Some Applications of Model Atmospheres

1974 ◽  
Vol 2 (5) ◽  
pp. 230-235
Author(s):  
M. S. Bessell
Keyword(s):  
White Dwarf ◽  
Model Atmosphere ◽  
Stellar Atmosphere ◽  
List Type ◽  
Cool Star ◽  
Abundance Patterns ◽  
Synthetic Spectra

In the five years since the last invited paper on model stellar atmosphere applications there have been many significant advances made on all fronts. The five aspects which I will cover in this paper are: (1) the results of white dwarf model atmosphere investigations;(2) the results of the inclusion of non LTE phenomena in the atmosphere computations of hot (T > 15,000 K) stars;(3) the probable understanding of the cause of peculiar abundance patterns in the Ap and Bp and Am stars;(4) the advances in theory and observations of cool star atmospheres; and(5) the use of synthetic spectra and colours.

scholarly journals White Dwarf Model Atmospheres: Synthetic Spectra for Supersoft Sources

2011 ◽  
Vol 7 (S281) ◽  
pp. 60-63
Author(s):  
Thomas Rauch
Keyword(s):  
Stellar Evolution ◽  
White Dwarf ◽  
Model Atmosphere ◽  
Parameter Range ◽  
Spectral Energy ◽  
Virtual Observatory ◽  
Metal Line ◽  
Energy Distributions ◽  
Synthetic Spectra ◽  
High Level

AbstractThe Tübingen NLTE Model-Atmosphere Package (TMAP) calculates fully metal-line blanketed white dwarf model atmospheres and spectral energy distributions (SEDs) at a high level of sophistication. Such SEDs are easily accessible via the German Astrophysical Virtual Observatory (GAVO) service TheoSSA. We discuss applications of TMAP models to (pre) white dwarfs during the hottest stages of their stellar evolution, e.g. in the parameter range of novae and supersoft sources.

scholarly journals The effects of different Type Ia SN yields on Milky Way chemical evolution

2021 ◽  
Vol 503 (3) ◽  
pp. 3216-3231
Author(s):  
Marco Palla
Keyword(s):  
Chemical Evolution ◽  
White Dwarf ◽  
Time Distribution ◽  
Milky Way ◽  
Massive Stars ◽  
Abundance Patterns ◽  
Type Ia ◽  
Low Metallicity ◽  
Explosion Mechanism ◽  
Chemical Evolution Model

ABSTRACT We study the effect of different Type Ia SN nucleosynthesis prescriptions on the Milky Way chemical evolution. To this aim, we run detailed one-infall and two-infall chemical evolution models, adopting a large compilation of yield sets corresponding to different white dwarf progenitors (near-Chandrasekar and sub-Chandrasekar) taken from the literature. We adopt a fixed delay time distribution function for Type Ia SNe, in order to avoid degeneracies in the analysis of the different nucleosynthesis channels. We also combine yields for different Type Ia SN progenitors in order to test the contribution to chemical evolution of different Type Ia SN channels. The results of the models are compared with recent LTE and NLTE observational data. We find that ‘classical’ W7 and WDD2 models produce Fe masses and [α/Fe] abundance patterns similar to more recent and physical near-Chandrasekar and sub-Chandrasekar models. For Fe-peak elements, we find that the results strongly depend either on the white dwarf explosion mechanism (deflagration-to-detonation, pure deflagration, double detonation) or on the initial white dwarf conditions (central density, explosion pattern). The comparison of chemical evolution model results with observations suggests that a combination of near-Chandrasekar and sub-Chandrasekar yields is necessary to reproduce the data of V, Cr, Mn and Ni, with different fractions depending on the adopted massive stars stellar yields. This comparison also suggests that NLTE and singly ionized abundances should be definitely preferred when dealing with most of Fe-peak elements at low metallicity.

scholarly journals Influence of Abundances Upon Stellar Atmosphere Calculations

1976 ◽  
Vol 72 ◽  
pp. 3-15
Author(s):  
B. Baschek
Keyword(s):  
Model Atmosphere ◽  
Stellar Atmosphere ◽  
Theoretical Spectrum ◽  
Differential Analysis ◽  
Absorption Coefficients ◽  
Quantitative Classification ◽  
Element Abundances ◽  
Scattering Coefficients ◽  
Basic Equations ◽  
Bolometric Correction

