1 M ⊙ yr ⁻¹ ). For LAMOST we predict ≲14 detectable nBHBs, which is lower partially because its field of view covers just ∼6% of the Galaxy.
The INT Galactic Plane Survey (IGAPS) is the merger of the optical photometric surveys, IPHAS and UVEX, based on data from the Isaac Newton Telescope (INT) obtained between 2003 and 2018. Here, we present the IGAPS point source catalogue. It contains 295.4 million rows providing photometry in the filters, i , r , narrow-band H α , g , and U RGO . The IGAPS footprint fills the Galactic coordinate range, | b | < 5° and 30° < ℓ < 215°. A uniform calibration, referred to as the Pan-STARRS system, is applied to g , r , and i , while the H α calibration is linked to r and then is reconciled via field overlaps. The astrometry in all five bands has been recalculated in the reference frame of Gaia Data Release 2. Down to i ∼ 20 mag (Vega system), most stars are also detected in g , r , and H α . As exposures in the r band were obtained in both the IPHAS and UVEX surveys, typically a few years apart, the catalogue includes two distinct r measures, r I and r U . The r 10 σ limiting magnitude is approximately 21, with median seeing of 1.1 arcsec. Between approximately 13th and 19th mag in all bands, the photometry is internally reproducible to within 0.02 mag. Stars brighter than r = 19.5 mag are tested for narrow-band H α excess signalling line emission, and for variation exceeding | r I − r U | = 0.2 mag. We find and flag 8292 candidate emission line stars and over 53 000 variables (both at > 5 σ confidence).
In our Galaxy, star formation occurs in a variety of environments, with a large fraction of stars formed in clusters hosting massive stars. OB stars have an important feedback on the evolution of protoplanetary disks around nearby young stars and likely on the process of planet formation occurring in them. The nearby massive association Cygnus OB2 is an outstanding laboratory to study this feedback. It is the closest massive association to our Sun, and hosts hundreds of massive stars and thousands of low mass members. In this paper, we analyze the spatial variation of the disk fraction in Cygnus OB2 and we study its correlation with the local values of Far and Extreme ultraviolet radiation fields and the local stellar surface density. We present definitive evidence that disks are more rapidly dissipated in the regions of the association characterized by intense local UV field and large stellar density. In particular, the FUV radiation dominates disks dissipation timescales in the proximity (i.e. within 0.5 pc) of the O stars. In the rest of the association, EUV photons potentially induce a significant mass loss from the irradiated disks across the entire association, but the efficiency of this process is reduced at increasing distances from the massive stars due to absorption by the intervening intracluster material. We find that disk dissipation due to close stellar encounters is negligible in Cygnus OB2, and likely to have affected 1% or fewer of the stellar population. Disk dissipation is instead dominated by photoevaporation. We also compare our results to what has been found in other young clusters with different massive populations, concluding that massive associations like Cygnus OB2 are potentially hostile to protoplanetary disks, but that the environments where disks can safely evolve in planetary systems are likely quite common in our Galaxy.
Our Galaxy, the Milky Way, is a benchmark for understanding disk galaxies. It is the only galaxy whose formation history can be studied using the full distribution of stars from white dwarfs to supergiants. The oldest components provide us with unique insight into how galaxies form and evolve over billions of years. The Galaxy is a luminous (L-star) barred spiral with a central box/peanut bulge, a dominant disk, and a diffuse stellar halo. Based on global properties, it falls in the sparsely populated "green valley" region of the galaxy colour-magnitude diagram. Here we review the key integrated, structural and kinematic parameters of the Galaxy, and point to uncertainties as well as directions for future progress. Galactic studies will continue to play a fundamental role far into the future because there are measurements that can only be made in the near field and much of contemporary astrophysics depends on such observations.
Following on from the second release of calibrated photometry from IPHAS, the INT/WFC Photometric Hα Survey of the Northern
Galactic Plane, we present incompleteness-corrected stellar density maps in the r and i photometric bands. These have been computed to a range of limiting magnitudes reaching to 20th magnitude in r and 19th in i (Vega system), and with different angular resolutions – the highest resolution available being 1 arcmin2. The maps obtained cover 94 per cent of the 1800 square degree IPHAS footprint, spanning the Galactic latitude range, −5°
< b < +5°, north of the celestial equator. The corrections for incompleteness, due to confusion and sensitivity loss at the faint
limit, have been deduced by the method of artificial source injection. The presentation of this method is preceded by a discussion
of other more approximate methods of determining completeness. Our method takes full account of position-dependent seeing
and source ellipticity in the survey data base. The application of the star counts to testing reddened Galactic disc models
is previewed by a comparison with predicted counts along three constant-longitude cuts at ℓ ≃ 30°, 90° and 175°: some overprediction
of the most heavily reddened ℓ ≃ 30° counts is found, alongside good agreement at ℓ ≃ 90° and 175°.
We present predictions for the surface density of ultracool dwarfs (with
spectral types M8-T8) for a host of deep fields that are likely to be observed
with the James Webb Space Telescope. Based on simple thin and thick/thin disk
(exponential) models, we show the typical distance modulus is mu~9.8 mag, which
at high Galactic latitude is 5log(2 z_scl)-5. Since this is a property of the
density distribution of an exponential disk, it is independent of spectral type
or stellar sample. Using the published estimates of the ultracool dwarf
luminosity function, we show that their number counts typically peak around
J~24 mag with a total surface density of Sigma ~ 0.3 arcmin^-2, but with a
strong dependence on galactic coordinate and spectral type. Owing to the
exponential shape of the disk, the ultracool dwarfs are very rare at faint
magnitudes (J>~27 mag), with typical densities of Sigma~0.005 arcmin^-2 (or
~20% of the total contribution within the field). Therefore in the very narrow
and deep fields, we predict there are only a few ultracool dwarfs, and hence
these stars are likely not a severe contaminant in searches for high-redshift
galaxies. Furthermore the ultracool dwarfs are expected to be considerably
brighter than the high-redshift galaxies, so samples near the faint-end of the
high-redshift galaxy population will be the purest. We present the star-count
formalism in a simplified way so that observers may easily predict the number
of stars for their conditions (field, depth, wavelength, etc.).
We map the distribution of dust in M31 at 25pc resolution, using stellar
photometry from the Panchromatic Hubble Andromeda Treasury. We develop a new
mapping technique that models the NIR color-magnitude diagram (CMD) of red
giant branch (RGB) stars. The model CMDs combine an unreddened foreground of
RGB stars with a reddened background population viewed through a log-normal
column density distribution of dust. Fits to the model constrain the median
extinction, the width of the extinction distribution, and the fraction of
reddened stars. The resulting extinction map has >4 times better resolution
than maps of dust emission, while providing a more direct measurement of the
dust column. There is superb morphological agreement between the new map and
maps of the extinction inferred from dust emission by Draine et al. 2014.
However, the widely-used Draine & Li (2007) dust models overpredict the
observed extinction by a factor of ~2.5, suggesting that M31's true dust mass
is lower and that dust grains are significantly more emissive than assumed in
Draine et al. (2014). The discrepancy we identify is consistent with similar
findings in the Milky Way by the Planck Collaboration (2015), but has a more
complex dependence on parameters from the Draine & Li (2007) dust models. We
also show that the discrepancy with the Draine et al. (2014) map is lowest
where the interstellar radiation field has a harder spectrum than average. We
discuss possible improvements to the CMD dust mapping technique, and explore
further applications.
Column density (N) PDFs serve as a powerful tool to characterize the physical
processes that influence the structure of molecular clouds. Star-forming clouds
can best be characterized by lognormal PDFs for the lower N range and a
power-law tail for higher N, commonly attributed to turbulence and self-gravity
and/or pressure, respectively. We report here on PDFs obtained from
observations of 12CO, 13CO, C18O, CS, and N2H+ in the Cygnus X North region and
compare to a PDF derived from dust observations with the Herschel satellite.
The PDF of 12CO is lognormal for Av~1-30, but is cut for higher Av due to
optical depth effects. The PDFs of C18O and 13CO are mostly lognormal up for
Av~1-15, followed by excess up to Av~40. Above that value, all CO PDFs drop,
most likely due to depletion. The high density tracers CS and N2H+ exhibit only
a power law distribution between Av~15 and 400, respectively. The PDF from dust
is lognormal for Av~2-15 and has a power-law tail up to Av~500. Absolute values
for the molecular line column densities are, however, rather uncertain due to
abundance and excitation temperature variations. Taken the dust PDF face value,
we 'calibrate' the molecular line PDF of CS to the one of the dust and
determined an abundance [CS]/[H2] of 10^-9. The slopes of the power-law tails
of the CS, N2H+, and dust PDFs are -1.6, -1.4, and -1.9, respectively, and are
thus consistent with free-fall collapse of filaments and clumps. A quasi static
configuration of filaments and clumps can possibly also account for the
observed N-pdfs, as long as they have a sufficiently condensed density
structure and external ram pressure by gas accretion is provided. The somehow
flatter slopes of N2H+ and CS can reflect an abundance change and/or subthermal
excitation at low column densities.
Selection effects can bedevil the inference of the properties of a population
of astronomical catalogues, unavoidably biasing the observed catalogue. This is
particularly true when mapping interstellar extinction in three dimensions:
more extinguished stars are fainter and so generally less likely to appear in
any magnitude limited catalogue of observations. This paper demonstrates how to
account for this selection effect when mapping extinction, so that accurate and
unbiased estimates of the true extinction are obtained. We advocate couching
the description of the problem explicitly as a Poisson point process, which
allows the likelihoods employed to be easily and correctly normalised in such a
way that accounts for the selection functions applied to construct the
catalogue of observations.
