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An IRAS survey of young stellar objects towards the Cygnus X region
Abstract
From a study of the Infrared Astronomical Satellite Point Source
Catalog, 159 objects similar to young B0 and early A type ZAMS stars
were identified in the Cygnus X region. Among the optical
identifications obtained are two unusual ringlike objects, IRAS 20286 +
4105 and 20236 + 4058; 13 compact nebulae often showing a biconical
shape; and 11 sources coincident with bright, starlike objects. It is
likely that the IRAS survey has detected all of the young, far-IR
emitting, B type stars within about 3 kpc, the majority coincident with
known molecular clouds. The inferred luminosity function implies a young
stellar object density of 0.0002-0.0003 stars/cu pc for L greater than
10 solar luminosities, similar to estimates for optically identifiable B
and early A type stars in the Galactic disk population as a whole.
... It is also coincident with a 6 cm emission radio source (GB6 B2010+3635; Gregory et al. 1996) and an IRAS source (IRAS 20103+3633). Odenwald (1989) noted the presence of several YSOs near the IRAS source, but we could not find any identification of a stellar cluster at this position. ...
Open stellar clusters are extremely valuable probes of Galactic structure, star formation, kinematics, and chemical abundance patterns. Near-infrared (NIR) data have enabled the detection of hundreds of clusters hidden from optical surveys, and mid-infrared (MIR) data are poised to offer an even clearer view into the most heavily obscured parts of the Milky Way. We use new MIR images from the Spitzer GLIMPSE-360, Cyg-X, and SMOG surveys to visually identify a large number of open cluster candidates in the outer disk of the Milky Way (65° < l < 265°). Using NIR color-magnitude diagrams, stellar isochrones, and stellar reddening estimates, we derive cluster parameters (metallicity, distance, reddening) for those objects without previous identification and/or parameters in the literature. In total, we present coordinates and sizes of 20 previously unknown open cluster candidates; for 7 of these we also present metallicity, distance, and reddening values. In addition, we provide the first estimates of these values for nine clusters that had been previously cataloged. We compare our cluster sizes and other derived parameters to those in the open cluster catalog of Dias et al. and find strong similarities except for a higher mean reddening for our objects, which signifies our increased detection sensitivity in regions of high extinction. The results of this cluster search and analysis demonstrate the ability of MIR imaging and photometry to augment significantly the current census of open clusters in the Galaxy.
Context. The Cygnus region, which dominates the local spiral arm of the Galaxy, is one of the nearest complexes of massive star formation, extending over several hundred parsecs. Its massive stellar content, regions of ongoing star formation, and molecular gas have been studied in detail at virtually all wavelengths. However, little is known of the history of the region beyond the past 10 Myr.
Aims. We use the correlations between age, mass and luminosity of red supergiants to explore the history of star formation in Cygnus previous to the formation of the present-day associations. The brightness and spectroscopic characteristics of red supergiants make it easy to identify them and build up a virtually complete sample of such stars at the distance of the Cygnus region, thus providing a record of massive star formation extending several tens of Myr into the past, a period inaccessible through the O and early B stars observable at present.
Methods. We have made a selection based on the 2MASS colors of a sample of bright, red stars in an area of 84 square degrees covering the whole present extension of the Cygnus association in the Local Arm. We have obtained spectroscopy in the red visible range allowing an accurate, homogeneous spectral classification as well as a reliable separation between supergiants and other cool stars. Our data are complemented with Gaia Data Release 2 astrometric data.
Results. We have identified 29 red supergiants in the area, 17 of which had not been previously classified as supergiants. Twenty-four of the 29 most likely belong to the Cygnus region and four of the remaining to the Perseus arm. We have used their derived luminosities and masses to infer the star formation history of the region. Intense massive star formation activity is found to have started approximately 15 Myr ago, and we find evidence for two other episodes, one taking place between 20 and 30 Myr ago and another one having ended approximately 40 Myr ago. There are small but significant differences between the kinematic properties of red supergiants younger or older then 20 Myr, hinting that stars of the older group were formed outside the precursor of the present Cygnus complex, possibly in the Sagittarius-Carina arm.
FU Orionis stars (FUors) are eruptive pre-main sequence objects thought to represent quasi-periodic or recurring stages of enhanced accretion during the low-mass star-forming process. We characterize the sample of known and candidate FUors in a homogeneous and consistent way, deriving stellar and circumstellar parameters for each object. We emphasize the analysis in those parameters that are supposed to vary during the FUor stage. We modeled the spectral energy distributions of 24 of the 26 currently known FUors, using the radiative transfer code of Whitney et?al. We compare our models with those obtained by Robitaille et?al. for Taurus class?II and?I sources in quiescence periods by calculating the cumulative distribution of the different parameters. FUors have more massive disks: we find that ?80% of the disks in FUors are more massive than any Taurus class?II and?I sources in the sample. Median values for the disk mass accretion rates are ?10?7M
??yr?1 versus ?10?5M
??yr?1 for standard young stellar objects (YSOs) and FUors, respectively. While the distributions of envelope mass accretion rates for class?I FUors and standard class?I objects are similar, FUors, on average, have higher envelope mass accretion rates than standard class?II and class?I sources. Most FUors (?70%) have envelope mass accretion rates above 10?7M
??yr?1. In contrast, 60% of the classical YSO sample has an accretion rate below this value. Our results support the current scenario in which changes experimented by the circumstellar disk explain the observed properties of these stars. However, the increase in the disk mass accretion rate is smaller than theoretically predicted, although in good agreement with previous determinations.