The basic equations for constructing a stellar atmosphere (hydrostatic equilibrium, flux constancy, radiative transfer, convective instability) are briefly summarized. While the parameters Teff (effective temperature) and g (surface gravity) are directly contained in these equations, the element abundances ∈i enter only indirectly through the thermodynamic properties (such as electron pressure, entropy, …) and the absorption and scattering coefficients of stellar matter.The equation of state, convection, the effects of the absorption coefficients (particularly of line absorption) on the temperature stratification, and the role of velocity fields (microturbulence) are discussed in some detail, emphasizing their dependence on the abundances.From a given model atmosphere, a ‘theoretical spectrum’ (colours, bolometric correction, line strengths etc.) can be calculated. The (relative) fluxes emerging at the surface are essentially determined by the temperature gradient and the absorption coefficients at the frequencies under consideration. The basic goal of quantitative classification, however, is the inverse problem, namely to deduce the stellar parameters from selected observed spectral criteria. Aspects relevant to this problem such as the question of uniqueness and the occurrence of possible systematic errors (even when using differential analysis techniques) are briefly sketched and illustrated by some examples.

scholarly journals Some Properties of Hot, High-Gravity, Pure Helium Atmospheres

1979 ◽  
Vol 53 ◽  
pp. 125-129
Author(s):  
F. Wesemael ◽  
H.M. Van Horn
Keyword(s):  
Effective Temperature ◽  
White Dwarf ◽  
White Dwarfs ◽  
Model Atmosphere ◽  
Surface Gravity ◽  
High Gravity ◽  
Pure Helium

Model atmosphere analyses of white dwarf spectra have contributed significantly to our understanding of the properties of degenerate stars.: In particular, the pioneering investigations of Bues (1970), Strittmatter and Wickramasinghe (1971) and Shipman (1972) have provided the first reliable determinations of the effective temperature and surface gravity of these objects (see Shipman 1979 and Weidemann 1978 for recent results). We now know with certainty that the hydrogen-rich white dwarf sequence extends at least over the range Te ∽ 6000 – 60.000K. In contrast, the hottest identified helium-rich white dwarfs seem to reach Te ~ 25.000K only, a puzzling result since the progenitors of DB white dwarfs should presumably also be helium-rich.

scholarly journals The Distribution of Masses and Radii of White-Dwarf Stars

1978 ◽  
Vol 80 ◽  
pp. 117-120
Author(s):  
Harry L. Shipman
Keyword(s):  
White Dwarf ◽  
White Dwarfs ◽  
Model Atmosphere ◽  
Selection Effects ◽  
Dwarf Stars ◽  
White Dwarf Stars ◽  
The Status ◽  
The Mean

The status of determinations of white dwarf radii by model atmosphere methods is reviewed in this paper. Details will appear elsewhere (Shipman 1978). In brief, the results are that (i) the mean radius of a sample of 95 hydrogen-rich stars with parallaxes is 0.0131 R⊙; (ii) the mean radius of a sample of 13 helium-rich stars is 0.011 R⊙, indistinguishably different from the radius of the hydrogen-rich stars; and (iii) that the most serious limitation on our knowledge of the mean radius of white dwarfs is the influence of selection effects. An estimate of the selection effects indicates that the true mean white dwarf radius is near 0.011 R⊙.

scholarly journals Observational properties of massive black hole binary progenitors

2018 ◽  
Vol 609 ◽  
pp. A94 ◽  
Author(s):  
R. Hainich ◽  
L. M. Oskinova ◽  
T. Shenar ◽  
P. Marchant ◽  
J. J. Eldridge ◽  
...  
Keyword(s):  
Mass Loss ◽  
Binary Systems ◽  
Spectral Characteristics ◽  
Broad Band ◽  
Stellar Atmosphere ◽  
High Mass ◽  
Massive Black Hole ◽  
Synthetic Spectra ◽  
Binary Evolution ◽  
Loss Rates