The young massive OB association Cygnus OB2, in the Cygnus X complex, is the
closest (1400 pc) star forming region to the Sun hosting thousands of young low
mass stars and up to 1000 OB stars, among which are some of the most massive
stars known in our Galaxy. This region holds great importance for several
fields of modern astrophysics, such as the study of the physical properties of
massive and young low-mass stars and the feedback provided by massive stars on
star and planet formation process. Cygnus OB2 has been recently observed with
Chandra/ACIS-I as part of the 1.08Msec Chandra Cygnus OB2 Legacy Project. This
survey detected 7924 X-ray sources in a square degree area centered on Cyg OB2.
Since a proper classification and study of the observed X-ray sources also
requires the analysis of their optical and infrared counterparts, we combined a
large and deep set of optical and infrared catalogs available for this region
with our new X-ray catalog. In this paper we describe the matching procedure
and present the combined catalog containing 5703 sources. We also briefly
discuss the nature of the X-ray sources with optical and infrared counterparts
using their position in the color-magnitude and color-color diagrams.
We present a method for obtaining the likelihood function of distance and
extinction to a star given its photometry. The other properties of the star
(its mass, age, metallicity and so on) are marginalised assuming a simple
Galaxy model. We demonstrate that the resulting marginalised likelihood
function can be described faithfully and compactly using a Gaussian mixture
model. For dust mapping applications we strongly advocate using monochromatic
over bandpass extinctions, and provide tables for converting from the former to
the latter for different stellar types.
We present the first results of our search for new, extended planetary nebulae (PNe) based on careful, systematic, visual
scrutiny of the imaging data from the Isaac Newton Telescope Photometric Hα Survey of the Northern Galactic plane (IPHAS).
The newly uncovered PNe will help to improve the census of this important population of Galactic objects that serve as key
windows into the late-stage evolution of low- to intermediate-mass stars. They will also facilitate study of the faint end
of the ensemble Galactic PN luminosity function. The sensitivity and coverage of IPHAS allows PNe to be found in regions of
greater extinction in the Galactic plane and/or those PNe in a more advanced evolutionary state and at larger distances compared
to the general Galactic PN population. Using a set of newly revised optical diagnostic diagrams in combination with access
to a powerful, new, multiwavelength imaging data base, we have identified 159 true, likely and possible PNe for this first
catalogue release. The ability of IPHAS to unveil PNe at low Galactic latitudes and towards the Galactic Anticentre, compared
to previous surveys, makes this survey an ideal tool to contribute to the improvement of our knowledge of the whole Galactic
PN population.
The INT/WFC Photometric Hα Survey of the Northern Galactic Plane (IPHAS) is a 1800 deg2 imaging survey covering Galactic latitudes |b| < 5° and longitudes ℓ = 30°–215° in the r, i, and Hα filters using the Wide Field Camera (WFC) on the 2.5-m Isaac Newton Telescope (INT) in La Palma. We present the first
quality-controlled and globally calibrated source catalogue derived from the survey, providing single-epoch photometry for
219 million unique sources across 92 per cent of the footprint. The observations were carried out between 2003 and 2012 at
a median seeing of 1.1 arcsec (sampled at 0.33 arcsec pixel−1) and to a mean 5σ depth of 21.2 (r), 20.0 (i), and 20.3 (Hα) in the Vega magnitude system. We explain the data reduction and quality control procedures, describe and
test the global re-calibration, and detail the construction of the new catalogue. We show that the new calibration is accurate
to 0.03 mag (root mean square) and recommend a series of quality criteria to select accurate data from the catalogue. Finally,
we demonstrate the ability of the catalogue's unique (r − Hα, r − i) diagram to (i) characterize stellar populations and extinction regimes towards different Galactic sightlines and (ii) select
and quantify Hα emission-line objects. IPHAS is the first survey to offer comprehensive CCD photometry of point sources across
the Galactic plane at visible wavelengths, providing the much-needed counterpart to recent infrared surveys.
We present a three-dimensional extinction map in the r band. The map has a spatial angular resolution, depending on latitude, between 3 and 9 arcmin and covers the entire Xuyi
Schmidt Telescope Photometric Survey of the Galactic Anticentre (XSTPS-GAC) survey area of over 6000 deg2 for Galactic longitude 140 < l < 240 deg and latitude −60 < b < 40 deg. By cross-matching the photometric catalogue of the
XSTPS-GAC with those of 2MASS and WISE, we have built a multiband photometric stellar sample of about 30 million stars and applied spectral energy distribution
(SED) fitting to the sample. By combining photometric data from the optical to the near-infrared, we are able to break the
degeneracy between the intrinsic stellar colours and the amounts of extinction by dust grains for stars with high photometric
accuracy, and trace the extinction as a function of distance for low Galactic latitude and thus highly extincted regions.
This has allowed us to derive the best-fitting extinction and distance information of more than 13 million stars, which are
used to construct the three-dimensional extinction map. We have also applied a Rayleigh–Jeans colour excess (RJCE) method
to the data using the 2MASS and WISE colour (H − W2). The resulting RJCE extinction map is consistent with the integrated two-dimensional map deduced using the best-fitting
SED algorithm. However for individual stars, the amounts of extinction yielded by the RJCE method suffer from larger errors
than those given by the best-fitting SED algorithm.
We present a scheme for using stellar catalogues to map the three-dimensional distributions of extinction and dust within
our Galaxy. Extinction is modelled as a Gaussian random field, whose covariance function is set by a simple physical model
of the interstellar medium that assumes a Kolmogorov-like power spectrum of turbulent fluctuations. As extinction is modelled
as a random field, the spatial resolution of the resulting maps is set naturally by the data available; there is no need to
impose any spatial binning. We verify the validity of our scheme by testing it on simulated extinction fields and show that
its precision is significantly improved over previous dust-mapping efforts. The approach we describe here can make use of
any photometric, spectroscopic or astrometric data; it is not limited to any particular survey. Consequently, it can be applied
to a wide range of data from both existing and future surveys.
We present a 3D map of extinction in the northern Galactic plane derived using photometry from the INT/WFC Photometric Hα
Survey of the northern Galactic plane. The map has fine angular ( ∼ 10 arcmin) and distance (100 pc) sampling allied to a
significant depth (≳5 kpc). We construct the map using a method based on a hierarchical Bayesian model described in a previous
article by Sale. In addition to mean extinction, we also measure differential extinction, which arises from the fractal nature
of the interstellar medium, and show that it will be the dominant source of uncertainty in estimates of extinction to some
arbitrary position. The method applied also furnishes us with photometric estimates of the distance, extinction, effective
temperature, surface gravity, and mass for ∼38 million stars. Both the extinction map and the catalogue of stellar parameters
are made publicly available via http://www.iphas.org/extinction.
We present a map of the dust reddening to 4.5 kpc derived from Pan-STARRS1
stellar photometry. The map covers almost the entire sky north of declination
-30 degrees at a resolution of 7' to 14', and is based on the estimated
distances and reddenings to more than 500 million stars. The technique is
designed to map dust in the Galactic plane, where many other techniques are
stymied by the presence of multiple dust clouds at different distances along
each line of sight. This reddening-based dust map agrees closely with the
Schlegel, Finkbeiner, and Davis (SFD; 1998) far-infrared emission-based dust
map away from the Galactic plane, and the most prominent differences between
the two maps stem from known limitations of SFD in the plane. We also compare
the map with Planck, finding likewise good agreement in general at high
latitudes. The use of optical data from Pan-STARRS1 yields reddening
uncertainty as low as 25 mmag E(B-V).
The VST Photometric Hα Survey of the Southern Galactic Plane and Bulge (VPHAS+) is surveying the southern Milky Way in u, g, r, i and Hα at ∼1 arcsec angular resolution. Its footprint spans the Galactic latitude range −5o < b < +5° at all longitudes south of the celestial equator. Extensions around the Galactic Centre to Galactic latitudes ±10°
bring in much of the Galactic bulge. This European Southern Observatory public survey, begun on 2011 December 28, reaches
down to ∼20th magnitude (10σ) and will provide single-epoch digital optical photometry for ∼300 million stars. The observing
strategy and data pipelining are described, and an appraisal of the segmented narrow-band Hα filter in use is presented. Using
model atmospheres and library spectra, we compute main-sequence (u − g), (g − r), (r − i) and (r − Hα) stellar colours in the Vega system. We report on a preliminary validation of the photometry using test data obtained
from two pointings overlapping the Sloan Digital Sky Survey. An example of the (u − g, g − r) and (r − Hα, r − i) diagrams for a full VPHAS+ survey field is given. Attention is drawn to the opportunities for studies of compact nebulae
and nebular morphologies that arise from the image quality being achieved. The value of the u band as the means to identify planetary-nebula central stars is demonstrated by the discovery of the central star of NGC
2899 in survey data. Thanks to its excellent imaging performance, the VLT Survey Telescope (VST)/OmegaCam combination used
by this survey is a perfect vehicle for automated searches for reddened early-type stars, and will allow the discovery and
analysis of compact binaries, white dwarfs and transient sources.
The science goals for ground-based large-area surveys, such as the Dark
Energy Survey, Pan-STARRS, and the Large Synoptic Survey Telescope, require
calibration of broadband photometry that is stable in time and uniform over the
sky to precisions of a per cent or better. This performance will need to be
achieved with data taken over the course of many years, and often in less than
ideal conditions. This paper describes a strategy to achieve precise internal
calibration of imaging survey data taken in less than photometric conditions,
and reports results of an observational study of the techniques needed to
implement this strategy. We find that images of celestial fields used in this
case study with stellar densities of order one per arcmin-squared and taken
through cloudless skies can be calibrated with relative precision of 0.5 per
cent (reproducibility). We report measurements of spatial structure functions
of cloud absorption observed over a range of atmospheric conditions, and find
it possible to achieve photometric measurements that are reproducible to 1 per
cent in images that were taken through cloud layers that transmit as little as
25 per cent of the incident optical flux (1.5 magnitudes of extinction). We
find, however, that photometric precision below 1 per cent is impeded by the
thinnest detectable cloud layers. We comment on implications of these results
for the observing strategies of future surveys.