An important factor is the relation of HAEBE stars with other groups of peculiar early type stars, such as Be and B[e] stars, luminous blue variables (LBVs) etc. In many cases Polarimetric properties of HAEBE stars and B[e] (Be) stars are similar. We have carried out a Polarimetric study of peculiar early-type stars with the following aims:
1.
First, to obtain new Polarimetrie (linear and circular) data for a large group of HAEBE stars and candidate members, as well as for some peculiar Be and B[e] stars and LBVs. The whole list of programme stars consists of 60 objects located mainly in the southern hemisphere. For 40 of the 60 objects Polarimetrie data have been obtained for the first time.
2.
Second, to study Polarimetrie variability of the programme stars on different time-scales (minutes ... hours ...days).
The Great Cygnus Rift harbors numerous very active regions of current or recent star formation. In this part of the sky we look down a spiral arm, s= o regions from only a few hundred pc to several kpc are superposed. The North America and Pelican nebulae, parts of a single giant HII region, are the best known of the Cygnus regions of star formation and are located at a distance of only about 600 pc. Adjacent, but at a distance of about 1.7 kpc, is the Cygnus X region, a ˜10° complex of actively star forming molecular clouds and young clusters. The most massive of these clusters is the 3-4 Myr old Cyg OB2 association, containing several thousand OB stars and akin to the young globular clusters in the LMC. The rich populations of young low and high mass stars and protostars associated with the massive cloud complexes in Cygnus are largely unexplored and deserve systematic study.
The Cygnus X region is one of the most nearby star formation regions
within our Galaxy, that is recognised by prominent emission throughout
the entire electromagnetic spectrum, from radio to gamma-ray waves. The
centre is populated by the peculiar stellar cluster Cyg OB2,
traditionally classified as OB association, but in the light of modern
data more likely the prototype of a young globular cluster within our
own Galaxy. Heavily shredded by dense molecular clouds this object has
been less thoroughly studied than other, more distant star formation
complexes in the Milky Way. I will review our current knowledge about
this object, by exposing its properties and that of its environment
throughout the entire electromagnetic spectrum. Particular emphasis
will be given to infrared, radio-continuum, X-ray, and gamma-ray line
observations that find in Cyg OB2 a text-book case for multiwavelength
studies.
We present the results of an unbiased survey of a flux limited sample of
red IRAS point sources [Fν(12 μm) ≥ 1.0 Jy] in a
region of the southern sky being situated close to the plane of the
Galaxy and containing the Giant Molecular Clouds (GMCs) of the Vela
Molecular Ridge (VMR). From multifrequency observations we identify the
Lada & Wilking (LW) Class I sources of low to intermediate
luminosity in the actively star forming VMR. These clouds have probably
been forming stars for at least some 106 years and the
overall efficiency is estimated to lie in the range 5 - 15%, i.e. being
comparable to the average value found for the solar neighbourhood. The
shape of the bolometric luminosity function of the Class I sources of
the VMR is similar to that observed in other star forming regions of the
solar neighbourhood. These LFs are commonly very fiat and can be fit by
a mean power law of index 0.4±0.2 over five orders of magnitude
in luminosity, i.e. the form of the Class I LFs appears invariant under
luminosity translation. Such similarity suggests that the shape of the
Class I LFs is not strongly dependent on the different cloud properties,
which can vary appreciably from cloud to cloud. For the VMR, we have
also obtained a statistically more reliable Class I LF, containing more
than 102 sources and which is the result of subtracting the
source content of an adjacent reference field being void of molecular
material at the LSR-velocities of the VMR. Besides of also being fiat,
this LF does furthermore reveal that a single power law representation
seems inadequate over the observed range in luminosity. We identify the
indicated power law break tentatively as the result of a turnover of the
Class I LF at lower luminosities. Such turnover has been predicted by
some recent theories of protostellar evolution and is introduced by the
development of a characteristic mass scale during the mass accretion
time of the star forming cloud. Our observations would thus lend support
to the evolutionary model according to which the LW-Class I sources, as
a class, are protostars acereting matter at (normally) relatively high
rates. It would further be implied that the prevailing accretion rates
are not predominantly determined by the mass of the acereting protostar,
but largely by the effective isothermal sound speeds of the cloud. For a
given star forming cloud, this would determine the exact location of the
peak of the LF, thus introducing a dependence on global properties of
the cloud. We argue that in typical GMCs like the VMR a Salpeter-like
IMF is determined already by the time of appearance of the Class I
sources, whose mass spectrum is that observed for molecular cloud
clumps. Any initial differences in local IMFs produced by, e.g., less
massive molecular clouds would become `ironed out' with time, as the
GMCs develop stellar mass functions similar to the field star IMF at
comparable efficiency. We present, finally, some heuristic arguments
which indicate that the average mass column through a molecular cloud
provides an observable measure of the efficiency at which the cloud is
producing stars.
