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Hubble Space Telescope Observations of Planetary Nebulae in the Magellanic Clouds. III. Ultraviolet Spectroscopy Using the Faint Object Spectrograph
Abstract
The Faint Object Spectrograph on board the Hubble Space Telescope (HST) is used to obtain spectra in the wavelength range λλ1150-4800 of three planetary nebulae in the Small Magellanic Cloud and 10 planetary nebulae in the Large Magellanic Cloud. This sample extends the sample of 12 objects previously observed with HST and reported in the third paper of this series. Observed and dereddened emission-line fluxes are presented. Reddening estimates from the He II λ1640/λ4686 flux ratio are generally up to 0.2 dex lower than the reddening derived from the Balmer decrement. Nebular temperatures are estimated from the N+ λλ2138, 2142/λλ6548, 6583 flux ratio. Nebular densities are calculated from the O IV] complex at λ1400, the N IV] λλ1483, 1487 doublet, and the Si III] λλ1883, 1892 doublet. Densities calculated using the oxygen lines are comparable to those determined from the optical lines. Densities calculated from the nitrogen lines show a scatter of over 3 dex, which is relatively large compared to the optically derived densities. Three of the five densities derived from the silicon lines are greater than 10,000 cm-3. The C+2/O+2, Si+2/C+2, and N+2/O+2 ionic abundance ratios are calculated using the available ultraviolet emission features. The C/O and N/O ratios are anticorrelated, which supports the premise that third dredge-up has taken place during the asymptotic giant branch phase. In contrast to the third paper in this series, type I classification does not imply the presence of Si III] emission. Three objects show P Cygni-like line profiles at C IV λλ1548, 1551, indicative of stellar winds.
... Several explanations for the abundance discrepancies have Table 4. Effective temperature of the central star (T eff ), nebular radius (r), and nebular elemental abundances in SMC C 60 PNe. ǫ(C) are derived from C CELs. ǫ(C) in SMC1 was estimated using the O abundances of and the C/O ratios of Vassiliadis et al. (1998). The ǫ(C) in Lin49 is an expected value when we adopted the average CEL C/O ratio amongst the other SMC C 60 PNe. ...
We performed a detailed spectroscopic analysis of the fullerene C60-containing planetary nebula (PN) Lin49 in the Small Magellanic Cloud using XSHOOTER at the ESO VLT and the Spitzer/IRS instruments. We derived nebular abundances for nine elements. We used TLUSTY to derive photospheric parameters for the central star. Lin49 is C-rich and metal-deficient PN (Z~0.0006). The nebular abundances are in good agreement with Asymptotic Giant Branch nucleosynthesis models for stars with initial mass 1.25 Msun and metallicity Z = 0.001. Using the TLUSTY synthetic spectrum of the central star to define the heating and ionising source, we constructed the photoionisation model with CLOUDY that matches the observed spectral energy distribution (SED) and the line fluxes in the UV to far-IR wavelength ranges simultaneously. We could not fit the ~1-5 um SED using a model with 0.005-0.1 um-sized graphite grains and a constant hydrogen density shell owing to the prominent near-IR excess, while at other wavelengths the model fits the observed values reasonably well. We argue that the near-IR excess might indicate either (1) the presence of very small particles in the form of small carbon clusters, small graphite sheets, or fullerene precursors, or (2) the presence of a high-density structure surrounding the central star. We found that SMC C60 PNe show a near-IR excess component to lesser or greater degree. This suggests that these C60 PNe might maintain a structure nearby their central star.
The Magellanic Clouds are uniquely placed to study the stellar contribution to dust emission. Individual stars can be resolved
in these systems even in the mid-infrared, and they are close enough to allow detection of infrared excess caused by dust.
We have searched the Spitzer Space Telescope data archive for all Infrared Spectrograph (IRS) staring-mode observations of the Small Magellanic Cloud
(SMC) and found that 209 Infrared Array Camera (IRAC) point sources within the footprint of the Surveying the Agents of Galaxy
Evolution in the Small Magellanic Cloud (SAGE-SMC) Spitzer Legacy programme were targeted, within a total of 311 staring-mode observations. We classify these point sources using a
decision tree method of object classification, based on infrared spectral features, continuum and spectral energy distribution
shape, bolometric luminosity, cluster membership and variability information. We find 58 asymptotic giant branch (AGB) stars,
51 young stellar objects, 4 post-AGB objects, 22 red supergiants, 27 stars (of which 23 are dusty OB stars), 24 planetary
nebulae (PNe), 10 Wolf–Rayet stars, 3 H ii regions, 3 R Coronae Borealis stars, 1 Blue Supergiant and 6 other objects, including 2 foreground AGB stars. We use these
classifications to evaluate the success of photometric classification methods reported in the literature.
