Model Atmospheres For Stars Research Articles

Spherically Symmetric Model Atmospheres for Low‐Mass Pre–Main‐Sequence Stars with Effective Temperatures between 2000 and 6800 K

We present a grid of spherically symmetric model atmospheres for young pre-MS stars. This grid spans the parameter range $2000\K \leq \Teff \leq 6800\K$ and $2.0 \leq \logg \leq 3.5$ for $M=0.1\Msun$, appropriate for low mass stars and brown dwarfs. A major improvement is the replacement of TiO and \water line lists with the newer line list calculated by the NASA-AMES group, for TiO (about 175 million lines of 5 isotopes) and for \water (about 350 million lines in 2 isotopes). We provide the model structures, spectra and broad-band colors in standard filters in electronic form.

NLTE Model Atmospheres of Irradiated Stars in B Binaries

AbstractModel atmospheres of B stars irradiated by their companion are calculated using our model atmosphere computer code under the assumption of hydrostatic, radiative, and statistical equilibrium (NLTE). The external source of radiation (a hot white dwarf) is able to change the temperature structure of the outer atmospheric layers of B stars significantly. The changes of the temperature structure cause changes in the profiles of some lines, especially in those of hydrogen.

Non-LTE effects in neutron star atmospheres

AbstractWe present the first non-LTE model atmospheres for neutron stars (NS). We study their structure and NLTE effects on the emergent thermal radiation of old isolated NS.

TheNextGenModel Atmosphere Grid. II. Spherically Symmetric Model Atmospheres for Giant Stars with Effective Temperatures between 3000 and 6800 K

We present the extension of our NextGen model atmosphere grid to the regime of giant stars. The input physics of the models presented here is nearly identical to that of the NextGen dwarf atmosphere models; however, spherical geometry is used self-consistently in the model calculations (including the radiative transfer). We revisit the discussion of the effects of spherical geometry on the structure of the atmospheres and the emitted spectra and discuss the results of non-LTE calculations for a few selected models.

The sphericity effects in the NLTE model atmospheres of hot post-AGB stars

The assumption of a plane-parallel geometry is tested for the case of NLTE model atmospheres of hot post-AGB stars. As a test sample, two stars of this type are chosen, namely Barnard 29 and LS 4825. It is found that the differences between line profiles calculated for plane-parallel and spherically symmetric atmospheres are about several percent in the centers of hydrogen Balmer and Paschen lines, and, consequently, cannot be neglected. In addition, LTE approximation is also checked and it is found that this assumption is completely unacceptable for the analysis of the atmospheres of hot post-AGB stars.

Metal abundances in the Galactic Center

Recent progress in model atmospheres for hot stars is presented. The impact of nlte wind-blanketed models is shown by comparison with the ‘old’ standard model. In order to confindently use the IR-spectra of objects at the Galactic Center (with high extinction) to derive their physical properties and trace the metal content, the consistency between results from optical and IR analysis is checked in a calibration study using the B1.5Ia+ star HDE 316285. With the above consistency proved, we started a detail investigation of the Pistol Star. Preliminary results are presented.

MODEL ATMOSPHERES OF VERY LOW MASS STARS AND BROWN DWARFS

▪ Abstract As progressively cooler stellar and substellar objects are discovered, the presence first of molecules and then of condensed particulates greatly complicates the understanding of their physical properties. Accurate model atmospheres that include these processes are the key to establishing their atmospheric parameters. They play a crucial role in determining structural characteristics by setting the surface conditions of model interiors and providing transformations to the various observational planes. They can reveal the spectroscopic properties of brown dwarfs and help establish their detectability. In this paper, we review the current state-of-the-art theory and modeling of the atmospheres of very low mass stars, including the coolest known M dwarfs, M subdwarfs, and brown dwarfs, i.e. Teff≤ 4,000 K and −4.0 ≤ [M/H] ≤ +0.0. We discuss ongoing efforts to incorporate molecular and grain opacities in cool stellar spectra, as well as the latest progress in (a) deriving the effective temperature scale of M dwarfs, (b) reproducing the lower main sequences of metal-poor subdwarfs in the halo and globular clusters, and (c) results of the models related to the search for brown dwarfs.