Five compact CO sources associated with unresolved IRAS sources in the
Cygnus X region have been detected. The compact CO sources are about 1-2
arcmin in diameter and are associated with IRAS sources that have FIR
continua indicative of young stellar objects. It is found that one of
the sources, IRAS 20286 + 4105, is surrounded by a ring-shaped optical
nebula circumscribing the region containing the CO and FIR emission.
We have undertaken a search for submillimetre (submm) embedded cores associated with known water maser sources. For 44 water
maser sources, we found submm continuum emission from 40 (91 per cent) using the James Clerk Maxwell Telescope (JCMT). Some
of the sources were then observed further. Maps were made in CO and C18O ($J=2\rightarrow1$) and CS ($J=5\rightarrow4$), using the JCMT, so that the molecular environment of the sources could be examined. In addition the Very Large Array (VLA)
was used to find accurate positions for the water maser emission and hence any association with compact radio continuum sources.
A preliminary examination of the data shows that most of the water masers are on average closer than 0.1 pc to the corresponding
submm source. A more detailed analysis will follow in a later paper.
We present matching-beam 6 and 2 cm radio continuum observations made
with the Very Large Array, and ammonia and water maser observations made
at the Haystack Observatory of 12 Infrared Astronomical Satellite (IRAS)
point sources selected from the survey of Scalise, Rodriguez, &
Mendoza-Torres (1989) of bright, unassociated IRAS point sources. These
sources have 60 or 100 micrometer flux densities in excess of
103 Jy and have no previous reference in any of the 37
catalogs considered for association of IRAS sources with known sources.
Six of the 12 sources have associated radio continuum, ammonia, and
water maser emission, and all of them show at least one of these three
emissions. In all sources detected, the ammonia is warm (T approximately
20 K) and suggests the association of dense molecular gas with embedded
heating sources. It is argued that all sources in the sample could be
associated with time-variable H2O maser emission. The radio and
far-infrared data appear to indicate that these sources are star-forming
regions, powered by a late O or early B-type star. Several of the
sources of lower luminosity (approximately 5 x 103 solar
luminosity) appear to have ionizing photon fluxes in excess of those
expected for a zero-age main-sequence star. Possible explanations for
this discrepancy are discussed.
The distribution of young stellar objects (YSOs) toward the Cygnus X
region obtained from the Infrared Astronomical Satellite (IRAS) is
combined with a recent (C-12)O (J = 2-1) survey. The most luminous YSOs
are examined for membership in the Local arm and the Perseus arm. The
far-infrared, radio, and molecular emission is also analyzed from a
global perspective. Seventy YSOs have been identified, corresponding to
stars earlier than about B7 ZAMS, primarily associated with molecular
clouds in the Local arm. The (C-12)O (J = 2-1) emission associated with
these YSOs reveals 23 instances of broad-line wings including 10 with
spatially complex, extended high-velocity molecular emission. We have
identified a new Perseus arm star-forming region, IRAS 20144+3726, and
have also identified 20264+4042 and 20321+4112 near the H II regions DR
7 and 18P61 as broad-wing outflow sources also located in the Perseus
arm. We conclude that no more than three YSOs earlier than approximately
B0 ZAMS currently exist in the Perseus arm region located behind Cygnus
X.
Near infrared observations of DR 18, a HII region in the Cygnus X
molecular complex, are presented in this paper. These observations
reveal DR 18 as an arc-shaped nebula in the 2.2 mu m region, with a
central star of V=15.6 obscured by A_V =~ 8 magnitudes. Visible and
near-infrared spectroscopy and photometry indicate a spectral type
around B0.5V for this star, while a near-infrared color-color diagram of
the stars in the area shows that the central star is the most luminous
one of a loose aggregate. Analysis of the narrow band imaging in the K
band suggests that the arc nebulosity is principally due to emission by
small grains, heated by the central star, in a photodissociation region.
We interpret the arc nebula as the interface between a molecular cloud
that is being eroded by the central star and the resulting HII region.
Using published models of photodissociation regions, we estimate the
density in the arc nebula to be a few times 10(3) cm(-3) . We briefly
discuss the possible relation of the structures observed in the near
infrared with the source IRAS 20333+4102, which has been included in
several far infrared and radio studies of the area. We conclude that
IRAS 20333+4102 is not directly related to any of the structures that we
describe here, and could be an intermediate mass protostar embedded
deeper in the molecular cloud. The emission associated to ionized gas in
DR 18 has a morphology fairly different from that of the arc nebula,
being brighter near the position of the central star. A crescent-shaped
peak is observed beside the central star and facing the arc nebula,
suggesting an interaction between a stream of ionized gas from the
nebula and the wind from the central star. We present two dimensional
gas dynamical simulations which successfully reproduce such gas stream,
the bow shock ahead of the central star, and the overall appearance of
the nebula. An essential component of our model is the existence of an
outward-decreasing density stratification in the cloud being eroded, as
is commonly observed in dense molecular clumps. The simple geometry of
the nebula and the observability of the central star at short
wavelengths make the derivation of the physical conditions of the region
and the modeling of its dynamical evolution comparatively easier than in
other, similar regions. DR 18 thus provides a good case study of several
features associated to the interaction of an early B star with a
molecular cloud. Based on observations collected at the German-Spanish
Astronomical Center (Calar Alto, Spain), and at the European Southern
Observatory (La Silla, Chile)
We present a radio continuum survey at 1400 and 350 MHz of a region of 2° × 2° centered on the Cygnus OB2 association (d = 1.7 kpc), using the Westerbork Synthesis Radio Telescope (WSRT) with angular resolutions of, respectively, 13'' and 55''. The resulting 5 σ flux-density limits of, respectively, ~2 mJy and ~10-15 mJy are a significant improvement over previous surveys. We detected 210 discrete sources with sizes less than 1.9θbeam (beam size), 98 of which at both frequencies. We also detected 28 resolved sources (sizes > 1.9θbeam) still having well-defined peak intensities. The observed spectral index α distribution and source count strongly suggest an excess of sources of Galactic origin in the direction of Cyg OB2.
