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Weak metal lines in optical high-resolution Very Large Telescope and Keck spectra of “cool” PG?1159 stars
摘要： PG 1159 stars are very hot (effective temperatures Teff = 75 000–200 000 K), hydrogen-deficient (pre-) white dwarfs. They probably are the result of a late helium-shell flash that laid bare the He, C, and O rich intershell matter of the progenitor Asymptotic Giant Branch (AGB) star. Their chemical surface composition thus allows to conclude on details of AGB-star nucleosynthesis. Due to their very high effective temperatures, detailed spectral analyses are usually completely reliant on ultraviolet observations, except for some species in the hottest PG 1159 stars (Teff >～ 130 000 K), which do exhibit highly excited lines from the CNO elements and neon (C iv, N v, O vi, Ne vii-viii) in optical spectra. Particularly problematic are, however, the coolest members of the PG 1159 class that exclusively show C iv lines in the optical. Access to the nitrogen abundance is important to decide which of the late-thermal pulse evolutionary scenarios was experienced by a particular star, while a high oxygen abundance is an important marker that the star could pulsate. In the present paper, we investigate high-resolution high signal-to-noise optical spectra of three “cool” PG 1159 stars (PG 0122+200, PG 2131+066, MCT 0130?1937, Teff = 80 000–95 000 K). With the help of non-LTE model atmospheres and synthetic spectra, we are able to identify a large number of weak CNO lines (C iii, N iv, O iii-v) that were not detected before in these stars. They allow abundance determinations and enable us to constrain the effective temperature to high precision through ionization equilibria without the requirement to access the ultraviolet spectral range.
关键词： stars: abundances,white dwarfs,stars: evolution,stars: AGB and post-AGB,stars: atmospheres
更新于2025-09-23 15:23:52
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Synthetic photometry for carbon-rich giants
摘要： Context. The evolution and spectral properties of stars on the asymptotic giant branch (AGB) are significantly affected by mass loss through dusty stellar winds. Dynamic atmosphere and wind models are an essential tool for studying these evolved stars, both individually and as members of stellar populations, to understand their contribution to the integrated light and chemical evolution of galaxies. Aims. This paper is part of a series with the purpose of testing state-of-the-art atmosphere and wind models of C-type AGB stars against observations, and making them available to the community for use in various theoretical and observational studies. Methods. We have computed low-resolution spectra and photometry (in the wavelength range 0.35–25 μm) for a grid of 540 dynamic models with stellar parameters typical of solar-metallicity C-rich AGB stars and with a range of pulsation amplitudes. The models cover the dynamic atmosphere and dusty outflow (if present), assuming spherical symmetry, and taking opacities of gas-phase species and dust grains consistently into account. To characterize the time-dependent dynamic and photometric behaviour of the models in a concise way we defined a number of classes for models with and without winds. Results. Comparisons with observed data in general show a quite satisfactory agreement for example regarding mass-loss rates vs. (J ? K) colours or K magnitudes vs. (J ? K) colours. Some exceptions from the good overall agreement, however, are found and attributed to the range of input parameters (e.g. relatively high carbon excesses) or intrinsic model assumptions (e.g. small particle limit for grain opacities). Conclusions. While current results indicate that some changes in model assumptions and parameter ranges should be made in the future to bring certain synthetic observables into better agreement with observations, it seems unlikely that these pending improvements will significantly affect the mass-loss rates of the models.
关键词： stars: carbon,stars: mass-loss,stars: AGB and post-AGB,stars: atmospheres,circumstellar matter,stars: variables: general
更新于2025-09-23 15:21:21
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A new method to identify subclasses among AGB stars using <i>Gaia</i> and 2MASS photometry
摘要： Aims. We explore the wealth of high-quality photometric data provided by data release 2 (DR2) of the Gaia mission for long-period variables (LPVs) in the Large Magellanic Cloud (LMC). Our goal is to identify stars of various types and masses along the asymptotic giant branch. Methods. For this endeavour, we developed a new multi-band approach combining Wesenheit functions WRP,BP?RP and WKs,J?Ks in the Gaia BP, RP, and 2MASS J, Ks spectral ranges, respectively, and use a new diagram, (WRP,BP?RP ? WKs,J?Ks ) versus Ks, to distinguish between di?erent kinds of stars in our sample of LPVs. We used stellar population synthesis models to validate our approach. Results. We demonstrate the ability of the new diagram to discriminate between O- and C-rich objects, and to identify low-mass, intermediate-mass, and massive O-rich red giants, as well as extreme C-rich stars. Stellar evolution and population synthesis models guide the interpretation of the results, highlighting the diagnostic power of the new tool to discriminate between stellar initial masses, chemical properties, and evolutionary stages.