The results of CCD photometry in the Vilnius seven-color system down to
V = 18 mag are presented for 242 stars in the direction of the young
open cluster IC 1805 that is located in the active star-forming region
W4 in the Cas OB6 association. Photometric data were used to classify
stars into spectral and luminosity classes, and to determine their
interstellar reddenings, extinctions and distances. We confirm the
CH3OH and H2O maser VLBA parallax results that the
cluster is located close to the front side of the Perseus arm, at a
distance about 2.0 kpc. In the color-magnitude diagram, zero-age main
sequence (ZAMS) stars of the cluster extend to spectral class A0. The
extinction values for the majority of the cluster stars are between 2.2
and 2.7 mag, with a mean value of 2.46 mag. This extinction originates
mainly between the Sun and the outer edge of the Local arm, in
accordance with the distribution of CO clouds. In the Perseus arm and
beyond, the extinction was investigated using the classification and
reddening determination for A0-F0 stars measured in the r, i, Hα
system of the IPHAS survey to r = 19 mag. The extinction AV
within the Perseus arm ranges from 2.5-4.5 mag at the front edge to
3.0-5.0 mag at the far edge. Possibly, we have found about 20 early
A-type stars located in the Outer arm. The 2MASS JHKs
photometry for red giants gives much higher extinction values (up to
about 6 mag), which would correspond to the stars located behind dense
clouds of both arms. In the area, using the WISE, 2MASS, and IPHAS
photometry data, 18 possible young stellar objects (YSOs) of low masses
are identified. Six high-mass YSOs (five Ae/Be stars and a F6e star) are
known from previous investigations.
Full Table 1 is only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr
(ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/554/A3
We present the high-dispersion spectra of the elliptical ring shaped
planetary nebula NGC 6803, secured with the Hamilton Echelle
Spectrograph attached to the 3-m Shane telescope of Lick Observatory.
Numerous lines from neutral to quadruply ionized ions are presented in
the wavelength region from 3650 to 9900 Å. We also use the low
dispersion UV spectral data obtained with the 60-cm interstellar
ultraviolet explorer. In spite of its simplistic symmetrical bilateral
shape, the diagnostics imply that the physical condition of the nebular
shell is very complex with a huge density range of 1300-80 000
cm-3. A comparison of the 1995 and 2001 [Ar iv] data suggests
that the density increase occurred near the inner shell boundary. In
spite of a huge ionization potential range, the average electron
temperature indicated by primary diagnostic lines is relatively low,
i.e., Te ≤ 9500 K, except for [Cl iv], from which we
derive a temperature that is around 11 500 K. We derived the chemical
abundances of He, C, N, O, Ne, S, Ar, Cl, and K, based on the physical
condition suggested by diagnostics and photo-ionization analysis. The
chemical abundances of NGC 6803 are mostly enhanced when compared with
the average Galactic planetary nebula. The effective temperature of its
central star appears to be about 90 000 K and its luminosity about 2400
L⊙, assuming a distance of 3000 pc. The evolutionary
track implies that NGC 6803 might have been evolved from a companion
star of about 1.0 M⊙ in a binary system, or from a single
progenitor of about 1.5 M⊙, born in a metal-rich zone
near the Galactic plane.
Table 2 is available in electronic form at http://www.aanda.org
Over the next few years, the optical surveys IPHAS, UVEX and VPHAS+ will
provide complete photometric coverage of the Galactic Plane within the
latitude range -{5}^{circ } < b < +{5}^{circ }. Of these surveys,
IPHAS is the most complete. Drawing on the work that has already been
embarked upon using IPHAS r, i and narrowband Hα data, examples of
the application of these very wide area surveys to the study of star
clusters and the structure of the Milky Way disc are outlined.
Research aiming to describe the spiral structure of the Milky Way disc
has to confront both difficulties in obtaining distances and the high
interstellar extinction found in the Galactic plane. We present a study
of a section of the Perseus Arm, between Galactic longitudes
(120∘, 140∘) and Galactic latitudes
(-{1}^{circ },+{4}^{circ }), including a number of young clusters.
Determination of physical parameters of young stellar objects (YSO) and
mapping of the A-star surface density are used to analyse evidence of
young structures (typically 10-100 Myrs old) defined in space.
Until recently, Hα has been seen as the tracer of ionized gas,
picking out both star formation and the late stages of stellar
evolution. This has been reaffirmed, spectacularly, by the recent WHAM
and SHS surveys. But the advent of large-area digital detectors creates
a new role for narrowband Hα as a direct, simultaneous, measure of
intrinsic stellar colour and reddening when e.g. r'-Hα is combined
with a nearby broad band colour e.g. r'- i'. This new capability has
been clearly demonstrated by the nearly-complete IPHAS survey.
The science goals for future ground-based all-sky surveys, such as the Dark Energy Survey, PanSTARRS, and the Large Synoptic Survey Telescope, require calibration of broadband photometry that is stable in time and uniform over the sky to precisions of a per cent or better, and absolute calibration of color measurements that are similarly accurate. This performance will need to be achieved with measurements made from multiple images taken over the course of many years, and these surveys will observe in less than ideal conditions. This paper describes a technique to implement a new strategy to directly measure variations of atmospheric transmittance at optical wavelengths and application of these measurements to calibration of ground-based observations. This strategy makes use of measurements of the spectra of a small catalog of bright 'probe' stars as they progress across the sky and back-light the atmosphere. The signatures of optical absorption by different atmospheric constituents are recognized in these spectra by their characteristic dependences on wavelength and airmass. State-of-the-art models of atmospheric radiation transport and modern codes are used to accurately compute atmospheric extinction over a wide range of observing conditions. We present results of an observing campaign that demonstrate that correction for extinction due to molecular constituents and aerosols can be done with precisions of a few millimagnitudes with this technique.
Aims: Eleven new symbiotic stars have recently been discovered with IPHAS, the INT Halpha survey of the Northern Galactic plane. The star IPHAS J190832.31+051226.6 was proposed as an additional candidate on the basis of the existing spectrum. Here, we investigate the nature of this source by means of additional observations. Methods: Photometric data, optical spectra obtained in 2006 and 2009, a higher resolution spectrum resolving the Halpha profile, and near-IR spectra of IPHAS J190832.31+051226.6 are all presented. Results: The source brightened in the r band by 2.3 mag from 2004 to 2009. From 2006 to 2009, the spectrum has evolved from one with the obvious continuum of an M giant star plus HI and HeI lines in emission to a lower excitation nebular spectrum with HI, CaII, and FeII emission and a bluer continuum in which the absorption bands of the red giant are only visible at wavelengths longer than 7500 Å. The Halpha line is broad with a deep central absorption and extended wings. Conclusions: The averaged rate of the brightness increase, the rise of a blue continuum overwhelming the absorption bands of the M giant, and the corresponding decline of the ionization condition of the emission-line spectrum, are all consistent with the hypothesis that IPHAS J190832.31+051226.6 is a new symbiotic star picked up during the onset of a symbiotic nova outburst that is still in progress at the time of writing. Based on observations obtained at; the 2.6 m Nordic Optical Telescope operated by NOTSA, and the 2.5 m INT and 4.2 m WHT telescopes of the Isaac Newton Group of Telescopes in the Spanish Observatorio del Roque de Los Muchachos of the Instituto de Astrofísica de Canarias.
The formation of stars in massive clusters is one of the main modes of the
star formation process. However, the study of massive star forming regions is
hampered by their typically large distances to the Sun. One exception to this
is the massive star forming region Cygnus OB2 in the Cygnus X region, at the
distance of about 1400 pc. Cygnus OB2 hosts very rich populations of massive
and low-mass stars, being the best target in our Galaxy to study the formation
of stars, circumstellar disks, and planets in presence of massive stars. In
this paper we combine a wide and deep set of photometric data, from the r band
to 24 micron, in order to select the disk bearing population of stars in Cygnus
OB2 and identify the class I, class II, and stars with transition and
pre-transition disks. We selected 1843 sources with infrared excesses in an
area of 1 degree x 1 degree centered on Cyg OB2 in several evolutionary stages:
8.4% class I, 13.1% flat-spectrum sources, 72.9% class II, 2.3% pre-transition
disks, and 3.3% transition disks. The spatial distribution of these sources
shows a central cluster surrounded by a annular overdensity and some clumps of
recent star formation in the outer region. Several candidate subclusters are
identified, both along the overdensity and in the rest of the association.
Context. Be stars are probably the most rapid rotators among stars in
the main sequence (MS) and, as such, are excellent candidates to study
the incidence of the rotation on the characteristics of their non-radial
pulsations, as well as on their internal structure. Pulsations are also
thought to be possible mechanisms that help the mass ejection needed to
build up the circumstellar disks of Be stars. Aims: The purpose
of this paper is to identify a number of faint Be stars observed with
the CoRoT satellite and to determine their fundamental parameters, which
will enable us to study their pulsation properties as a function of the
location in the HR diagram and to search for correlations with the light
outbursts, which are possibly produced by discrete mass ejections. Methods: We identified those objects in the exofields of CoRoT
presenting the Be phenomenon using Hα surveys, as well as
automated methods based on pulsation properties that we finally
confirmed with FLAMES/GIRAFFE and X-shooter spectroscopic observations
at VLT/ESO, and with near-IR photometry. The spectra were 1) corrected
for the veiling effect, 2) treated with the GIRFIT code to determine
apparent fundamental parameters, and 3) corrected with the FASTROT code
for effects induced by the rapid rotation. Results: A list of 41
Be star candidates were found from photometric and spectroscopic
criteria. The spectral coverage useful for determining the fundamental
parameters was obtained for only about half of them. We then
spectroscopically identified 21 Be stars, two probable Be stars, and two
B stars contaminated by the Sh 2-284 nebulosity. A
short description of the spectral characteristics of each star is given.