Non‐LTE Line‐blanketed Model Atmospheres of Hot Stars. III. Hot Subdwarfs: The sdO Star BD +75o325

We have made a detailed comparison of results of spectroscopic analysis using three differents types of model atmospheres: classical non-LTE H-He models; approximate non-LTE line-blanketed models (with only a subset of Fe and Ni lines: those originating from transitions between levels with measured energies); and non-LTE fully blanketed models. The three models were applied to the sdO star BD +75°325, adopted as a test case.

The sphericity effects in model atmospheres of central stars of Planetary Nebulae

The model atmospheres of central stars of planetary nebulae are often calculated under the assumption of the plane-parallel geometry. This assumption seems to be reasonable for stars with relatively thin atmospheres like white dwarfs, where the thickness of the atmosphere is only few kilometers while the corresponding radius is about several thousands of kilometers. Nevertheless, calculations of a grid of pure hydrogen model atmospheres of hot white dwarfs demonstrated that this assumption may fail even for thin atmospheres (Kubát 1995). We found small differences between line profiles (about 1%) calculated under the assumption of plane-parallel and spherically symmetric atmospheres. Such differences are detectable by contemporary observational technique, and, consequently, may not be neglected. In addition, sphericity effects are important also in stars of other types (Kubát 1996). We decided to test the assumption of the plane parallel geometry also for sample model atmospheres of central stars of planetary nebulae. We assumed static atmospheres in radiative, hydrostatic, and statistical equilibrium (NLTE) consisting of hydrogen and helium. We calculated spherically symmetric and plane parallel model atmospheres of two central stars, namely LoTr4 and K1-27. Basic atmospheric parameters of these stars were adopted from Rauch et al. (1996, 1994), respectively. The atmospheric parameters for LoTr4(Teff = 120000K, log g = 5.5, M = 0.65M⊙, n(H)/n(He) = 0.5 yield a model with an extension (r(τR = 10–5)/r(τR = 2/3)) of 1.028. For K1-27, the atmospheric parameters (Teff = 100000K, log g = 6.5, M = 0.52M⊙, n(H)/n(He) = 0.2 produce only a small extension of 1.0035. The differences in temperature structure of our model atmospheres are more pronounced for a star with lower gravity and, consequently, larger extension. Continuum flux is lower for spherical atmospheres. This difference is larger for LoTr4, i.e. a star with more extended atmosphere, and almost negligible for K1-27. Paschen and Pickering He ii lines also show differences. These differences are quite large (several per cent) for α lines, and they decrease towards higher series members. Using plane-parallel model atmospheres instead of spherically symmetric ones introduces a systematic error into results. This error is present also in highly sophisticated NLTE line blanketed models. Due to the above mentioned differences one should avoid using lower series members (e.g. He ii 4686 å) for determination of atmospheric parameters (Teff, log g, abundances,…) from plane-parallel atmospheres. Details of our calculations will be presented in Astronomy and Astrophysics.

Model Atmospheres of AGB Stars: Dynamics, Molecules and Dust

The atmospheres and circumstellar envelopes of AGB stars are characterized by complex physical phenomena like shock waves caused by stellar pulsation or formation of molecules and dust which often lead to a heavy mass loss and have a strong influence on IR properties as observed by ISO. To allow a physical interpretation of various observations we have constructed improved dynamical model atmospheres of long-period variables. In this contribution we mainly investigate the dependence of the atmospheric structure and its variability on stellar pulsation, molecular opacities and time-dependent dust formation. IR spectra resulting from our models are discussed in detail by Loidl et al. (1997b) and compared to ISO-SWS spectra obtained by Hron et al. (1997).