We have searched for positional coincidences of the detected sources in our list with other radio, infrared, and optical objects from various surveys by using the likelihood ratio (LR) method. Furthermore, we looked for objects that show characteristics of either optically thick stellar winds (α +0.6), or nonthermal emission (-1 α +0.6) and/or variable spectral flux density. The LR method resulted in 108 identifications. Eighty unidentified sources, i.e., ~2/3, show characteristics of sources of Galactic origin, 10 of which may be stars. The remaining unidentified sources are probably of extragalactic origin.
We identified one source with the O7 star Cyg OB2-335 and consider it to be a candidate colliding-wind binary. We also identify 19 point sources with known infrared and optical objects: these have nearly flat or inverted spectral indices, and some of them show flux-density variability. Follow-up multifrequency monitoring of these sources will be important in establishing the reality of the flux variabilities and to assess the nature of these sources.
We present a near-infrared spectroscopic survey of a large area centered on the Cygnus OB2 association aimed at constraining its massive star contents. Our goal is to establish a nearly complete list of O-type members of the association, both to examine recent claims based on starcounts that suggest a richer content than previously thought, and to provide a suitable database for further studies of the entire high-mass end of one of the richest associations of the Galaxy. The target selection is based on the JHK photometry published in the 2MASS all-sky survey. We identify 46 new early-type candidates, most of them expected to be O-type stars, plus 16 new stars with emission in Brγ and often in other lines as well, characteristic of evolved massive stars undergoing intense mass loss. We also present spectra of three luminous stars with CO overtone emission, one of them having also intense H2 emission and being associated with compact nebulosity. By considering our findings, those of other authors, and plausible completeness corrections, we estimate the number of O-type stars or stars having evolved from a O-type progenitor to be 90–100, slightly below, but compatible with, most recent starcounts estimates by Knödlseder (2000, A&A, 360, 539). These results support the notion that Cygnus OB2 may be considered as a young globular cluster. The lists of new members that we provide, in particular those with emission lines, should be a useful resource for future investigations of Cygnus OB2 itself, as well as of very massive stellar evolution by providing a nearby, abundant sample of stars sharing a common environment.
Whether the Cygnus X complex consists of one physically connected region of
star formation or of multiple independent regions projected close together on
the sky has been debated for decades. The main reason for this puzzling
scenario is the lack of trustworthy distance measurements. We aim to understand
the structure and dynamics of the star-forming regions toward Cygnus X by
accurate distance and proper motion measurements. To measure trigonometric
parallaxes, we observed 6.7 GHz methanol and 22 GHz water masers with the
European VLBI Network and the Very Long Baseline Array. We measured the
trigonometric parallaxes and proper motions of five massive star-forming
regions toward the Cygnus X complex and report the following distances within a
10% accuracy: 1.30+-0.07 kpc for W 75N, 1.46^{+0.09}_{-0.08} kpc for DR 20,
1.50^{+0.08}_{-0.07} kpc for DR 21, 1.36^{+0.12}_{-0.11} kpc for
IRAS20290+4052, and 3.33+-0.11kpc for AFGL 2591. While the distances of W 75N,
DR 20, DR 21, and IRAS 20290+4052 are consistent with a single distance of
1.40+-0.08 kpc for the Cygnus X complex, AFGL 2591 is located at a much greater
distance than previously assumed. The space velocities of the four star-forming
regions in the Cygnus X complex do not suggest an expanding Stroemgren sphere.
During a synoptic survey of the North American Nebula region, the Palomar Transient Factory (PTF) detected an optical outburst (dubbed PTF10nvg) associated with the previously unstudied flat or rising spectrum infrared source IRAS 20496+4354. The PTF R-band light curve reveals that PTF10nvg brightened by more than 5 mag during the current outburst, rising to a peak magnitude of R
PTF ≈ 13.5 in 2010 September. Follow-up observations indicate that PTF10nvg has undergone a similar ~5 mag brightening in the K band and possesses a rich emission-line spectrum, including numerous lines commonly assumed to trace mass accretion and outflows. Many of these lines are blueshifted by ~175 km s–1 from the North American Nebula's rest velocity, suggesting that PTF10nvg is driving an outflow. Optical spectra of PTF10nvg show several TiO/VO band heads fully in emission, indicating the presence of an unusual amount of dense (>1010 cm–3), warm (1500-4000 K) circumstellar material. Near-infrared spectra of PTF10nvg appear quite similar to a spectrum of McNeil's Nebula/V1647 Ori, a young star which has undergone several brightenings in recent decades, and 06297+1021W, a Class I protostar with a similarly reached near-infrared emission line spectrum. While further monitoring is required to fully understand this event, we conclude that the brightening of PTF10nvg is indicative of enhanced accretion and outflow in this Class-I-type protostellar object, similar to the behavior of V1647 Ori in 2004-2005.
