- 标题
- 摘要
- 关键词
- 实验方案
- 产品
-
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
-
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
-
Empirical photometric calibration of the <i>Gaia</i> red clump: Colours, effective temperature, and absolute magnitude
摘要： Context. Gaia Data Release 1 allows the recalibration of standard candles such as the red clump stars. To use those stars, they first need to be accurately characterised. In particular, colours are needed to derive interstellar extinction. As no filter is available for the first Gaia data release and to avoid the atmosphere model mismatch, an empirical calibration is unavoidable. Aims. The purpose of this work is to provide the first complete and robust photometric empirical calibration of the Gaia red clump stars of the solar neighbourhood through colour–colour, effective temperature–colour, and absolute magnitude–colour relations from the Gaia, Johnson, 2MASS, Hipparcos, Tycho-2, APASS-SLOAN, and WISE photometric systems, and the APOGEE DR13 spectroscopic temperatures. Methods. We used a 3D extinction map to select low reddening red giants. To calibrate the colour–colour and the effective temperature–colour relations, we developed a MCMC method that accounts for all variable uncertainties and selects the best model for each photometric relation. We estimated the red clump absolute magnitude through the mode of a kernel-based distribution function. Results. We provide 20 colour versus G ? Ks relations and the first Teff versus G ? Ks calibration. We obtained the red clump absolute magnitudes for 15 photometric bands with, in particular, MKs = (?1.606 ± 0.009) and MG = (0.495 ± 0.009) + (1.121 ± 0.128) (G ? Ks ? 2.1). We present a dereddened Gaia-TGAS HR diagram and use the calibrations to compare its red clump and its red giant branch bump with Padova isochrones.
关键词： stars: fundamental parameters,stars: atmospheres,dust, extinction,stars: abundances
更新于2025-09-23 15:21:21
-
Theoretical gravity and limb-darkening coefficients for the MOST satellite photometric system
摘要： Aims. We present new calculations of limb and gravity-darkening coe?cients to be used as input in many ?elds of stellar physics such as synthetic light curves of double-lined eclipsing binaries and planetary transits, studies of stellar diameters or line pro?les in rotating stars. Methods. We compute the limb-darkening coe?cients speci?cally for the photometric system of the satellite MOST (Microvariability and Oscillations in STars). All computations were performed by adopting the least-square method, but for completeness we also performed calculations for the linear and bi-parametric approaches by adopting the ?ux conservation method. The passband gravity-darkening coe?cients y(λ) were computed by adopting a more general di?erential equation, which also takes the e?ects of convection into account. Results. We used two stellar atmosphere models: ATLAS (plane-parallel) and PHOENIX (spherical and quasi-spherical). We adopted six laws to describe the speci?c intensity distribution: linear, quadratic, square root, logarithmic, exponential, and a more general one with four terms. The covered ranges of Te?, log g, metallicities, and microturbulent velocities are (1500–50 000 K, 0–5.5, ?5.0–+1.0, 0–8 km s?1), respectively.
关键词： binaries: close,binaries: eclipsing,stars: evolution,stars: atmospheres,planetary systems
更新于2025-09-23 15:21:21
-
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
-
Solar twins in the ELODIE archive
摘要： A large dataset of ～2800 spectra extracted from the ELODIE archive was analysed in order to ?nd solar twins. Stellar spectra closely resembling the spectrum of the Sun were selected by applying a purely di?erential method, directly on the ?uxes. As solar reference, 18 spectra of asteroids, Moon, and blue sky were used. Atmospheric parameters and di?erential abundances of eight chemical elements were determined for the solar twin candidates after a careful selection of appropriate lines. The Li feature of the targets was investigated and additional information on absolute magnitude and age was gathered from the literature. HIP 076114 (HD 138573) is our best twin candidate; it looks exactly like the Sun in all these properties.
关键词： stars: fundamental parameters,stars: solar-type,stars: atmospheres,stars: abundances
更新于2025-09-23 15:19:57
-
Deep SDSS optical spectroscopy of distant halo stars
摘要： Aims. We analyze a sample of tens of thousands of spectra of halo turno? stars, obtained with the optical spectrographs of the Sloan Digital Sky Survey (SDSS), to characterize the stellar halo population “in situ” out to a distance of a few tens of kpc from the Sun. In this paper we describe the derivation of atmospheric parameters. We also derive the overall stellar metallicity distribution based on F-type stars observed as ?ux calibrators for the Baryonic Oscillations Spectroscopic Survey (BOSS). Methods. Our analysis is based on an automated method that determines the set of parameters of a model atmosphere that reproduces each observed spectrum best. We used an optimization algorithm and evaluate model ?uxes by means of interpolation in a precomputed grid. In our analysis, we account for the spectrograph’s varying resolution as a function of ?ber and wavelength. Our results for early SDSS (pre-BOSS upgrade) data compare well with those from the SEGUE Stellar Parameter Pipeline (SSPP), except for stars with log g (cgs units) lower than 2.5. Results. An analysis of stars in the globular cluster M 13 reveals a dependence of the inferred metallicity on surface gravity for stars with log g < 2.5, con?rming the systematics identi?ed in the comparison with the SSPP. We ?nd that our metallicity estimates are signi?cantly more precise than the SSPP results. We also ?nd excellent agreement with several independent analyses. We show that the SDSS color criteria for selecting F-type halo turno? stars as ?ux calibrators e?ciently excludes stars with high metallicities, but does not signi?cantly distort the shape of the metallicity distribution at low metallicity. We obtain a halo metallicity distribution that is narrower and more asymmetric than in previous studies. The lowest gravity stars in our sample, at tens of kpc from the Sun, indicate a shift of the metallicity distribution to lower abundances, consistent with what is expected from a dual halo system in the Milky Way.
关键词： techniques: spectroscopic,stars: atmospheres,methods: observational,Galaxy: halo,stars: Population II,stars: fundamental parameters
更新于2025-09-23 15:19:57
-
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
-
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
-
The centre-to-limb variations of solar Fraunhofer lines imprinted upon lunar eclipse spectra
摘要： The atmospheres of exoplanets are commonly studied by observing the transit of the planet passing in front of its parent star. The obscuration of part of the stellar disk during a transit will reveal aspects of its surface structure resulting from general centre-to-limb variations (CLVs). These become apparent when forming the ratio between the stellar light in and out of transit. These phenomena can be seen particularly clearly during the progress of a penumbral lunar eclipse, where the Earth transits the solar disk and masks different regions of the solar disk as the eclipse progresses. When inferring the properties of the planetary atmosphere, it is essential that this effect originating at the star is properly accounted for. Using the data observed from the 2014-April-15 lunar eclipse with the ESPaDOnS spectrograph mounted on the Canada France Hawaii Telescope (CFHT), we have obtained for the first time a time sequence of the penumbral spectra. These penumbral spectra enable us to study the centre-to-limb variations of solar Fraunhofer lines when the Earth is transiting Sun. The Na i and Ca ii absorption features reported from previous lunar eclipse observations are demonstrated to be CLV features, which dominate the corresponding line profiles and mask possible planetary signal. Detecting atmospheric species in exoplanets via transit spectroscopy must account for the CLV effect.
关键词： Earth,eclipses,stars: atmospheres,planets and satellites: atmospheres,Moon
更新于2025-09-19 17:15:36