Magnetized Iron Atmospheres for Neutron Stars

Using a Hartree-Fock formalism, we estimate energy levels and photon cross sections for atomic iron in magnetic fields B ~ 10^13 G. Computing ionization equilibrium and normal mode opacities with these data, we construct LTE neutron star model atmospheres at 5.5 < Log(T_eff) < 6.5 and compute emergent spectra. We examine the dependence of the emergent spectra on T_eff and B. We also show the spectral variation with the angle between the magnetic field and the atmosphere normal and describe the significant limb darkening in the X-ray band. These results are compared with recent detailed computations of neutron star H model atmospheres in high fields and with low field Fe and H model atmospheres constructed from detailed opacities. The large spectral differences for different surface compositions may be discernible with present X-ray data; we also note improvements needed to allow comparison of Fe models with high quality spectra.

Analysis of Keck high-resolution spectra of VB 10

We use a preliminary version of our ``NextGen'' grid of cool star model atmospheres to compute synthetic line profiles which fit high resolution Keck spectra of the cool M~dwarf VB10 satisfactorily well. We show that the parameters derived from the Keck data are consistent with the parameters derived from lower resolution spectra with larger wavelength coverage. We discuss the treatment of van der Waals broadening in cool, molecular (mostly ${\rm H_2}$) dominated stellar atmospheres. The line profiles are dominated by van der Waals pressure broadening and are a sensitive indicator for the gravity and metallicity. Therefore, the high-resolution Keck spectra are useful for determining the parameters of M dwarfs. There is some ambiguity between the metallicity and gravity. For VB10, we find from the high-resolution spectra that $ 5.0 < \log(g) < 5.5$ and $0 < \left[\case{{\rm M}}{{\rm H}}\right] < +0.5$ for an adopted fixed effective temperature of 2700~K (Schweitzer, 1995), which is consistent with recent interior calculations (Baraffe etal, 1995).

Spectral analysis and model atmospheres of WR type central stars

Wolf-Rayet type spectra of central stars are compared with spectra from Pop. I objects. Non-LTE models for expanding atmospheres are applied for analyzing these spectra quantitatively.

Non-LTE line-blanketed model atmospheres of hot stars. 1: Hybrid complete linearization/accelerated lambda iteration method

A new munerical method for computing non-Local Thermodynamic Equilibrium (non-LTE) model stellar atmospheres is presented. The method, called the hybird complete linearization/accelerated lambda iretation (CL/ALI) method, combines advantages of both its constituents. Its rate of convergence is virtually as high as for the standard CL method, while the computer time per iteration is almost as low as for the standard ALI method. The method is formulated as the standard complete lineariation, the only difference being that the radiation intensity at selected frequency points is not explicity linearized; instead, it is treated by means of the ALI approach. The scheme offers a wide spectrum of options, ranging from the full CL to the full ALI method. We deonstrate that the method works optimally if the majority of frequency points are treated in the ALI mode, while the radiation intensity at a few (typically two to 30) frequency points is explicity linearized. We show how this method can be applied to calculate metal line-blanketed non-LTE model atmospheres, by using the idea of 'superlevels' and 'superlines' introduced originally by Anderson (1989). We calculate several illustrative models taking into accont several tens of thosands of lines of Fe III to Fe IV and show that the hybrid CL/ALI method provides a robust method for calculating non-LTE line-blanketed model atmospheres for a wide range of stellar parameters. The results for individual stellar types will be presented in subsequent papers in this series.

Non-LTE line-blanketed model atmospheres of hot stars. 2: Hot, metal-rich white dwarfs

We present several model atmospheres for a typical hot metal-rich DA white dwarf, T(sub eff) = 60,000 K, log g = 7.5. We consider pure hydrogen models, as well as models with various abundances of two typical 'trace' elements-carbon and iron. We calculte a number of Local Thermodynamic Equilibrium (LTE) and non-LTE models, taking into account the effect of numerous lines of these elements on the atmospheric structure. We demostrate that while the non-LTE effects are notvery significant for pure hydrogen models, except for describing correctly the central emission in H-alpha they are essential for predicting correctly the ionization balance of metals, such as carbon and iron. Previously reported discrepancies in LTE abundances determinations using C III and C IV lines are easily explained by non-LTE effects. We show that if the iron abundance is larger than 10(exp -5), the iron line opacity has to be considered not only for the spectrum synthesis, but also in the model construction itself. For such metal abundances, non-LTE metal line-blanketed models are needed for detailed abundance studies of hot, metal-rich white dwarfs. We also discuss the predicted Extreme Ultraviolet (EUV) spectrum and show that it is very sensitive to metal abundances, as well as to non-LTE effects.