The morphology and stellar content of 22 clusters in the Cygnus area has been determined using 2MASS infrared observations in the $J$ and $K$ bands. 7 of the clusters are well-known objects and our results are in good agreement with previous work. 12 objects are recently-discovered cluster candidates (Dutra & Bica [CITE]) and 3 are new cluster candidates. Adopting distance estimates from the literature or by considering cluster counterparts, we derive a consistent set of parameters suitable for analysis and comparison: center coordinates, radius, stellar population, IMF slope and mass range. We find a mean IMF slope of $\Gamma=-1.30$ for the known clusters and $\Gamma=-1.40$ for the others. Infrared data having better completeness, we derive larger stellar contents and cluster masses than previous works. An important fraction of the cluster stellar population in Cygnus is hidden in very obscured areas. A clear tendency towards mass segregation appears for most of the clusters. Some of the new clusters seem to be quite massive, probably hosting very massive stars at the center. They are privileged targets for more detailed investigations using infrared spectroscopy.
We present a near-infrared imaging survey of compact HII regions in the direction of the Cygnus X complex, for which no previous observations at those wavelengths have been published so far. The targets have been selected from a catalog of sources in that region having a thermal spectral energy distribution between 408 and 4800 MHz (Wendker et al. 1991), and an inferred angular size smaller than 5 arcmin across. We present images in the $JHK$ filters, color-magnitude, and color-color diagrams for each region. We also suggest and apply a method for estimating the distance by comparing the dereddened $H$-band flux from all the stars in the area of the HII region and the radio-continuum flux. Many of the regions imaged are clearly associated with stellar aggregates with different degrees of concentration, whose components show varying amounts of extinction. Some objects are often found in the region of the $(J-H)$, $(H-K)$ diagram indicating excess emission of circumstellar nature. A detailed discussion on each object is provided in the context of existing published observations at different wavelengths, in particular regarding the existence of ultracompact components. A number of ultracompact HII regions are found to have clearly visible unresolved or nearly unresolved $K$-band counterparts characterized by very red $H-K$ colors, suggesting that the extinction may be low enough in their direction so as to allow the direct observation of the star ionizing the ultracompact component.
We present a survey for OH maser emission at 18 cm in the direction of 196 IRAS point sources. The primary aim of the survey was to detect OH maser emission from post-asymptotic giant branch (post-AGB) stars. OH emission and/or absorption was detected in the direction of 77 sources of which 57 OH detections were previously unknown, giving an overall detection rate of 39 per cent. Fourteen maser sources associated with evolved stars or planetary nebulae were found, corresponding to 7 per cent of the total observed. These included three planetary nebulae (two new OH detections), nine post-AGB stars (three new OH detections) and two cold OH/IR stars (both previously known). Two sources were of uncertain identification (both new OH detections). The remainder of the OH sources were identified with young stellar objects and with Hii regions. The survey included the detection of OH maser emission from HD 101584, an early-type supergiant at high Galactic latitude. For several of the post-AGB stars, changes have occurred in the OH spectral profiles over a period of several years. This is interpreted as evidence for rapid changes in the stellar winds, due either to interactions with stellar companions, novae-like events or rapid stellar evolution.
New observations of DR 15 and 20 are reported as part of a study of compact H II regions in the Cyg X region. The radio and FIR data for these objects, when combined with (C-12)O maps, IRAS imagery, and optical photographs, provide new insights into the structure of this complex region and the nature of the star-formation process there. The observations show that DR 15 may consist of one or two B0 ZAMS stars whose H I regions have formed a low-density cavity within a molecular cloud. DR 20 appears to be a young OB cluster. The cluster is dominated by an O5.5 ZAMS star and also contains an approximately 3500-yr-old B0 star appearing as a compact H II region, along with weak FIR sources that may be B0-star candidates.