The chemical evolution of the Universe is governed by the chemical yields
from stars, which in turn is determined primarily by the initial stellar mass.
Even stars as low as 0.9Msun can, at low metallicity, contribute to the
chemical evolution of elements. Stars less massive than about 10Msun experience
recurrent mixing events that can significantly change the surface composition
of the envelope, with observed enrichments in carbon, nitrogen, fluorine, and
heavy elements synthesized by the slow neutron capture process (the s-process).
Low and intermediate mass stars release their nucleosynthesis products through
stellar outflows or winds, in contrast to massive stars that explode as
core-collapse supernovae. Here we review the stellar evolution and
nucleosynthesis for single stars up to ~10Msun from the main sequence through
to the tip of the asymptotic giant branch (AGB). We include a discussion of the
main uncertainties that affect theoretical calculations and review the latest
observational data, which are used to constrain uncertain details of the
stellar models. We finish with a review of the stellar yields available for
stars less massive than about 10Msun and discuss efforts by various groups to
address these issues and provide homogeneous yields for low and
intermediate-mass stars covering a broad range of metallicities.
We present UV observations of the symbiotic stars Hen 1213, Hen 1341, and Hen 1761, made almost simultaneously with optical observations during 1995; previous optical observations were made during 1987 and 1991. The UV spectrum of Hen 1213 was badly affected by background radiation, and only a rough description of the spectrum could be given here; for this reason, only the observations of Hen 1341 and Hen 1761 are discussed. They have the typical spectra of symbiotic stars, with a late giant component, with many emission lines superimposed. It is found that for both objects the reddening determined from He II λ1640/He II λ4686 is smaller than that obtained from the Balmer decrement, including optical depth effects. Hen 1341 does not show important variations for the three observing periods. Hen 1761 shows P Cygni profiles for He II λ4686 and He I λ4471, with expansion velocities of the order of 900 and 600 km s-1, respectively. The 1991 observations give the lower temperature of the hot component with the larger size of the nebula and the lowest He contents. Rough estimates indicate that the hot component of Hen 1761 is smaller and less luminous than that of Hen 1341.
Three planetary nebulae in the Large Magellanic Cloud (LMC) were observed with the Faint Object Spectrograph on board the Hubble Space Telescope. Their central stars display P Cygni-like features in the ultraviolet. We modeled these profiles to derive the stellar wind velocities. The wind velocities of the LMC planetary nebula nuclei (PNNs) are comparable to those of some Galactic counterparts, but other Galactic objects of similar temperatures exist with higher wind velocities. One object, LMC-SMP 61, has a Wolf-Rayet (WR) type central star. For this nucleus we derived a wind velocity of -1600 km s-1 and an upper limit to the mass-loss rate of ≤ 7 × 10-7M☉ yr-1 from the He and C lines.
The observed continuum emission in the UV and optical range is modeled with a combination of stellar and nebular contributions, yielding independent determinations of stellar and nebular quantities. The sample is numerically too limited for a significant comparison between LMC and Galactic PNN populations, but represents the first exploration of mass-loss properties of evolved stellar objects beyond the Milky Way.
Using the Goddard High Resolution Spectrograph (GHRS) and the Faint Object Spectrograph (FOS) on the Hubble Space Telescope, we measured the flux of the N II] (2s2p35S2 → 2s22p23P2,1) lines at λvac = 2143.45, 2139.68 Å in the Orion Nebula—the first detection of these lines in an H II region. In order to assess the N+/O+ ratio, we also measured the flux of the [O II] (2p32Po1/2,3/2→2p34So3/2) lines at λvac = 2471.05, 2471.12 Å. In addition, with the FOS, other emission lines were measured in the same aperture in order to assess the average electron temperature and mean-square temperature variation (t2) in the N+ region, as well as the N+/O+ ratio. When we require that the empirically determined values be equal for (N+/O+)uv (obtained from the N II] 2142 and [O II] 2471 lines) and (N+/O+)opt (obtained from the [N II] 6585 and [O II] 3728 lines), we obtain the following. For the (N+, O+) zone, the average electron density is ~7000 cm-3, the average electron temperature is 9500 K, t2 = 0.032, and N+/O+ = 0.14.