The effect of constant horizontial magnetic fields on model atmospheres of AP stars

view Abstract Citations (21) References (22) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Effect of Constant Horizontal Magnetic Fields on Model Atmospheres of AP Stars LeBlanc, F. ; Michaud, G. ; Babel, J. Abstract Model atmospheres have been constructed in LTE for A-type magnetic stars. The Lorentz force caused by the interaction between the ambipolar diffusion of hydrogen and the magnetic field, has been taken into account in the hydrostatic equation. Calculations were carried out using a modified version of the ATLAS line blanketed program with metal solar abundances, but no helium. Models with Teff = 8500, 10,000 and 12,000 K, log g = 4.0 and constant horizontal magnetic fields up to 10 kG in the 12,000 K model and up to 8 kG in the cooler ones are presented. The Lorentz force increases effective gravity in the hydrogen ionization region and thus modifies the structureof magnetic atmospheres. In a Teff = 10,000 K star, effective gravity is up to 7 times greater than gravity in the presence of a 5 kG horizontal field in the hydrogen ionization zone. The pressure gradient is proportionately increased in the ionization region of these magnetic Ap stars. The effect is present in stars with Teff up to at least 12,000 K but the effect decreases as Teff further increased and hydrogen is completely ionized in the atmosphere. Balmer lines are broadened in these magnetic atmospheres leading to a variation of 0.01 mag in the beta index in the Teff - 10,000 and 12,000 K models. This is smaller than the variation of 0.02 mag with rotational period typically observed in Ap stars. This still might play a role in explaining the beta variations of stars like 56 Ari and CU Vir. Observed metallicity gradients and magnetic field configurations could also play a role. The effect on Balmer lines in the Teff = 8500 K model is considerably reduced, since the ionization zone occurs at too large an optical depth for the pressure gradient increase to affect observations. Publication: The Astrophysical Journal Pub Date: August 1994 DOI: 10.1086/174492 Bibcode: 1994ApJ...431..388L Keywords: Abundance; Ambipolar Diffusion; Magnetic Stars; Stellar Atmospheres; Stellar Magnetic Fields; Stellar Models; Atmospheric Ionization; Balmer Series; Computational Astrophysics; Lorentz Force; Pressure Gradients; Astrophysics; DIFFUSION; STARS: ATMOSPHERES; STARS: MAGNETIC FIELDS; STARS: PECULIAR full text sources ADS | data products SIMBAD (2)

Line blanketing by iron group elements in non-LTE model atmospheres of hot stars

We report on our recent progress in modeling non-LTE atmospheres of O-stars including blanketing by lines from the iron group elements. The numerical method to account for the huge number of atomic levels and line transitions is presented. Results of exploratory model calculations examining the effects on the temperature structure, the hydrogen and helium line profiles and UV/EUV fluxes are discussed.

The Population of Massive Stars in R136 from Faint Object Camera Ultraviolet Observations

New ultraviolet (λ ≃ 1300 Å, λ ≃ 3400 Å), HST Faint Object Camera observations have been used to derive the UV color-magnitude diagram (CMD) of R136. The main scientific goal is the study of the upper end of the stellar mass function at ultraviolet wavelengths where the color degeneracy encountered in visual CMDs is less severe. <P />The CMD has been compared to a set of theoretical isochrones, which have been computed using the latest generation of evolutionary models and model atmospheres for early-type stars. Wolf- Rayet stars are included, and their emergent fluxes are calculated with the theoretical continuum energy distributions of Schmutz et al. Comparison of the theoretical and observed CMD suggests that there are no stars brighter than M<SUB>130</SUB> ≃ -11. <P />We use the observed main sequence turnoff and the known spectroscopic properties of the stellar population to derive constraints on the most probable age of R136. The presence of WNL stars and the lack of red supergiants suggests a most likely age of 3±1 Myr. A theoretical isochrone of 3±1 Myr is consistent with the observed stellar content of R1 36 if the most massive stars have initial masses around ≃50 M<SUB>sun</SUB>.