Two ultracompact H II regions (IRAS 19181+1349 and 20178+4046) and one compact molecular clump (20286+4105) have been observed at far infrared wavelengths using the TIFR 1 m balloon-borne telescope and at mid infrared wavelengths using ISO. Far infrared observations have been made simultaneously in two bands with effective wavelengths of ~ 150 and ~ 210 micron, using liquid 3He cooled bolometer arrays. ISO observations have been made in seven spectral bands using the ISOCAM instrument; four of these bands cover the emission from Polycyclic Aromatic Hydrocarbon (PAH) molecules. In addition, IRAS survey data for these sources in the four IRAS bands have been processed using the HIRES routine. In the high resolution mid infrared maps as well as far infrared maps multiple embedded energy sources have been resolved. There are structural similarities between the images in the mid infrared and the large scale maps in the far infrared bands, despite very different angular resolutions of the two. Dust temperature and optical depth (tau_150 um) maps have also been generated using the data from balloon-borne observations. Spectral energy distributions (SEDs) for these sources have been constructed by combining the data from all these observations. Radiation transfer calculations have been made to understand these SEDs. Parameters for the dust envelopes in these sources have been derived by fitting the observed SEDs. In particular, it has been found that radial density distribution for three sources is diffrent. Whereas in the case of IRAS 20178+4046, a steep distribution of the form r^-2 is favoured, for IRAS 20286+4105 it is r^-1 and for IRAS 19181+1349 it the uniform distribution (r^0). Line ratios for PAH bands have generally been found to be similar to those for other compact H II regions but different from general H II regions. Comment: To appear in Astronomy & Astrophysics; (19 pages including 14 Figures and 6 Tables)
The Cygnus X region is one of the most nearby massive star forming regions within our Galaxy, recognised by prominent emission throughout the entire electromagnetic spectrum, from radio to gamma-ray waves. This paper reviews our current knowledge about this region by describing its multi-wavelength characteristics. Particular emphasis will be given to the central stellar cluster Cyg OB2 that dominates the energetics and kinematics in the area. The cluster is also believed to be an active site of nucleosynthesis, as traced by the observation of the 1809 keV gamma-ray line, attributed to radioactive decay of 26Al. New observations obtained by the SPI telescope onboard the INTEGRAL gamma-ray observatory will be presented that corroborate this hypothesis, and that for the first time allow to measure the kinematics of the gas in the hot Cygnus X superbubble.
The authors present plots of all 5425 spectra in the IRAS catalogue of
low-resolution spectra. The LRS catalogue contains the average spectra
of most IRAS point sources with 12 μm flux densities above 10 Jy.
More than 95 percent of the LRS sources are stars, most of them with
circumstellar envelopes.
Results of a new far-infrared survey of Cygnus X conducted with 12-arcmin resolution are presented. Forty-nine sources were observed, 39 of them for the first time. Associations are made with sources from mid-infrared, radio continuum, and CO radio line surveys.
IRAS data have been used to examine the vicinity of nearly 100 dense clumps of molecular gas observed in lines of CO and NH3. More than one-third of these molecular cloud 'cores' contain infrared sources that appear to be newly formed, or forming, low mass stars. While about one-third of the infrared sources are associated with visible stars and have properties that resemble T Tauri stars, the remainder of the infrared sources have no optical counterparts and are probably embedded within the molecular cloud itself. These invisible sources lie close to the molecular cloud peaks and have cold, massive shells of material around them. Some of the cores without infrared sources have gas properties similar to those with embedded sources and so may be on the verge of gravitational collapse. The selection of the IRAS sample and its properties are described. The spectral energy distributions of the sources and their nature are discussed along with their physical properties, the properties of cores with and without infrared sources, and the time scale for the evolution of the sources.
To assess whether B335 is unique among dark clouds or whether CO-12 pedestal features are quite common, 180 opacity class 5 and 6 Lynds clouds were surveyed. From this set of data, three additional sources were found to have pedestal features. These suggest the presence of embedded low-mass stars, though a hot differentially rotating disk cannot be excluded for B335. Estimates of the mass-loss rate required to produce stellar winds consistent with observations are comparable with mass-loss rates for T Tauri stars. Further, the pedestal feature formation rate is similar to the local low-mass star formation rate.
A second list of new candidate young stellar objects found by searching the vicinities of IRAS positions along the galactic plane is presented. The list comprises 30 new objects with 2.2-micron magnitudes brighter than 12.0, and with near-infrared colors similar to those of the objects discussed by Persson and Campbell (1987) that have red H - K colors at a given J - H. These objects have a high probability of being analogs of well-known fiducial objects such as GI 490. Positions accurate to + or - 2 arcsec and deep CCD pictures of all the fields are given.
The authors surveyed a sample of IRAS sources within the boundaries of
dark clouds for broad-winged CO emission. Five of the 19 selected
objects have broad-winged CO emission characteristic of molecular
outflow. These objects are similar in characteristics to outflow sources
associated with pre-main-sequence stars. These results add to the
growing body of evidence that molecular outflows accompany the
pre-main-sequence evolution of low-mass stars.
A previous search for high-velocity molecular gas in the vicinity of
Herbig-Haro objects is extended. New observations at the (C-12)O J = 1-0
transition obtained with the 14 m telescope of the Five College Radio
Astronomy Observatory reveal four sites of spatially limited supersonic
molecular gas which are tentatively identified as additional energetic
outflows in the vicinity of Herbig-Haro objects. One is in the NGC 1999
region, two are in the neighborhood of RNO 43, and one is coincident
with the newly discovered H2O maser in NGC 2068. A summary of 17
energetic molecular outflows in the vicinity of Herbig-Haro objects is
presented, and the association between optically emitting shocks and
large-scale expanding gas is discussed. The previous estimate that at
least all stars as massive as 1 solar mass undergo a phase of mass loss
of not less than about 10 to the -7th solar mass/yr for a period of
about 10,000 yr is upheld by the new observations.