The fundamental parameters and, in particular, the rotation frequency
νr (cycles per day) were all corrected for rotational
effects at rotation rates ranging from Ω/Ωc = 0.8
to 1.0. We have determined the positions of Be stars in the HR diagram
and find two of them located beyond the MS phase. Conclusions:
The well-determined fundamental parameters presented in this paper for a
statistically reliable sample of Be stars will enable us in forthcoming
papers to compare the properties of stars as pulsators either according
to their location in different regions of the HR diagram or with those
predicted from theory.
Appendices are only available in electronic form at http://www.aanda.org
We present a determination of the average space density and birth rate of hydrogen-line (DA) white dwarfs within a radius
of 1 kpc around the Sun, based on an observational sample of 360 candidate white dwarfs with g < 19.5 and (g − r) < 0.4, selected from the UV-Excess (UVEX) survey of the northern Galactic plane, in combination with a theoretical white
dwarf population that has been constructed to simulate the observations, including the effects of reddening and observational
selection effects. The main uncertainty in the derivation of the white dwarf space density and current birth rate lies in
the absolute photometric calibration and the photometric scatter of the observational data, which influences the classification
method on colours, the completeness and the pollution. Corrections for these effects are applied. We derive an average space
density of DA white dwarfs with Teff > 10 000 K (MV < 12.2) of (3.8 ± 1.1) × 10−4 pc−3, and an average DA white dwarf birth rate over the last 7×107 yr of (5.4 ± 1.5) × 10−13 pc−3 yr−1. Additionally, we show that many estimates of the white dwarf space density from different studies are consistent with each
other, and with our determination here.
Aiming to explore weak spectral features of stellar and interstellar origin
we used the NES echelle spectrograph of the 6-m telescope to obtain
high-resolution spectra for 13 hot O3 - B4 stars in the Cyg OB2 association,
including a high luminous star No. 12. Velocity fields in the atmospheres and
interstellar medium, characteristics of optical spectra and line profiles are
investigated. The cascade star formation scheme for the association is
confirmed. Evidence is presented suggesting that the hypergiant Cyg OB2 No.12
is an LBV object and that its anomalous reddening has a circumstellar nature.
We investigate a region of the Galactic plane, between 120° ≤ ℓ ≤ 140° and −1° ≤ b ≤ +4°, and uncover a population of moderately reddened (E(B − V) ∼ 1) classical Be stars within and beyond the Perseus and Outer Arms. 370 candidate emission-line stars (13 ≲ r ≲ 16) selected from the Isaac Newton Telescope Photometric Hα Survey of the Northern Galactic plane have been followed up
spectroscopically. A subset of these, 67 stars with properties consistent with those of classical Be stars, have been observed
at sufficient spectral resolution (Δλ ≈ 2–4 Å) at blue wavelengths to narrow down their spectral types. We determine these
to a precision estimated to be ±1 subtype and then we measure reddenings via spectral energy distribution fitting with reference
to appropriate model atmospheres. Corrections for contribution to colour excess from circumstellar discs are made using an
established scaling to Hα emission equivalent width. Spectroscopic parallaxes are obtained after luminosity class has been
constrained via estimates of distances to neighbouring A/F stars with similar reddenings. Overwhelmingly, the stars in the
sample are confirmed as luminous classical Be stars at heliocentric distances ranging from 2 kpc up to ∼12 kpc. However, the
errors are presently too large to enable the cumulative distribution function with respect to distance to distinguish between
models placing the stars exclusively in spiral arms, or in a smooth exponentially declining distribution.
Cygnus OB2 is the nearest example of a massive star-forming region (SFR), containing over 50 O-type stars and hundreds of B-type stars. We have analyzed the properties of young stars in two fields in Cyg OB2 using the recently published deep catalog of Chandra X-ray point sources with complementary optical and near-IR photometry. Our sample is complete to ~1 M ☉ (excluding A- and B-type stars that do not emit X-rays), making this the deepest study of the stellar properties and star formation history in Cyg OB2 to date. From Siess et al. isochrone fits to the near-IR color-magnitude diagram, we derive ages of 3.5+0.75 –1.0 and 5.25+1.5 –1.0 Myr for sources in the two fields, both with considerable spreads around the pre-main-sequence isochrones. The presence of a stellar population somewhat older than the present-day O-type stars, also fits in with the low fraction of sources with inner circumstellar disks (as traced by the K-band excess) that we find to be very low, but appropriate for a population of age ~5 Myr. We also find that the region lacks a population of highly embedded sources that is often observed in young SFRs, suggesting star formation in the vicinity has declined. We measure the stellar mass functions (MFs) in this limit and find a power-law slope of Γ = –1.09 ± 0.13, in good agreement with the global mean value estimated by Kroupa. A steepening of the slope at higher masses is observed and suggested as due to the presence of the previous generation of stars that have lost their most massive members. Finally, combining our MF and an estimate of the radial density profile of the association suggests a total mass of Cyg OB2 of ~3 × 104 M ☉, similar to that of many of our Galaxy's most massive SFRs.
We present a complete library of synthetic spectra based on Kurucz's
codes that covers the 2500 10 500 Å wavelength range at resolving
powers R{P}=20 000, 11 500 (≡GAIA), 8500 (≡RAVE),
2000 (≡SLOAN) and uniform dispersions of 1 and 10 Å/pix. The
library maps the whole HR diagram, exploring 51 288 combinations of
atmospheric parameters spanning the ranges: 3500 ≤ T_eff ≤ 47 500
K, 0.0≤ log g ≤ 5.0 , -2.5 ≤ [M/H] ≤ 0.5, [ α/Fe] =
0.0,+0.4, ξ = 1, 2, 4 km s-1, 0 ≤ V_rot ≤ 500 km
s-1. The spectra are available both as absolute fluxes as
well as continuum normalized. Performance tests and spectroscopic
applications of the library are discussed, including automatic
classification of data from spectroscopic surveys like RAVE, SLOAN,
GAIA. The entire library of synthetic spectra is accessible via the web.
During the past decade or so, measurements of Galactic HI absorption using VLBI against extragalactic sources, as well as multi-epoch observations in pulsar directions, have detected small-scale transverse variations corresponding to tens of AU at the distance of the absorbing matter. Hitherto these measurements have been interpreted as small-scale structure in the HI distribution with densities nHI˜ 104-105 cm-3, orders of magnitude greater than those of the parsec-scale structure. Naturally it is difficult to imagine how such structures could exist in equilibrium with other components of the ISM. In this talk we show that structure on all scales contributes to the differences on neighbouring lines of sight, and that the observed differences can be accounted for by a natural extension of the distribution of irregularities in the distribution of HI opacities at larger scales, using a single power law. Appreciating certain similarity of the contexts, we also examine if a suitable single power-law description of the electron density distribution in the ISM is consistent with the observations of the Extreme Scattering Events. In either case, we find a) no compelling evidence for over-dense, over-pressured structures, and b) no surprise in these apparent density enhancements on small transverse scales.
An extensive grid of spherically symmetric model atmospheres of stars
with
1. 2500 K ≤ Teff ≤ 8000 K, 2. -1.0 ≤ log g (= log
GM/R2) ≤ 5.0 (cgs units), 3. different combinations of M
and R, 4. -5 ≤ [A/H] ≤ 1, and 5. a number of CNO abundance
combinations
is being constructed with an updated version of the MARCS program.
Special efforts are made to reach accuracy and completeness in opacity
data. Opacity sampling is used with 10,000 and (for a minority of
models) 90,000 wavelength points. Synthetic spectra are also provided.
We shall show how these classical models may be used to illustrate
important physical properties of cool star atmospheres.
We present the first deep observations with the new Hitchhiker parallel CCD camera. In parallel with scheduled observations,
we have observed a field near the North Galactic Pole in four wavebands, with an equivalent total exposure time of 5 h, and
obtained galaxy number counts to limiting magnitudes of $m_B=26.4, m_V=24.9, m_R=26.3$ and $m_I=22.5$. The data should be complete to about 0.5 mag above these faint limits. We also present B–V, B–R, B–I, V–R and R–I colour histograms for the faint galaxies as a function of B, V or R magnitude. The observations are in good agreement with previous deep CCD data, showing a steep number-count slope in the
blue, which gradually decreases for longer wavelength observations, and a gradual blueing (in terms of B–R) of the mean galaxy population at fainter magnitudes. Although the V frame is less deep, so limiting our sensitivity, there is no significant sign of equivalent blueing in V–R, suggesting that the main effect is at the shorter (observed) wavelengths, and supporting the idea that most of the excess
blue galaxies are dwarfs at moderately low redshifts. We then present a simple model, based on significant numbers of dwarf
galaxies, which accounts successfully for all the number-count and colour data with no evolution. The model underestimates the observed redshift distribution, and appears to be inconsistent with measures of the
local luminosity function. We conclude that some level of evolution is required, unless these surveys are biased against dwarf
galaxies.
Colour excesses and photometric distances of open clusters compiled by Janes & Adler and Lyngå have been used to study the
distribution of interstellar matter within a few kiloparsecs of the Sun. In general the reddening material is highly concentrated
near the galactic plane. It is found that the plane defined by the distribution of interstellar matter is inclined to the
formal galactic plane by an angle of $0_{.}^{\circ}8\pm0_{.}^{\circ}2$ and shows a maximum upward tilt towards l~60°. The scale height of the reddening material is found to be 0.16±0.02 kpc. Based on the distribution of whole interstellar
matter, the Sun is found to be situated at a distance of about 10 pc above the galactic plane of symmetry defined by the reddening
material, while from a subsample in the direction of maximum tilt this figure is found to be 3±4 pc.