By comparing our FOS observations to predicted fluxes, utilizing our two previous photoionization models, we are able to derive the N/O ratio. There is fairly good agreement between (N/O)uv and (N/O)opt as derived from the two models with a range between 0.13 and 0.18. This range also encompasses our model-derived values for (N/O)ir (0.17-0.18), which fit the observed far-infrared line ratio [N III] 57 μm/[O III] 52 μm. The empirically derived N+/O+ value requires a correction for the possibility that the N+ and O+ regions are not identical. Our overall results place the gas-phase Orion N/O ratio in the range 0.13-0.18, which is somewhat higher than solar.
R-matrix calculations of electron impact excitation rates among the 2s22p2 3P, 1D, 1S, and 2s2p3 5S levels of N II are presented. These results are used in conjunction with other recent calculations of electron impact excitation rates and Einstein A-coefficients for N II to derive the emission-line ratio: ratio diagrams (R1, R2) and (R1, R3), where R1 = I(5756.2 Å)/I(6549.9 + 6585.2 Å), R2 = I(2143.5 Å)/I(6549.9 + 6585.2 Å), and R3 = I(2139.7 Å)/I(6549.9 + 6585.2 Å), for a range of electron temperatures (Te = 5000-20,000 K) and electron densities (Ne = 102-107 cm-3) appropriate to gaseous nebulae. These diagrams should, in principle, allow the simultaneous determination of Te and Ne from measurements of the [N II] lines in a spectrum. Plasma parameters deduced for a sample of gaseous nebulae, using observational data obtained from ground-based telescopes plus the International Ultraviolet Explorer and Hubble Space Telescope satellites, are found to show generally excellent internal consistency and to be in good agreement with the values of Te and Ne estimated from other line ratios. These results provide observational support for the accuracy of the theoretical ratios and hence the atomic data adopted in their derivation. Theoretical ratios are also presented for the infrared line pair R4 = I(122 μm)/I(205 μm), and the usefulness of R4 as an electron density diagnostic is briefly discussed.
A photoionization analysis of Hubble Space Telescope UV and ground-based optical spectrophotometry is given for eight more planetary nebulae (PNs) in the Large Magellanic Cloud (LMC). This allows the central stars to be placed accurately on the H-R diagram and permits the determination of the He, C, N, O, Ne, S, and Ar abundances. In some cases, the gas-phase abundances of Mg and Si may also be determined. We have combined these results with the analysis of two other objects published by us in the first two papers of this series. The observed abundance patterns are qualitatively consistent with the (mass-dependent) operation of the various chemical dredge-up processes as predicted by theory. Dredge-up of C during the thermal pulsing stage appears to be most important, and "hot bottom burning" transforms much of this C to N in the more massive stars. There is no sign of dredge-up of 22Ne. We show that the spread in the α-process element abundances can be understood as being due to differences in core mass of the planetary nebula nucleus, which is related directly to initial mass of the precursor star. This is, therefore, a tracer of the age-metallicity relationship for stars in general, and we derive, for the first time, the chemical history of the LMC based on PNs. We find that the base metallicity of the LMC almost doubled ~2 Gyr ago. This is consistent with studies of field stars and of clusters that show that there was a major burst of star formation at that time.
We acquired spectra of 24 LMC planetary nebulae (PNs) in the 1150-3000 Å range in order to determine carbon and other ionic abundances. The sample more than doubles the number of LMC PNs with high-quality UV spectra in this wavelength range and whose optical images are available in the Hubble Space Telescope archive. The Space Telescope Imaging Spectrograph was used with a very large aperture to obtain virtually slitless spectra; thus, the monochromatic images in the major nebula emission lines are also available. The analysis of the data shows extremely high quality spectra. This paper presents the emission lines identified and measured and the calculation of the ionic abundances of the emitting carbon and other ions, as well as total carbon abundance. P Cygni profiles have been found in a fraction of the nebulae, and the limiting velocities of the stellar winds estimated. The total carbon abundance can be inferred reliably in most nebulae. We found that the average carbon abundance in round and elliptical PNs is one order of magnitude larger than that of the bipolar PNs, while elliptical and round PNs with a bipolar core have a bimodal behavior. This results confirm that bipolarity in LMC PNs is tightly correlated with high-mass progenitors. When compared with predicted yields, we found that the observed abundance ratio shows a shift toward higher carbon abundances, which may be due to initial conditions assumed in the models not appropriate for LMC PNs.