The results of a search for high-velocity molecular gas associated with
star forming molecular clouds are reported. Faint, high-velocity
(ΔV ≥ 30 km s-1)12CO emission is
frequently found near buried infrared sources. In most clouds, the
high-velocity gas is observed over an extended area having a typical
radius around 1018 cm and exhibits bipolar structure with the
redshifted emission formed in a different region from the corresponding
blueshifted emission. Analysis of the CO line emission indicates that
typical flows have masses in the range M ≈ 0.3 - 100 M_sun;,
velocities in the range V ≈ 10 - 50 km s-1, and lifetimes
as distinctly recognizable dynamical entities of around t ≈
104 yr. The kinematics of the high-velocity gas are best
explained as outflows from the vicinity of the associated infrared
sources. However, it appears that the ionized stellar winds proposed to
explain infrared line and radio continuum observations toward many of
these sources cannot be driving the energetic molecular flows. The
presence of high-velocity outflow in molecular clouds indicates an
unanticipated phase of energetic mass loss associated with the birth and
early evolution of stars.
The authors present CCD images, surface polarimetry, and long-slit
spectrograms of a hitherto unknown cometary reflection nebula associated
with a dense dust cloud. A bright, compact Herbig-Haro object is
embedded in its brightest part. The highly reddened illuminating star of
about 3 - 5 M_sun; is located near the apex of the nebula; it emits a
collimated bipolar high-velocity flow whose blueshifted component feeds
the Herbig-Haro object. The redshifted component can be traced toward
the interior of the dark cloud, where the density exceeds
105cm-3.
A systematic search was carried out for high-velocity CO emission
associated with bright 100-micron sources from the IRAS Point Source
Catalog, in an effort to increase the understanding of the nature and
evolutionary status of the objects producing molecular outflows.
Eighteen sources with 100-micron flux densities greater than 500 Jy were
selected and maps were made in the J = 1-0 (C-12) line around each
source. Almost every source observed was found to lie toward, or in the
immediate vicinity of, strong CO emission and in most cases close to the
maximum of this emission. Thus, all of the far-infrared sources can be
associated with molecular clouds. Five of the sources show clear
evidence of high-velocity molecular emission and at least three have
bipolar morphologies. The newly detected outflow sources are all
intrinsically luminous objects, and their outflows energetic. The
infrared characteristics of all the sources in the present survey
suggest that they are young stellar objects still embedded in their
parent molecular clouds. Statistics on the occurrence of outflows from
bright far-infrared sources are used to set an upper limit of 400,000 yr
for the dispersal time scale of material around young, luminous stellar
objects.
An overview of the Milky Way Galaxy is provided and aspects of
astronomical background are considered, taking into account positions
and coordinate systems, proper motions, parallax, radial velocities,
stellar spectra, magnitudes and colors, absolute energy distributions,
and astronomical catalogs and atlases. The physical properties of stars
and the interstellar medium are examined, giving attention to stellar
distances, stellar masses, stellar radii, an analysis of stellar
spectra, spiral arm and disk stars, spheroidal-component stars, stellar
structure and evolution, pulsating variable stars, and questions of
interstellar absorption. The space distribution of stars and the
chemical elements in the Milky Way Galaxy are discussed along with the
large-scale structure and stellar content of galaxies, the solar motion,
the stellar residual-velocity distribution, and the rotation of
galaxies. A description is presented of the large-scale distribution of
gas in galaxies, taking into consideration the distribution of neutral
hydrogen in the Milky Way Galaxy and in other galaxies, molecular clouds
in the Milky Way Galaxy, and the galactic center.
A wealth of data is now available on the energetic mass outflows that
are associated with young stellar objects. This phenomenon is thought to
occur at a very early stage in the evolution of stars of almost all
masses. The discovery of this energetic event was first made through
observations of the rapidly expanding molecular gas that surrounds many
of these young stellar objects. A review of the physical properties,
including the energetics and morphology, of the expanding molecular gas
is presented in this paper. In addition, the role these energetic winds
play in affecting the dynamics of the parental molecular clouds is also
discussed. Finally, the results of detailed studies of the structure and
kinematics of the high velocity molecular gas are reviewed, and the
evidence for existence of wind-swept cavities and molecular shells
within the clouds are presented.
A sample of young stellar objects selected from the IRAS Point Source
Catalog (1985) has been surveyed for the presence of high-velocity
(C-12)O J = 1-0 emission with the Five College Radio Astronomy
Observatory 14-m telescope. Three new molecular outflows have been
detected from this survey. The high-velocity emission from these regions
is dominated by redshifted material indicative of monopolar outflow. The
energy injection rate from all stellar winds within the Taurus cloud
complex is compared to the energy dissipation rate due to radiative
collisions of clumps. It is uncertain, based on these observations, that
the stellar winds can provide the energy necessary to sustain the
supersonic turbulent motions which characterize molecular clouds. From a
map of (C-13)O J = 1-0 emission of the B18 cloud, the ring of enhanced
line widths detected from an earlier study of (C-12)O J = 1-0 emission
is confirmed. This ring may be due to the cumulative interaction of
accelerated shells driven by stellar winds from several T Tauri stars
associated with the cloud.