A numerical algorithm has been developed for calculating
three-dimensional visual interstellar extinction and its error from
inputs of Galactic longitude, Galactic latitude, and distance. The code
synthesizes the results of several published studies and can be used
both for making corrections to individual observations and for
correcting statistical samples. The studies have been modified to
introduce a statistical extinction correction at large (poorly-sampled)
distances, and to truncate values at the (assumed) edge of the Milky
Way. When combined, the studies allow individual absorbing clouds in the
interstellar medium to be identified within S kpc. Several observational
selection biases are noted and discussed in view of this combined
analysis. The FORTRAN-coded algorithm EXTINCT.FOR (v. 2.0.2) can be
obtained via anonymous ftp from site ftp.mankato.msus.edu/pub/astro.
We describe a sample of 13 bright (18.5 < BJ < 20.1), compact galaxies at low redshift (0.05 < z < 0.21) behind the Fornax Cluster. These galaxies are unresolved on UK Schmidt sky survey plates, and so they would be missing from most galaxy catalogs compiled from this material. The objects were found during initial observations of The Fornax Spectroscopic Survey. This project is using the Two-degree Field spectrograph on the Anglo-Australian Telescope to obtain spectra for a complete sample of all 14,000 objects, stellar and nonstellar, with 16.5 < BJ < 19.7, in a 12 deg2 area centered on the Fornax Cluster of galaxies. The surface density of compact galaxies with magnitudes 16.5 < BJ < 19.7 is 7 ± 3 deg-2, representing 2.8% ± 1.6% of all local (z < 0.2) galaxies to this limit. There are 12 ± 3 deg-2 with 16.5 < BJ < 20.2. They are luminous (-21.5 < MB < -18.0, for H0 = 50 km s-1 Mpc-1), and most have strong emission lines (Hα equivalent widths of 40-200 Å) and small sizes typical of luminous H II galaxies and compact narrow emission line galaxies. Four out of 13 have red colors and early-type spectra, and so they are unlikely to have been detected in any previous surveys.
We have conducted a detailed new survey of the local population of white dwarfs lying within 20 pc of the Sun. A new revised catalog of local white dwarfs containing 122 entries (126 individual degenerate stars) is presented. This list contains 27 white dwarfs not included in a previous list from 2002, as well as new and recently published trigonometric parallaxes. In several cases new members of the local white dwarf population have come to light through accurate photometric distance estimates. In addition, a suspected new double degenerate system (WD 0423+120) has been identified. The 20 pc sample is currently estimated to be 80% complete. Using a variety of recent spectroscopic, photometric, and trigonometric distance determinations, we re-compute a space density of 4.8 ± 0.5 × 10–3 pc–3 corresponding to a mass density of 3.2 ± 0.3 × 10–3 M ☉ pc–3 from the complete portion of the sample within 13 pc. We find an overall mean mass for the local white dwarfs of 0.665 M ☉, a value larger than most other non-volume-limited estimates. Although the sample is small, we find no evidence of a correlation between mass and temperature in which white dwarfs below 13,000 K are systematically more massive than those above this temperature. Within 20 pc 25% of the white dwarfs are in binary systems (including double degenerate systems). Approximately 6% are double degenerates and 6.5% are Sirius-like systems. The fraction of magnetic white dwarfs in the local population is found to be 13%.
Databases of observed stellar spectra are continuously being published and made publicly available, and the average number of stars per database is increasing. This paper reviews the current status. The Asiago Database of Spectroscopic Databases (ADSD) aims to provide a census of publicly available libraries of observed stellar spectra, to document their content and to homogenize their parameters for easier consultation and access. Refereed journals, conference proceedings and personal web pages have been searched for libraries of a given minimum size, properly documented and with data made publicly and directly accessible. A total of 294 databases (54 ultraviolet, 183 optical, 50 infrared and 7 combined) have been found to match the selection criteria and have been included in ADSD. They provide spectra of 16046 different stars in electronic or printed formats. A card for each library describes in a homogeneous way its aims, content, type of data, caveats, data download links, source paper, properties of included stars and more. A dedicated web page allows direct access to ADSD, plans future updates, and provides interrogation tools to search all the libraries matching given characteristics or including any given star.
The CCD observations of the rich open star cluster NGC 2099 and its surrounding field region have been carried out up to a limiting magnitude of V ∼ 22 mag in B, V and I passbands for the first time. A total of ∼12 000 stars have been observed in the area of about 24 × 34 in the cluster region, as well as ∼2180 stars in the ∼12 × 12 area of the field region located ∼45 away from the cluster center. The cluster parameters determined by fitting the convective core overshoot isochrones in the V, (B − V) and V, (V − I) diagrams are E(B − V) = 0.30 ± 0.04 mag, distance = 1360 ± 100 pc, age = 400 Myr and metallicity Z = 0.008. A well-defined cluster main sequence spread over about 8 mag in range is observed for the first time. Its intrinsic spread amounting to ∼0.06 mag in colour is almost the same over the entire brightness and can be understood in terms of the presence of physical/optical binaries. The core and cluster radii determined from the radial stellar density profiles are 185 and 1000 respectively. Only about 22% of cluster members are present in the core region. The effects of mass segregation, most probably due to dynamical evolution, have been observed in the cluster. The mass function slope of the entire cluster is ∼−0.67 ± 0.12. It becomes closer to the Salpeter value of −1.35, if flattening in the cluster mass function due to presence of both binaries and a much more extended corona is considered.
Aims.The Two Micron All Sky Survey, along with the Stellar Population Synthesis Model of the Galaxy, developed in Besançon, is used to calculate the extinction distribution along different lines of sight. By combining many lines of sight, the large scale distribution of interstellar material can be deduced.Methods.The Galaxy model is used to provide the intrinsic colour of stars and their probable distances, so that the near infrared colour excess, and hence the extinction, may be calculated and its distance evaluated. Such a technique is dependent on the model used, however we are able to show that moderate changes in the model parameters result in insignificant changes in the predicted extinction. Results.This technique has now been applied to over 64 000 lines of sight, each separated by 15´, in the inner Galaxy ($|l|\le$ 100°, $|b|\le$ 10°). We have projected our three dimensional results onto a two dimensional plane in order to compare them with existing two dimensional extinction maps and CO surveys. We find that although differences exist due to the different methods used or the medium traced, the same large scale structures are visible in each of the different maps. Using our extinction map, we have derived the main characteristics of the large scale structure of the dust distribution. The scale height of the interstellar matter is found to be $125^{+17}_{-7}$ pc. The dust distribution is found to be asymmetrically warped, in agreement with CO and HI observations of the ISM. However, the slope of the dust warp and the galactocentric distance where it starts are found to be smaller than the values measured for the external HI disc: for positive longitudes the angle is $\theta=$ $89\degr$, it starts at 8.7 kpc from the Galactic center and grows with a slope of 0.14, while at negative longitudes, the angle of the maximum is at $\theta=$ $272\degr$, the starting radius 7.8 kpc and the slope 0.11. Finally, the presence of dust is detected in the Galactic bulge. It forms an elongated structure approximately 5.2 kpc long and lies at an angle of $30$ $\pm$ $5\degr$ with respect to the Sun-Galactic centre direction. This may be interpreted as a dust lane along the Galactic bar. This resulting extinction map will be useful for studies of the inner Galaxy and its stellar populations.
The distance dependence of interstellar extinction in the direction of the Aquila Rift is investigated using 473 stars observed in the Vilnius photometric system. The front edge of the dark clouds in the area is found to be at $225\pm55$ pc and the thickness of the cloud system is about 80 pc. The maximum extinction $A_V$ in the clouds is close to 3.0 mag. Two stars with larger extinction are found and discussed. Since the new distance of the clouds is larger than the previously accepted distance, the cloud system mass should be increased to $2.7\times10^5~M_{\odot}$ which is close to the virial mass estimated from the CO velocity dispersion. Additional arguments are given in favor of the genetic relation between the Serpens and the Scorpio-Ophiuchus dark clouds.
We present the kinematics of a sample of 398 DA white dwarfs from the SPY project (ESO SN Ia Progenitor surveY) and discuss kinematic criteria for distinguishing of thin-disk, thick-disk, and halo populations. This is the largest homogeneous sample of white dwarfs for which 3D space motions have been determined. Since the percentage of old stars among white dwarfs is higher than among main-sequence stars, they are presumably valuable tools in studies of old populations, such as the halo and the thick disk. Studies of white-dwarf kinematics can help to determine the fraction of the total mass of our Galaxy contained in the form of thick-disk and halo white dwarfs, an issue which is still under discussion. Radial velocities and spectroscopic distances obtained by the SPY project were combined with our measurements of proper motions to derive 3D space motions. Galactic orbits and further kinematic parameters were computed. We calculated individual errors of kinematic parameters by means of a Monte Carlo error propagation code. Our kinematic criteria for assigning population membership were deduced from a sample of F and G stars taken from the literature, for which chemical criteria can be used to distinguish between a thin-disk, a thick-disk and a halo star. Our kinematic population classification scheme is based on the position in the U - V - velocity diagram, the position in the J(z)-eccentricity diagram, and the Galactic orbit. We combined this with age information and found seven halo and 23 thick-disk white dwarfs in this brightness limited sample. Another four rather cool white dwarfs probably also belong to the thick disk. Correspondingly 2% of the white dwarfs belong to the halo and 7% to the thick disk. The mass contribution of the thick-disk white dwarfs is found to be substantial, but is insufficient to account for the missing dark matter.