关键词： stars: evolution,Magellanic Clouds,stars: AGB and post-AGB,stars: variables: general
更新于2025-09-23 15:21:21
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SiO masers from AGB stars in the vibrationally excited <i>v</i> = 1, <i>v</i> = 2, and <i>v</i> = 3 states
摘要： Context. The v = 1 and v = 2 J = 1–0 (43 GHz), and v = 1 J = 2–1 (86 GHz) SiO masers are intense in asymptotic giant branch (AGB) stars and have been mapped using very long baseline interferometry (VLBI) showing ring-like distributions. Those of the v = 1, v = 2 J = 1–0 masers are similar, but the spots are rarely coincident, while the v = 1 J = 2–1 maser arises from a well-separated region farther out. These relative locations can be explained by models tools that include the overlap of two IR lines of SiO and H2O. The v = 3 J = 1–0 line is not directly affected by any line overlap and its spot structure and position, relative to the other lines, is a good test to the standard pumping models. Aims. The aim of this project are to gain insight into the properties and the general theoretical considerations of the different SiO masers that can help to understand them. Methods. We present single-dish and simultaneous VLBI observations of the v = 1, v = 2, and v = 3 J = 1–0 maser transitions of 28SiO in several AGB stars. The results are compared to the predictions of radiative models of SiO masers that both include and not include the effect of IR line overlap. Results. The spatial distribution of the SiO maser emission in the v = 3 J = 1–0 transition from AGB stars is systematically composed of a series of spots that occupy a ring-like structure (as often found in SiO masers). The overall ring structure is extremely similar to that found in the other 43 GHz transitions and is very different from the structure of the v = 1 J = 2–1 maser. The positions of the individual spots of the different 43 GHz lines are, however, very rarely coincident, which in general is separated by about 0.3 AU (between 1 and 5 mas). These results are very difficult to reconcile with standard pumping models, which predict that the masers of rotational transitions within a given vibrational state require very similar excitation conditions (since the levels are placed practically at the same energy from the ground), while the transitions of different vibrational states (which are separated by an energy of 1800 K) should appear in different positions. However, models including line overlap tend to predict v = 1, v = 2, v = 3 J = 1–0 population inversion to occur under very similar conditions, while the requirements for v = 1 J = 2–1 appear clearly different, and are compatible with the observational results.
关键词： masers,stars: AGB and post-AGB,techniques: interferometric,circumstellar matter,radio lines: stars
更新于2025-09-23 15:21:21
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The X-shooter Spectral Library (XSL)
摘要： We present the ?rst release of the X-shooter Spectral Library (XSL). This release contains 237 stars. The spectra in this release span a wavelength range of 3000–10 200 ? and have been observed at a resolving power of R ≡ λ/Δλ ～ 10 000. The spectra were obtained at ESO’s 8-m Very Large Telescope (VLT). The sample contains O–M, long-period variable, C and S stars. The spectra are ?ux-calibrated and telluric-corrected. We describe a new technique for the telluric correction. The wavelength coverage, spectral resolution, and spectral type of this library make it well suited to stellar population synthesis of galaxies and clusters, kinematical investigation of stellar systems, and the study of the physics of cool stars.
关键词： stars: fundamental parameters,galaxies: stellar content,stars: atmospheres,stars: AGB and post-AGB,stars: abundances
更新于2025-09-23 15:19:57
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Spectral analysis of the extremely hot DA white dwarf PG?0948+534
摘要： There is a striking paucity of hydrogen-rich (DA) white dwarfs (WDs) relative to their hydrogen-deficient (non-DA) counterparts at the very hot end of the WD cooling sequence. The three hottest known DAs (surface gravity log g ≥ 7.0) have effective temperatures around Teff = 140 000 K, followed by only five objects in the range 104 000–120 000 K. They are by far outnumbered by 40 non-DAs with Teff = 100 000–250 000 K, giving a DA/non-DA ratio of 0.2. In contrast, this ratio is the inverse of that for the cooler WDs. One reason for this discrepancy could be uncertainties in the temperature determination of hot DAs using Balmer line spectroscopy. Recent investigations involving metal-ionization balances in ultraviolet (UV) spectra indeed showed that the temperatures of some DAs were underestimated, but the paucity of extremely hot DAs prevailed. Here we present the results of a UV spectral analysis of one of the three hottest DAs, PG 0948+534. We find that its temperature was strongly overestimated by recent Balmer line analyses. We correct it downward to 105 000 ± 5000 K, aggravating the hot-DA paucity. The Balmer line problem encountered previously is not resolved by our non-LTE line-blanketed model atmospheres. We speculate that it might be related to the possible presence of a magnetosphere. This is supported by the V-band variability that shows a period of P = 3.45 d (amplitude 0.19 mag), which we interpret as the star's rotation period. The metal abundances in PG 0948+534 are affected by atomic diffusion and we conclude that the onset of diffusion in hot DAs occurs when they cool below Teff ≈105 000 K. We discuss the possibility that the paucity of very hot DAs is a consequence of their fast evolutionary rate.