Infrared images of the young object S106 (AFGL 2584) have been produced
at 3.6 microns, 10 microns (N band), and 19.5 microns (Q band) with 5
arcsec spatial resolution using the Wyoming infrared telescope.
Photometry from 2.3 to 23 microns is presented for eight compact sources
which were identified in the infrared nebula. There is remarkable
spatial coincidence among compact optical, infrared, and radio knots,
suggesting that the gas and dust in the nebula are well mixed and that
Lyman photons may be important in heating the nebular dust. The infrared
images are consistent with a model in which a recently evolved star
excites a biconical nebula from within a large irregular disk of gas and
dust, which is a remnant of the stellar collapse process. It is
concluded that a single source (IRS 4) is responsible for exciting the
entire optical, infrared, and radio structure of S106. The exciting
object is probably a single luminous star of spectral type O7-O9. The
dust constituents in S106 do not appear to be depleted appreciably below
cosmic abundance. Several dust knots may be sites where low luminosity
companions to the central source have condensed.
The results of a survey of bright IRAS point sources along the southern
Milky Way are presented. The survey was designed to search in the near
infrared for new examples of young stellar objects, many of whose
northern counterparts are responsible for driving bipolar outflows in
molecular clouds. Photometry in the J, H, and K bands is given for
nearly all of the 113 near-infrared sources that were found to be
associated with the candidate IRAS sources, L magnitudes are given for
60, and M magnitudes for 14. Positions accurate to ≤2arcsec are
presented for all the objects detected at K. Forty-five new YSO
candidates have been found; the remaining IRAS objects are probably
field stars suffering heavy extinction. Deep i band (8000 Å) CCD
frames are presented for most of the prime YSO candidates. These are
used to examine the surroundings of each source for associated stars and
nebulosity; at least half of the candidates have associated optical
objects. Low-resolution spectrophotometry in the 2.2 μm region of 15
of the primary candidates confirm that none is a compact H II region or
late-type star.
The results of a radio survey of the Cygnus X region at 6.2 cm are
presented and discussed. The observations and reductions are described,
and maps of the area are presented. A bewildering amount of details and
structures are observed in the region. A simple count shows about 800
local maxima or 10 per square degree in the average. A few selected
objects are commented on.
Elias 1-12 is a heavily obscured star in the IC5146 dark cloud. It was
discovered during a 2 micron survey by Elias (1978). The photometric
history of the star and its 2 micron spectrum led to a probable
identification as only the fourth known FU Orionis star. The FU Orionis
phenomenon consists of a T Tauri star undergoing an outburst of 5-6 mag.
It was decided to include Elias 1-12 in a survey of CO emission-line
profiles toward pre-main-sequence objects. In the present investigation,
'CO' refers to (C-12)(O16). On the basis of the obtained results, it is
concluded that the FU Orionis star Elias 1-12 is surrounded by an area
of perturbed molecular gas. This gas extends up to 0.3 pc from the star,
has a total mass of 1.2 solar masses, and a kinetic temperature of 7-15
K. The observed characteristics can be explained on the basis of a
stellar wind driven shell.
Molecular outflows are used here to probe mass loss in premain sequence (PMS) stars. Mass-loss rates are determined for 26 objects ranging in luminosity from four to about 100,000 solar, in mass from 0.5 to 30 solar, and in age from about 10,000 to about a million years. The derived mass-loss rates range from 9 x 10 to the -9th to 9 x 10 to the -4th solar mass/yr, with a typical value of 3 x 10 to the -7th solar mass/yr. PMS objects showing mass loss fall in a clearly demarcated region of the H-R diagram. The mass loss is proportional to bolometric luminosity to the 0.6 power and to stellar mass to the 1.8 power. The implications of these findings for the nature of the PMS mass-loss mechanism, for self-regulated low-mass star formation, and for planetary formation are discussed.
An evolutionary sequence, from protostars to pre-main sequence stars, for the classification of young stellar objects is derived by comparing the predictions of the theoretical protostar models of Adams and Shu (AS, 1986) with the morphological classification scheme of Lada and Wilking (1984). It is shown that the AS models adequately explain the emergent spectral energy distributions of unidentified objects with negative spectral indices in the mid-IR and near-IR in both Taurus and Ophiuchus. If the infalling dust envelope is then completely removed, the spectra of the underlying stars and nebular disks used by AS provide a natural explanation for the near-IR and mid-IR excesses and the positive spectral indices of embedded T Tauri stars. It is found that the addition of a simple physical model for residual dust envelopes can reproduce the far-IR excesses found in some of these T Tauri stars.
Millimeter-wave continuum observations and spectral line observations of the J = 5-4 and J = 2-1 transitions of CS toward IRAS 16293-2422, an extremely cold infrared source associated with a high-velocity molecular outflow in the Rho Ophiuchi molecular cloud, are reported. The observations indicate that this source is a dust-enshrouded object embedded in a dense, elongated configuration of molecular gas whose major axis is roughly orthogonal to the direction of the double bipolar outflow observed in it. The observations appear to provide spectroscopic evidence for significant mass infall motions associated with a young stellar object, and suggest that IRAS 16293-2422 is a true protostar.