The Fornax Spectroscopic Survey will use the Two degree Field spectrograph (2dF) of the Angle-Australian Telescope to obtain spectra for a complete sample of all 14000 objects with 16.5 less than or equal to b(j) less than or equal to 19.7 in a 12 square degree area centred on the Fornax Cluster. The aims of this project include the study of dwarf galaxies in the cluster (both known low surface brightness objects and putative normal surface brightness dwarfs) and a comparison sample of background field galaxies. We will also measure quasars and other active galaxies, any previously unrecognised compact galaxies and a large sample of Galactic stars. By selecting all objects-both stars and galaxies-independent of morphology, we cover a much larger range of surface brightness and scale size than previous surveys. In this paper we first describe the design of the survey. Our targets are selected from UK Schmidt Telescope sky survey plates digitised by the Automated Plate Measuring (APM) facility. We then describe the photometric and astrometric calibration of these data and show that the APM astrometry is accurate enough for use with the 2dF. We also describe a general approach to object identification using cross-correlations which allows us to identify and classify both stellar and galaxy spectra. We present results from the first 2dF field. Redshift distributions and velocity structures are shown for all observed objects in the direction of Fornax, including Galactic stars? galaxies in and around the Fornax Cluster, and for the background galaxy population. The velocity data for the stars show the contributions from the different Galactic components, plus a small tail to high velocities. We find no galaxies in the foreground to the cluster in our 2dF field. The Fornax Cluster is clearly defined kinematically. The mean velocity from the 26 cluster members having reliable redshifts is 1560 +/- 80 km s(-1). They show a velocity dispersion of 380 +/- 50 km s(-1). Large-scale structure can be traced behind the cluster to a redshift beyond z = 0.3. Background compact galaxies and low surface brightness galaxies are found to follow the general galaxy distribution.
The parameterized extinction data of Fitzpatrick and Massa (1986, 1988) for the ultraviolet and various sources for the optical and near-infrared are used to derive a meaningful average extinction law over the 3.5 micron to 0.125 wavelength range which is applicable to both diffuse and dense regions of the interstellar medium. The law depends on only one parameter R(V) = A(V)/E(B-V). An analytic formula is given for the mean extinction law which can be used to calculate color excesses or to deredden observations. The validity of the law over a large wavelength interval suggests that the processes which modify the sizes and compositions of grains are stochastic in nature and very efficient.
We received a sample of 6840 Hipparcos stars south of declination -26 deg that (i) have MK spectral types in the Michigan catalogues and (ii) had spectroscopic parallaxes that placed them within 80 pc of the Sun. Of these, 3727 are well determined as luminosity class V and actually lie within 100 pc. From this subsample we can determine the distribution in M_V of main-sequence stars of given spectral type for spectral types that range from early F to early K. These distributions are significantly non-Gaussian, but when fitted to Gaussians they yield central values of M_V in good agreement with earlier estimates of the absolute magnitudes of main-sequence stars. We also determine anew the distribution of B-V at each spectral type. We find that the dispersion in B-V at given spectral type is very small.
We summarize radial velocity studies of selected stars in the old, distant clusters Berkeley 20, Berkeley 21, NGC 2141, Berkeley 29, and Berkeley 31. Three members of M67 were observed similarly, and those velocities compare extremely well with prior measures. We measured the chemical compositions of all six clusters for [Fe/H] as well as abundances of alpha elements, iron-peak elements, and those synthesized in either the s-process or the r-process. For the outer disk, the abundance gradient for [Fe/H] deviates from the trend defined near the solar neighborhood. Rather than declining with increasing Galactocentric distance, [Fe/H] appears to reach a "basement" at [Fe/H]=-0.5 beyond R_GC = 10 to 12kpc. We find enhanced [O/Fe], [alpha/Fe], and [Eu/Fe] in the outer disk revealing a rapid star formation history. An intriguing, but tentative, conclusion is that the outer disk open cluster abundance ratios are consistent with the outer disk being formed via a merger event. The basement in [Fe/H] and enhanced [alpha/Fe] suggest that the outer disk formed from a reservoir of gas with a star formation history distinct from the solar neighborhood. The open clusters may have formed as a result of star formation triggered by a merger event in the outer disk. The ages of the outer disk open clusters would then be a measure of when the merger occurred. [Abridged]
Aims:To constrain the formation and chemical evolution of the Galactic disk, we surveyed open clusters of different ages, metal contents, and distances from the Galactic centre. We employed FLAMES on VLT-UT2 to collect UVES spectra of five to ten giant stars in each of the selected clusters and used them to derive the iron abundance and the detailed chemical composition. Methods: Equivalent widths were measured and abundance analysis was performed using the MOOG code and Kurucz model atmospheres on all stars considered as cluster members on the basis of their radial velocity. Results: We derived the atmospheric parameters and the Fe abundance for NGC 2324 and NGC 2477 (average [Fe/H] = -0.17 with rms 0.05 dex, and +0.07 with rms 0.03 dex, respectively), two clusters never analyzed using high resolution spectra. We also derived the abundances of Mg, Al, Ca, Si, Ti, Cr, Ni, and Ba for these two clusters and for NGC 2660, NGC 3960, and Berkeley 32, whose atmospheric parameters and metallicities were measured in a previous paper. We determined the reddening values for the five clusters, based on the spectroscopic temperatures, literature photometry, and a colour-temperature relation. Conclusions: All clusters show solar-scaled abundances for alpha- and Fe-peak elements, while [Na/Fe] appears slightly enhanced and [Ba/Fe] significantly enhanced. Our findings were compared to thin-disk stars and other open clusters, and no significant deviation from the standard behavior was found.
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We give a brief overview of the INT Wide Field Camera (WFC) together with the automated pipeline processing developed specifically for the Wide Field Survey (WFS). The importance of accurate and complete FITS header information is stressed. Data processing products output from the complete pipeline are discussed.
The origin of helium-rich subdwarf O stars (He-sdO) is still unclear. Significant progress has been made recently in placing them in the Hertzsprung--Russell diagram. However, the question of their progenitors is still unsolved. It is unclear which types of stars evolve into He-sdOs and whether they are formed by all galactic populations. Here I will present results of a radial velocity (RV) survey of He-sdOs, which will be used to constrain the binary fraction and evaluate population membership.
We present a catalogue of point-source Hα emission-line objects selected from the INT/WFC Photometric Hα Survey (IPHAS) of the northern Galactic plane. The catalogue covers the magnitude range 13 ≤r′≤ 19.5 and includes Northern hemisphere sources in the Galactic latitude range −5° < b < 5°. It is derived from ∼1500 deg2 worth of imaging data, which represents 80 per cent of the final IPHAS survey area. The electronic version of the catalogue will be updated once the full survey data become available. In total, the present catalogue contains 4853 point sources that exhibit strong photometric evidence for Hα emission. We have so far analysed spectra for ∼300 of these sources, confirming more than 95 per cent of them as genuine emission-line stars. A wide range of stellar populations are represented in the catalogue, including early-type emission-line stars, active late-type stars, interacting binaries, young stellar objects and compact nebulae.
The spatial distribution of catalogue objects shows overdensities near sites of recent or current star formation, as well as possible evidence for the warp of the Galactic plane. Photometrically, the incidence of Hα emission is bimodally distributed in (r′−i′). The blue peak is made up mostly of early-type emission-line stars, whereas the red peak may signal an increasing contribution from other objects, such as young/active low-mass stars. We have cross-matched our Hα-excess catalogue against the emission-line star catalogue of Kohoutek & Wehmeyer, as well as against sources in SIMBAD. We find that fewer than 10 per cent of our sources can be matched to known objects of any type. Thus IPHAS is uncovering an order of magnitude more faint (r′ > 13) emission-line objects than were previously known in the Milky Way.
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Burstein and Heiles (1978) have reported the discovery of a new and
potentially accurate method for predicting line-of-sight reddening in
the galaxy. The present investigation is concerned with an assessment of
the accuracy of this method, taking into account a comparison of its
reddening predictions to those of two independent and highly accurate
methods, including the uvby-beta photometry of early-type stars and the
strong absorption line-strength-color relationship for the integrated
spectra of elliptical galaxies. On the basis of the results of the
investigation, it is concluded that a reasonable estimate of the
relative accuracy of the H I/galaxy counts method reported by Burstein
and Heiles is 0.01 mag in E(B-V) or 10% of the reddening, whichever is
larger.
We outline physical, numerical, and computational limits on Kurucz's
codes, for model atmospheres, ATLAS12 and ATLAS9, spectrum synthesis,
SYNTHE, and abundance analysis, WIDTH9.
Trumpler and Weaver (1953) discuss a systematic error which they attribute to the process of selecting stars by a lower limit in parallax values. The systematic error is introduced because, on the average, the observed parallaxes are larger than the true parallaxes. We demonstrate that the same type of systematic effect exists for all stars with observed t rigonometric parallaxes. The problem is treated analytically and can be put in a dimensionless form. The size of the systematic error depends only upon the ratio o/̌ where ó is the standard deviation of ̌0, the observed parallax. Corrections are obtained which are to be applied to absolute magnitudes derived from trigonometric parallaxes. The corrected absolute magnitudes are the proper values to use in luminosity calibrations.
Analytical expressions for interstellar extinction are convenient for
many numerical applications. Following the launch of the International
Ultraviolet Explorer (IUE), good quality UV data have become available
for individual objects in the Magellanic Clouds for the first time. The
main objective of the present investigation is related to a
parameterization of the UV LMC extinction law, as an aid in the analysis
of these data. Convenient formulas for the optical and IR extinction in
the LMC and the Galaxy are also provided.
We propose a new 3-dimensional extinction model based on the COBE/IRAS
all sky reddening map. Its application to globular and open cluster data
evidences that the COBE/IRAS reddening map has an accuracy of 18%, but
overestimates visual absorption by a factor of 1.16. This systematic
error does not change with galactic latitude and opacity significantly.