关键词： white dwarfs,stars: abundances,stars: individual: PG 0948+534,stars: AGB and post-AGB,stars: atmospheres
更新于2025-09-19 17:15:36
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High-resolution IR and radio observations of AGB stars
摘要： Aims. We present the results of observations with interferometers of a sample of pulsating asymptotic giant branch (AGB) stars in the infrared and at radio wavelengths. The goal of these observations is to explore the extended stellar atmospheres and to establish links between the spatial scales of molecular envelopes and of the dust shell. This is the key to better understand the process of dust formation and therefore of mass loss. Methods. We used the ESO VLTI/MIDI interferometer in the N band, the Keck Interferometer in the K band, and NRAO VLBA observations of SiO masers at 7 mm wavelength of a sample of AGB stars: U Ari, W Cnc, RX Tau, RT Tau, RT Aql, S Ser, and V Mon. The various instruments probe different altitudes of the atmosphere of the AGB stars. They are sensitive to regions below the silicate dust condensation distance and provide the opportunity of finding hints about how dust and its precursors form in the extended atmosphere of an AGB star. The K-band observations are sensitive to water and carbon-monoxyde vapors. Unfortunately, we were only able to observe S Ser in this wavelength range. Results. We find a ratio of 2.2 between the molecular envelope radius and the photospheric size, which is consistent with previous results. The N-band observations are mostly sensitive to vapors of SiO and water and to dust (alumina and silicate). The silicate dust shell is fully resolved, and no precise parameters can be deduced from the N-band observations other than a spatial extension of at least 12–16 R* for our sample. The sizes found for the SiO region are consistent with the radii of the SiO maser rings provided by the VLBA observations. The sizes of the alumina and water vapor regions are systematically found to be larger. There is clear evidence that SiO is absent from regions farther from the star where silicate dust condenses. Conclusions. These observations support a possible scenario in which SiO is adsorbed by species such as corundum. An alternative explanation could be that SiO has chemically disappeared at this range of distances.
关键词： infrared: stars,stars: mass-loss,masers,stars: AGB and post-AGB,techniques: interferometric,stars: atmospheres
更新于2025-09-19 17:15:36
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An imaging spectroscopic survey of the planetary nebula NGC 7009 with MUSE
摘要： Aims. The spatial structure of the emission lines and continuum over the 50'' extent of the nearby, O-rich, PN NGC 7009 (Saturn Nebula) have been observed with the MUSE integral field spectrograph on the ESO Very Large Telescope. This study concentrates on maps of line emission and their interpretation in terms of physical conditions. Methods. MUSE Science Verification data, in <0.6'' seeing, have been reduced and analysed as maps of emission lines and continuum over the wavelength range 4750–9350 ?. The dust extinction, the electron densities and temperatures of various phases of the ionized gas, abundances of species from low to high ionization and some total abundances are determined using standard techniques. Results. Emission line maps over the bright shells are presented, from neutral to the highest ionization available (He II and [Mn V]). For collisionally excited lines (CELs), maps of electron temperature (Te from [N II] and [S III]) and density (Ne from [S II] and [Cl III]) are available and for optical recombination lines (ORLs) temperature (from the Paschen jump and ratio of He I lines) and density (from high Paschen lines). These estimates are compared: for the first time, maps of the differences in CEL and ORL Te’s have been derived, and correspondingly a map of t2 between a CEL and ORL temperature, showing considerable detail. Total abundances of only He and O were formed, the latter using three ionization correction factors. However, the map of He/H is not flat, departing by ~2% from a constant value, with remnants corresponding to ionization structures. An integrated spectrum over an area of 2340 arcsec2 was also formed and compared to 1D photoionization models. Conclusions. The spatial variation of a range of nebular parameters illustrates the complexity of the ionized media in NGC 7009. These MUSE data are very rich with detections of hundreds of lines over areas of hundreds of arcsec2 and follow-on studies are outlined.