WOLF-RAYET STARS IN THE MILKY WAY, THE LARGE MAGELLANIC CLOUD, AND EMISSION-LINE GALAXIES

ABSTRACT Wolf-Rayet (W-R) stars are the short-lived endpoints of massive star evolution. Recent studies have identified a set of blue emission-line ''starburst'' galaxies whose integrated spectra reveal broad, resolved, emission features due to the presence of 102 - 105 such stars. We have performed a systematic study of the properties of a sample of such ''W-R galaxies''. The properties of W-R stars in the Galaxy and the Large Magellanic Cloud (LMC) provide the foundation for this analysis. Using a relation between the size of continuum discontinuities and the intrinsic (b-v)0 color found from model atmospheres of WN stars, we derive a formula for calculating the color excesses of such stars. We then calculate color excesses for nearly all WN stars in the Galaxy and the LMC. Our results are in good agreement with previous determinations of the color excesses for WN stars. Next, we derive average intrinsic He II 4686 and C IV lambda-5808 emission-line luminosities from the spectra of LMC WN and WC stars, respectively. Models of WN atmospheres are used to calculate effective temperatures, radii, luminosities, and Lyman continuum fluxes of LMC WNL stars. We have obtained a spatially integrated spectrum of the central 8' times 8' region of the giant H II region 30 Doradus in the LMC. From the observed line strengths and our average emission-line and Lyman-continuum luminosities we estimate the numbers of W-R and O stars present. Our results are in excellent agreement with direct star counts in the region. Finally, we have obtained moderate-resolution optical spectra of ten W-R galaxies and four comparison starburst galaxies. The spectra are very similar to those of giant H II regions; the various forbidden line ratios clearly indicate a stellar origin for the emission line spectrum. From the strong nebular emission lines, we determine the internal extinctions, densities, temperatures, oxygen abundances, and the Lyman continuum luminosity in these galaxies. From the flux in the broad He II lambda-4686 feature and its strength relative to H-beta, the number of W-R stars and the ratio of the number of W-R to the number of O stars present are determined. We calculate W-R/O number ratios that are an order of magnitude larger than predicted by current models of massive star evolution under conditions of continuous star formation with durations of 107 yr or longer. The observed number ratios can be reconciled with the model predictions only if the star formation in these galaxies occurs in ''bursts'' lasting </= 106 yr.

A deep envelope composition for TX Piscium?

view Abstract Citations (6) References (47) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Deep Envelope Composition for TX Piscium? Goebel, J. H. ; Goorvitch, D. ; Larson, H. P. ; Alexander, D. R. Abstract Results of observations of the carbon star TX Psc at high resolution in the 4000/cm spectral region and at moderate resolution from 2500 to 8000/cm are reported. The opacity sampling technique is employed to present model atmospheres of carbon stars with normal solar oxygen abundance and deep envelope composition that include HCN and C2H2. The oxygen abundance of TX Psc is found to be more accurately represented by a deep envelope composition depleted in oxygen by roughly two orders of magnitude than by a solar oxygen abundance. Polyatomic opacities influence the CO line depths and the derived oxygen abundance as a second-order effect. Polyatomic bands appear to be far too strong in the models, implying the need for another source of opacity to backwarm the outer atmosphere and thereby limit the polyatomic band depths. Dust, rather than polyatomic molecules, is a candidate for the backwarming opacity. Publication: The Astrophysical Journal Pub Date: January 1993 DOI: 10.1086/172169 Bibcode: 1993ApJ...402..680G Keywords: Carbon Stars; Stellar Composition; Stellar Envelopes; Abundance; Astronomical Models; Astronomical Spectroscopy; Opacity; Stellar Spectra; Stellar Temperature; Astrophysics; STARS: ABUNDANCES; STARS: ATMOSPHERES; STARS: CARBON; STARS: INDIVIDUAL CONSTELLATION NAME: TX PISCIUM full text sources ADS | data products SIMBAD (3)