The implementation of the new extinction model has optimized our
galactic structure and kinematic model to low-galactic latitudes. Four
star count samples distributed in different galactic directions have
been compared with galactic model simulations. Numerical experiments
allow us to constrain the radial distribution of the galactic disk. The
disk scale length is found to be 2250 +/- 50 pc and the displacement of
the Sun from the galactic plane ZSun = 27.5 +/- 6.0 pc.
The relationship between galaxy counts and galactic absorption has been
examined in the Lick astrographic survey. It is found that results of
previous workers are consistent with the survey being limited by the
isophotal size of galaxies rather than by their apparent magnitude. The
selection of galaxies by size is also consistent with the degree of
galaxy clustering observed in the sample.
This paper is reproduced from Publ. Astron. Soc. Pac., Vol. 42, p. 214 -
226 (1930) to celebrate the centenary of the Publications. See also
004.050.
We present a full-sky 100 μm map that is a reprocessed composite of the COBE/DIRBE and IRAS/ISSA maps, with the zodiacal foreground and confirmed point sources removed. Before using the ISSA maps, we remove the remaining artifacts from the IRAS scan pattern. Using the DIRBE 100 and 240 μm data, we have constructed a map of the dust temperature so that the 100 μm map may be converted to a map proportional to dust column density. The dust temperature varies from 17 to 21 K, which is modest but does modify the estimate of the dust column by a factor of 5. The result of these manipulations is a map with DIRBE quality calibration and IRAS resolution. A wealth of filamentary detail is apparent on many different scales at all Galactic latitudes. In high-latitude regions, the dust map correlates well with maps of H I emission, but deviations are coherent in the sky and are especially conspicuous in regions of saturation of H I emission toward denser clouds and of formation of H2 in molecular clouds. In contrast, high-velocity H I clouds are deficient in dust emission, as expected.
The influence of shifts in effective wavelengths on ratios of total to selective extinction is examined, primarily to determine how to evaluate the Galactic extinction of extragalactic bodies in a way that minimizes systematic errors. In the process, a new procedure is developed for evaluating the Galactic or extragalactic extinction of any source in any filter from any index of reddening. The amount of dust along a sightline is quantified by the optical depth at 1 μm, which has the advantage of being roughly equal numerically to E(B-V). The optical depth can be derived iteratively from a color excess using an appropriate spectral energy distribution (SED) for the reddening probe, and a monochromatic law of reddening which delivers a value of AV/E(B-V) characteristic of the obscuring medium when applied to the spectrum of a reference source for which this ratio is known. Knowledge of the optical depth then facilitates the determination of the extinction of any source in any filter without concern as to the shape of the spectrum of the probe. The ratio of total to selective extinction for stars and galaxies is synthesized for a variety of filter combinations in order to examine variations with type, tilt, optical depth, and redshift. For this purpose, representative integrated SEDs spanning the space ultraviolet to the near-infrared are constructed for galaxy types E, Sab, Sbc, Scd, and Im, all at well-defined inclinations. In addition, an algorithm to adjust the shapes of the SEDs for tilt is developed. Along the main sequence, the classical ratio of total to selective extinction, AV/E(B-V), increases by 23% from O5 to M6. At late types, there are differences as high as 17% between evolved and unevolved stars. Along the Hubble sequence, AV/E(B-V) decreases by 5% from E to Im. The value for elliptical galaxies falls near the locus for the main sequence, not the giant branch. Correlated against B-I, AV/E(B-V) for star-forming galaxies is systematically lower than for stars of the same color by up to 5%. It increases much more rapidly with tilt than with the optical depth of Galactic dust, although neither dependence is strong. For both stars and galaxies, AV/E(B-V) varies dramatically with the redshift. Changes of 16% for a Type Ia supernovae and 22% for a Cepheid are seen out to z = 0.4. For elliptical galaxies, a variation of 30% can be expected out to z = 1, the precise form of which being dependent upon the ultraviolet excess. Even infrared ratios of total to selective extinction, such as AH/E(B-V), change significantly with color and redshift because of differential shifts in the effective wavelengths of B and V. As a gauge of reddening, E(V-I) is greatly preferable to E(B-V), because it is much less sensitive to color and redshift, yet more sensitive to the optical depth of dust. A demonstration is given on how to quantify upper limits to Galactic extinction which should be placed on studies of high-redshift supernovae, to reduce the redshift dependence of extinction corrections to a range that is insignificant compared with residuals supporting accelerated universal expansion. When the new technique for evaluating extinction corrections is applied to Cepheids in M31, distances for fields at different radii become less dispersed, confirming that the period-luminosity relation is not very sensitive to metallicity. However, the discrepancy between the Cepheid and maser distances to NGC 4258 cannot be attributed to the method of handling the extinction.
We present a three-dimensional model for the Milky Way fitted to the far-infrared (FIR) and near-infrared (NIR) data from the COBE/DIRBE instrument for Galactic latitudes |b| < 30° and to within 20° of the Galactic center. Because of the low optical depth at 240 μm, the FIR emission traces the distribution of Galactic dust in the Galaxy. We model the dust distribution as due to three components: a warped exponential disk with scale length 0.28 R☉ and a flaring scale height, a spiral arm component with four arms as traced by Galactic H II regions, and the local (Orion) arm, which produces prominent emission features at Galactic longitudes l 80 and -100°. A cosmic infrared background of 1.07 MJy sr-1 is recovered, consistent with previous determinations. The dust distribution is then used to calculate absorption in J and K, and the stellar emission in these wavebands is modeled with two components: a warped exponential disk with a scale length of 0.28 R☉ and a spiral arm component dominated by two arms. This small scale length is consistent with a maximal disk model for our Galaxy, which is inconsistent with the cuspy dark matter halos predicted in CDM models. We find different amplitudes for the warp in the stars and dust, which starts within the solar circle.
Spectra of nearly 20000 point-like objects to a Galactic reddening corrected magnitude of i=19.1 have been obtained to test the completeness of the SDSS quasar survey. The spatially-unresolved objects were selected from all regions of color space, sparsely sampled from within a 278 sq. deg. area of sky covered by this study. Only ten quasars were identified that were not targeted as candidates by the SDSS quasar survey (including both color and radio source selection). The inferred density of unresolved quasars on the sky that are missed by the SDSS algorithm is 0.44 per sq. deg, compared to 8.28 per sq. deg. for the selected quasar density, giving a completeness of 94.9(+2.6,-3.8) to the limiting magnitude. Omitting radio selection reduces the color-only selection completeness by about 1%. Of the ten newly identified quasars, three have detected broad absorption line systems, six are significantly redder than other quasars at the same redshift, and four have redshifts between 2.7 and 3.0 (the redshift range where the SDSS colors of quasars intersect the stellar locus). The fraction of quasars missed due to image defects and blends is approximately 4%, but this number varies by a few percent with magnitude. Quasars with extended images comprise about 6% of the SDSS sample, and the completeness of the selection algorithm for extended quasars is approximately 81%, based on the SDSS galaxy survey. The combined end-to-end completeness for the SDSS quasar survey is approximately 89%. The total corrected density of quasars on the sky to i=19.1 is estimated to be 10.2 per sq. deg.
We present radial velocities and metallicities for a sample of 39 open clusters with ages greater than about 700 million years. For 24 clusters new moderate-resolution spectroscopic data obtained with multiobject spectrographs on the Kitt Peak National Observatory and the Cerro Tololo Inter-American Observatory 4 m telescopes are used to determine radial velocities and mean cluster metallicities. These new results are combined with data published previously by Friel & Janes to provide a sample of 459 giants in 39 old open clusters, which are used to investigate radial abundance gradients in the Galactic disk. Based on an updated abundance calibration of spectroscopic indices measuring Fe and Fe-peak element blends, this larger sample yields an abundance gradient of -0.06 ± 0.01 dex kpc-1 over a range in Galactocentric radius of 7 to 16 kpc. There is a slight suggestion of a steepening of the abundance gradient with increasing cluster age in this sample, but the significance of the result is limited by the restricted distance range for the youngest clusters. The clusters show no correlation of metallicity with age in the solar neighborhood. Consistent with the evidence for a steepening of the gradient with age, the clusters in the outer disk beyond 10 kpc show a suggestion at the 1.5 σ level of a dependence of metallicity on age.
We present two models for the interstellar extinction in the Galaxy that are based on the hypothesis that the interstellar dust is well mixed with the gas, with a constant ratio (except for a small dependence of metallicity on the Galactic radius), and therefore that the extinction is proportional to the column density of the gas. In the first model we assume that the Galaxy is axisymmetric; the gas density in the disk is a function of the Galactic radius and of the distance perpendicular to the Galactic plane, and the extinction is proportional to the column density of the gas. In the second model we take into account the spiral structure of the Galaxy. In this case, instead of increasing almost linearly with distance, the extinction increases by steps each time a spiral arm is crossed, but only increases slowly in the interarm regions. The gas density distribution is obtained from the Berkeley and Parkes H I surveys and from the Columbia University CO survey. The IRAS 100 μm brightness distribution is also used as a tracer of the interstellar dust column density. The predictions of the models are compared with data taken from a number of catalogs that present color excess and distances for large samples of stars. Our models are useful for estimating distances of objects and color corrections for objects for which the distance can be estimated by some other method, and also for star counts and brightness models of the Galaxy, among other applications.