关键词： planetary nebulae: individual: NGC 7009,dust, extinction,stars: AGB and post-AGB,ISM: abundances,atomic processes
更新于2025-09-19 17:15:36
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Clumpy dust clouds and extended atmosphere of the AGB star W Hydrae revealed with VLT/SPHERE-ZIMPOL and VLTI/AMBER
摘要： Aims. Our recent visible polarimetric images of the well-studied AGB star W Hya taken at pre-maximum light (phase 0.92) with VLT/SPHERE-ZIMPOL have revealed clumpy dust clouds close to the star at ～2 R(cid:63). We present second-epoch SPHERE-ZIMPOL observations of W Hya at minimum light (phase 0.54) as well as high-spectral resolution long-baseline interferometric observations with the AMBER instrument at the Very Large Telescope Interferometer (VLTI). Methods. We observed W Hya with VLT/SPHERE-ZIMPOL at three wavelengths in the continuum (645, 748, and 820 nm), in the Hα line at 656.3 nm, and in the TiO band at 717 nm. The VLTI/AMBER observations were carried out in the wavelength region of the CO ?rst overtone lines near 2.3 μm with a spectral resolution of 12 000. Results. The high-spatial resolution polarimetric images obtained with SPHERE-ZIMPOL have allowed us to detect clear time variations in the clumpy dust clouds as close as 34–50 mas (1.4–2.0 R(cid:63)) to the star. We detected the formation of a new dust cloud as well as the disappearance of one of the dust clouds detected at the ?rst epoch. The Hα and TiO emission extends to ～150 mas (～6 R(cid:63)), and the Hα images obtained at two epochs reveal time variations. The degree of linear polarization measured at minimum light, which ranges from 13 to 18%, is higher than that observed at pre-maximum light. The power-law-type limb-darkened disk ?t to the AMBER data in the continuum results in a limb-darkened disk diameter of 49.1 ± 1.5 mas and a limb-darkening parameter of 1.16±0.49, indicating that the atmosphere is more extended with weaker limb-darkening compared to pre-maximum light. Our Monte Carlo radiative transfer modeling shows that the second-epoch SPHERE-ZIMPOL data can be explained by a shell of 0.1 μm grains of Al2O3, Mg2SiO4, and MgSiO3 with a 550 nm optical depth of 0.6±0.2 and an inner and outer radii of 1.3 R(cid:63) and 10±2 R(cid:63), respectively. Our modeling suggests the predominance of small (0.1 μm) grains at minimum light, in marked contrast to the predominance of large (0.5 μm) grains at pre-maximum light. Conclusions. The variability phase dependence of the characteristic grain size implies that small grains might just have started to form at minimum light in the wake of a shock, while the pre-maximum light phase might have corresponded to the phase of e?cient grain growth.
关键词： stars: individual: W Hya,circumstellar matter,stars: imaging,stars: AGB and post-AGB,techniques: interferometric,techniques: polarimetric
更新于2025-09-12 10:27:22
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A CO-multilayer outer atmosphere for eight evolved stars revealed with VLTI/AMBER
摘要： We determine the physical parameters of the outer atmosphere of a sample of eight evolved stars, including the red supergiant α Scorpii, the red giant branch stars α Bootis and γ Crucis, the K giant λ Velorum, the normal M giants BK Virginis and SW Virginis, and the Mira star W Hydrae (in two different luminosity phases) by spatially resolving the stars in the individual carbon monoxide (CO) first overtone lines. We used the Astronomical Multi-BEam combineR (AMBER) instrument at the Very Large Telescope Interferometer (VLTI), in high-resolution mode (λ/(cid:5)λ ≈ 12 000) between 2.28 and 2.31 μm in the K band. The maximal angular resolution is 10 mas, obtained using a triplet telescope configuration, with baselines from 7 to 48 m. By using a numerical model of a molecular atmosphere in a spherical shells (MOLsphere), called PAMPERO (an acronym for the ‘physical approach of molecular photospheric ejection at high angular resolution for evolved stars’), we add multiple extended CO layers above the photospheric MARCS model at an adequate spatial resolution. We use the differential visibilities and the spectrum to estimate the size (R) of the CO MOLsphere, its column density (NCO) and temperature (Tmol) distributions along the stellar radius. The combining of the χ 2 minimization and a fine grid approach for uncertainty analysis leads to reasonable NCO and Tmol distributions along the stellar radius of the MOLsphere.
关键词： techniques: high angular resolution,stars: atmospheres,stars: AGB and post-AGB,stars: fundamental parameters,methods: numerical,methods: observational
更新于2025-09-11 14:15:04