[abridged] We present 52-93 micron spectra obtained with Spitzer in the MIPS-SED mode, of a representative sample of luminous compact far-IR sources in the LMC. These include carbon stars, OH/IR AGB stars, post-AGB objects and PNe, RCrB-type star HV2671, OH/IR red supergiants WOHG064 and IRAS05280-6910, B[e] stars IRAS04530-6916, R66 and R126, Wolf-Rayet star Brey3a, Luminous Blue Variable R71, supernova remnant N49, a large number of young stellar objects, compact HII regions and molecular cores, and a background galaxy (z~0.175). We use the spectra to constrain the presence and temperature of cold dust and the excitation conditions and shocks within the neutral and ionized gas, in the circumstellar environments and interfaces with the surrounding ISM. Evolved stars, including LBV R71, lack cold dust except in some cases where we argue that this is swept-up ISM. This leads to an estimate of the duration of the prolific dust-producing phase ("superwind") of several thousand years for both RSGs and massive AGB stars, with a similar fractional mass loss experienced despite the different masses. We tentatively detect line emission from neutral oxygen in the extreme RSG WOHG064, with implications for the wind driving. In N49, the shock between the supernova ejecta and ISM is revealed by its strong [OI] 63-micron emission and possibly water vapour; we estimate that 0.2 Msun of ISM dust was swept up. Some of the compact HII regions display pronounced [OIII] 88-micron emission. The efficiency of photo-electric heating in the interfaces of ionized gas and molecular clouds is estimated at 0.1-0.3%. We confirm earlier indications of a low nitrogen content in the LMC. Evidence for solid state emission features is found in both young and evolved object; some of the YSOs are found to contain crystalline water ice. Comment: Accepted for publication in The Astronomical Journal. This paper accompanies the Summer 2009 SAGE-Spec release of 48 MIPS-SED spectra, but uses improved spectrum extraction. (Fig. 2 reduced resolution because of arXiv limit.)
Self-consistent photoionization modeling of 44 Magellanic Cloud PNe is used to construct an H-R diagram for the central stars in both Clouds and is used to obtain the nebular chemical abundances and the physical parameters of the nebulae. The type I PNe are the youngest PNe in the clouds, have the highest core masses, and lie on the descending branch of the evolutionary tracks. These objects show correlated He and N/O abundance enhancements. The nebulae mass of the optically thick objects is well correlated with the nebular radius. Clear evidence is found for an abundance scatter in the material which formed the precursor star of the PNe in both Clouds. The metallicity range in the LMC is a factor of about 10, while that in the SMC is about 3-4. A core-mass:metallicity relationship is discovered for the LMC and is interpreted as a transformed age:metallicity relationship.
Optical spectroscopy in the range 3300-7800 A is presented for a total of 37 planetary nebulae in the LMC and seven in the SMC. Reddening estimates from Balmer line ratios have been determined, and the line intensities have been dereddened accordingly. Nebular forbidden O III electron temperatures, and where measurable forbidden S II densities, are derived. Forbidden O II electron densities are recalculated using appropriate electron temperatures. Zanstra temperatures are derived for 22 objects possessing detectable stellar continua. These temperatures are in agreement with photoionization modeling of this sample. 35 refs.
A graduate-level text and reference book on gaseous nebulae and the
emission regions in Seyfert galaxies, quasars, and other types of active
galactic nuclei (AGN) is presented. The topics discussed include:
photoionization equilibrium, thermal equilibrium, calculation of emitted
spectrum, comparison of theory with observations, internal dynamics of
gaseous nebulae, interstellar dust, regions in the galactic context,
planetary nebulae, nova and supernova remnants, diagnostics and physics
of AGN, observational results on AGN.
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A group of IRAS sources in the LMC and SMC have been monitored in the near infrared and pulsation period of over 1000 days have been determined for some of them. OH maser emission has been detected from some of the LMC objects. The OH line profiles indicate that stellar wind velocities in LMC stars are lower than in Galactic stars, a result attributed to the lower metallicity in the LMC. Evidence is presented that reducing metallicity lowers mass loss rates from AGB stars provided they do not turn into carbon stars. It is suggested that carbon star formation allows high mass loss rate to be maintained in AGB stars of low metallicity. There is no convincing evidence that AGB stars evolve significantly beyond the classical AGB limit of M(bol) about -7.1.
Narrow-band [OIII] 500.7 nm images and ultraviolet spectrophotometry are obtained for ˜20 planetary nebulae (PNe) in the Magellanic Clouds using the Hubble Space Telescope (HST). Four objects show P Cygni-like features in the ultraviolet HST data, and/or broad emission complexes near the HeII 468.6 nm and CIV 580.6 nm lines in ground-based spectra. All objects are of excitation class four or lower, and all are compact and dense compared to other objects in the sample. The likelihood of detecting ultraviolet P Cygni-like profiles in future HST spectroscopy of other objects is discussed.