A stellar spectral ux library of wide spectral coverage, and an example of its applica-tion are presented. The new library consists of 131 ux calibrated spectra, encompassing all normal spectral types and luminosity classes at solar abundance, and metal-weak and metal-rich FFK dwarf and GGK giant components. Each library spectrum was formed by combining data from several sources overlapping in wavelength coverage. The SIM-BAD database, measured colors and line strengths were used to check that each input component has closely similar stellar type. The library has complete spectral coverage from 1150|10620 A for all components, and to 25000 A for about half of them, mainly later types of solar abundance. Missing spectral coverage in the infrared currently con-sists of a smooth energy distribution formed from standard colors for the relevant t ypes. The library is designed to permit inclusion of additional digital spectra, particularly of non-solar abundance stars in the infrared, as they become available. The library spectra are each given as F vs. , from 1150|25000 A in steps of 5 A. A program to combine the library spectra in the ratios appropriate to a selected isochrone is described, and an example spectral component signature of a composite population of solar age and metallicity illustrated. The library spectra and associated tables are available as text les by remote electronic access.
In this Letter I compare recent findings suggesting a low binary star fraction for late-type stars with knowledge concerning the forms of the stellar initial and present-day mass functions for masses down to the hydrogen-burning limit. This comparison indicates that most stellar systems formed in the Galaxy are likely single and not binary, as has been often asserted. Indeed, in the current epoch two-thirds of all main-sequence stellar systems in the Galactic disk are composed of single stars. Some implications of this realization for understanding the star and planet formation process are briefly mentioned.
A universal initial mass function (IMF) is not intuitive, but so far no convincing evidence for a variable IMF exists. The detection of systematic variations of the IMF with star-forming conditions would be the Rosetta Stone for star formation.
In this contribution an average or Galactic-field IMF is defined, stressing that there is evidence for a change in the power-law index at only two masses: near 0.5 M⊙ and near 0.08 M⊙. Using this supposed universal IMF, the uncertainty inherent in any observational estimate of the IMF is investigated by studying the scatter introduced by Poisson noise and the dynamical evolution of star clusters. It is found that this apparent scatter reproduces quite well the observed scatter in power-law index determinations, thus defining the fundamental limit within which any true variation becomes undetectable. The absence of evidence for a variable IMF means that any true variation of the IMF in well-studied populations must be smaller than this scatter.
Determinations of the power-law indices α are subject to systematic errors arising mostly from unresolved binaries. The systematic bias is quantified here, with the result that the single-star IMFs for young star clusters are systematically steeper by Δα≈0.5 between 0.1 and 1 M⊙ than the Galactic-field IMF, which is populated by, on average, about 5-Gyr-old stars. The MFs in globular clusters appear to be, on average, systematically flatter than the Galactic-field IMF (Piotto & Zoccali; Paresce & De Marchi), and the recent detection of ancient white-dwarf candidates in the Galactic halo and the absence of associated low-mass stars (Ibata et al.; Méndez & Minniti) suggest a radically different IMF for this ancient population. Star formation in higher metallicity environments thus appears to produce relatively more low-mass stars. While still tentative, this is an interesting trend, being consistent with a systematic variation of the IMF as expected from theoretical arguments.
The Isaac Newton Telescope (INT) Photometric Hα Survey of the Northern Galactic Plane (IPHAS) is a 1800-deg2 CCD survey of the northern Milky Way spanning the latitude range −5° < b < + 5° and reaching down to r′≃ 20 (10σ). Representative observations and an assessment of point-source data from IPHAS, now underway, are presented. The data obtained are Wide Field Camera images in the Hα narrow-band, and Sloan r′ and i′ broad-band filters. We simulate IPHAS (r′−Hα, r′−i′) point-source colours using a spectrophotometric library of stellar spectra and available filter transmission profiles: this defines the expected colour properties of (i) solar metallicity stars, without Hα emission, and (ii) emission-line stars. Comparisons with observations of fields in Aquila show that the simulations of normal star colours reproduce the observations well for all spectral types earlier than M. A further comparison between colours synthesized from long-slit flux-calibrated spectra and IPHAS photometry for six objects in a Taurus field confirms the reliability of the pipeline calibration. Spectroscopic follow-up of a field in Cepheus shows that sources lying above the main stellar locus in the (r′− Hα, r′−i′) plane are confirmed to be emission-line objects with very few failures. In this same field, examples of Hα deficit objects (a white dwarf and a carbon star) are shown to be readily distinguished by their IPHAS colours. The role IPHAS can play in studies of spatially resolved northern Galactic nebulae is discussed briefly and illustrated by a continuum-subtracted mosaic image of Shajn 147 (a supernova remnant, 3° in diameter). The final catalogue of IPHAS point sources will contain photometry on about 80 million objects. Used on its own, or in combination with near-infrared photometric catalogues, IPHAS is a major resource for the study of stellar populations making up the disc of the Milky Way. The eventual yield of new northern emission-line objects from IPHAS is likely to be an order of magnitude increase on the number already known.
The UKIDSS Galactic Plane Survey (GPS) is one of the five near-infrared Public Legacy Surveys that are being undertaken by
the UKIDSS consortium, using the Wide Field Camera on the United Kingdom Infrared Telescope. It is surveying 1868 deg2 of the northern and equatorial Galactic plane at Galactic latitudes −5° < b < 5° in the J, H and K filters and a ∼200-deg2 area of the Taurus–Auriga–Perseus molecular cloud complex in these three filters and the 2.12 μm (1–0) H2 filter. It will provide data on ∼2 × 109 sources. Here we describe the properties of the data set and provide a user's guide for its exploitation. We also present
brief Demonstration Science results from DR2 and from the Science Verification programme. These results illustrate how GPS
data will frequently be combined with data taken in other wavebands to produce scientific results. The Demonstration Science
comprises six studies. (1) A GPS-Spitzer-GLIMPSE cross-match for the star formation region G28.983−0.603 to identify YSOs. This increases the number of YSOs identified
by a factor of 10 compared to GLIMPSE alone. (2) A wide-field study of the M17 nebula, in which an extinction map of the field
is presented and the effect of source confusion on luminosity functions in different subregions is noted. (3) H2 emission in the ρ Ophiuchi dark cloud. All the molecular jets are traced back to a single active clump containing only a
few protostars, which suggests that the duration of strong jet activity and associated rapid accretion in low-mass protostars
is brief. (4) X-ray sources in the nuclear bulge. The GPS data distinguishes local main-sequence counterparts with soft X-ray
spectra from nuclear bulge giant counterparts with hard X-ray spectra. (5) External galaxies in the zone of avoidance. The
galaxies are clearly distinguished from stars in fields at longitudes l > 90°. (6) IPHAS-GPS optical–infrared spectrophotometric typing. The (i′−J) versus (J−H) diagram is used to distinguish A–F type dwarfs, G dwarfs, K dwarfs and red clump giants in a field with high reddening.
The surface gravities and radii of stars are calculated for different MK spectral types using the masses of stars determined from their evolutionary tracks in the HR diagram and the most reliable values of effective temperatures and absolute bolometric magnitudes. MK spectral types are calibrated in absolute visual magnitudes using the studies ofM
v published since 1965. The calibration of MK types in temperatures is based on the newest investigations including the results both from the ultraviolet and the infrared. The obtained masses, gravities, and the mass-luminosity relationship show reasonable agreement with independent observational data.
We present an analysis of the dust optical properties at large scale, for the whole galactic anticenter hemisphere. We used the 2MASS Extended Source Catalog to obtain the total reddening on each galaxy line of sight and we compared this value to the IRAS 100 $\mu$m surface brightness converted to extinction by Schlegel et al. (1998, ApJ, 500, 525). We performed a careful examination and correction of the possible systematic effects resulting from foreground star contamination, redshift contribution and galaxy selection bias. We also evaluated the contribution of dust temperature variations and interstellar clumpiness to our method. The correlation of the near-infrared extinction to the far-infrared optical depth shows a discrepancy for visual extinction greater than 1 mag with a ratio $A_{\rm V}({\rm FIR}) / A_{\rm V}({\rm gal}) = 1.31$ $\pm$ $0.06$. We attribute this result to the presence of fluffy/composite grains characterized by an enhanced far-infrared emissivity. Our analysis, applied to half of the sky, provides new insights on the dust grains nature suggesting fluffy grains are found not only in some very specific regions but in all directions for which the visual extinction reaches about 1 mag.
With the advent of multi-fibre spectrographs such as the ‘Two-Degree Field’ (2dF) instrument at the Anglo-Australian Telescope, quasar surveys that are free of any pre-selection of candidates and any biases this implies have become possible for the first time. The first of these is that which is being undertaken as part of the Fornax Spectroscopic Survey, a survey of the area around the Fornax Cluster of galaxies, and aims to obtain the spectra of all objects in the magnitude range 16.5<bj<19.7. To date, 3679 objects in the central π-deg² area have been successfully identified from their spectral characteristics. Of these, 71 are found to be quasars, 61 with redshifts 0.3<z<2.2 and 10 with redshifts z>2.2. Using this complete quasar sample, a new determination of quasar number counts is made, enabling an independent check of existing quasars surveys. Cumulative counts per square degree at a magnitude limit of bj<19.5 are found to be 11.5±2.2 for 0.3<z<2.2,2.22±0.93 for z>2.2 and 13.7±3.1 for z>0.3.
Large-scale CO surveys of the entire Galactic plane and specific nearby clouds have been combined to produce a panorama of the entire Milky Way in molecular clouds at an angular resolution of 1/2 deg. This survey is the only molecular line survey to date with sky coverage and resolution comparable to that of the early 21 cm surveys. The telescopes and the observing techniques used for the individual surveys and the methods of reconciling the survey results are described. The composite survey is presented as a spatial map integrated over velocity and as a longitude-velocity map integrated over latitude; individual clouds and large-scale features of the maps are briefly discussed. The distribution and properties of molecular clouds within 1 kpc of the sun are investigated, and a comparison with other Population I tracers is used to demonstrate that the composite survey probably provides a nearly complete inventory of nearby molecular clouds. 92 references.