Context. The first directly detected gravitational waves (GW 150914) were emitted by two coalescing black holes (BHs) with masses of ≈ 36 M⊙ and ≈ 29 M⊙. Several scenarios have been proposed to put this detection into an astrophysical context. The evolution of an isolated massive binary system is among commonly considered models. Aims. Various groups have performed detailed binary-evolution calculations that lead to BH merger events. However, the question remains open as to whether binary systems with the predicted properties really exist. The aim of this paper is to help observers to close this gap by providing spectral characteristics of massive binary BH progenitors during a phase where at least one of the companions is still non-degenerate. Methods. Stellar evolution models predict fundamental stellar parameters. Using these as input for our stellar atmosphere code (Potsdam Wolf-Rayet), we compute a set of models for selected evolutionary stages of massive merging BH progenitors at different metallicities. Results. The synthetic spectra obtained from our atmosphere calculations reveal that progenitors of massive BH merger events start their lives as O2-3V stars that evolve to early-type blue supergiants before they undergo core-collapse during the Wolf-Rayet phase. When the primary has collapsed, the remaining system will appear as a wind-fed high-mass X-ray binary. Based on our atmosphere models, we provide feedback parameters, broad band magnitudes, and spectral templates that should help to identify such binaries in the future. Conclusions. While the predicted parameter space for massive BH binary progenitors is partly realized in nature, none of the known massive binaries match our synthetic spectra of massive BH binary progenitors exactly. Comparisons of empirically determined mass-loss rates with those assumed by evolution calculations reveal significant differences. The consideration of the empirical mass-loss rates in evolution calculations will possibly entail a shift of the maximum in the predicted binary-BH merger rate to higher metallicities, that is, more candidates should be expected in our cosmic neighborhood than previously assumed.

scholarly journals A Model-Atmosphere Analysis of the Spectrum of Arcturus

1976 ◽  
Vol 72 ◽  
pp. 19-20
Author(s):  
R. Mäckle ◽  
H. Holweger ◽  
R. and R. Griffin
Keyword(s):  
Effective Temperature ◽  
Isotope Ratio ◽  
Carbon Isotope Ratio ◽  
Model Atmosphere ◽  
Empirical Models ◽  
The Sun ◽  
Abundance Patterns ◽  
Cno Cycle ◽  
Average Factor ◽  
Low Mass

We have analysed the spectrum of Arcturus (K2 III) relatively to the Sun, using a differential technique employing empirical models for both stars. We derive an effective temperature of 4260 ± 50K and a surface gravity log g = +0.90 ±0.35; these in turn lead to a very low mass, in the range 0.1 to 0.6 M⊙. Elements are found to be underabundant by an average factor of 4 compared with the Sun. The abundance patterns in the two stars are significantly different, in keeping with the belief that Arcturus is a star of an older generation than the Sun. The carbon isotope ratio, which is as small as 5 or 6, shows that the atmospheric material of Arcturus has been processed through the CNO cycle, and theoretical arguments also indicate that Arcturus is somewhat evolved.

Model atmosphere analysis of the DZ white dwarf K789 - 37

1990 ◽  
Vol 350 ◽  
pp. 329 ◽  
Author(s):  
D. Koester ◽  
G. Wegner ◽  
D. Kilkenny
Keyword(s):  
White Dwarf ◽  
Model Atmosphere

scholarly journals Ellerman bombs and UV bursts: transient events in chromospheric current sheets

2019 ◽  
Vol 626 ◽  
pp. A33 ◽  
Author(s):  
V. Hansteen ◽  
A. Ortiz ◽  
V. Archontis ◽  
M. Carlsson ◽  
T. M. D. Pereira ◽  
...  
Keyword(s):  
Magnetic Flux ◽  
Current Sheet ◽  
Model Atmosphere ◽  
Opposite Polarity ◽  
Flux Emergence ◽  
Physical Relationship ◽  
Synthetic Spectra ◽  
Outer Atmosphere ◽  
Cool Gas ◽  
Ellerman Bombs

Context. Ellerman bombs (EBs), observed in the photospheric wings of the Hα line, and UV bursts, observed in the transition region Si IV line, are both brightenings related to flux emergence regions and specifically to magnetic flux of opposite polarity that meet in the photosphere. These two reconnection-related phenomena, nominally formed far apart, occasionally occur in the same location and at the same time, thus challenging our understanding of reconnection and heating of the lower solar atmosphere. Aims. We consider the formation of an active region, including long fibrils and hot and dense coronal plasma. The emergence of a untwisted magnetic flux sheet, injected 2.5 Mm below the photosphere, is studied as it pierces the photosphere and interacts with the preexisting ambient field. Specifically, we aim to study whether EBs and UV bursts are generated as a result of such flux emergence and examine their physical relationship. Methods. The Bifrost radiative magnetohydrodynamics code was used to model flux emerging into a model atmosphere that contained a fairly strong ambient field, constraining the emerging field to a limited volume wherein multiple reconnection events occur as the field breaks through the photosphere and expands into the outer atmosphere. Synthetic spectra of the different reconnection events were computed using the 1.5D RH code and the fully 3D MULTI3D code. Results. The formation of UV bursts and EBs at intensities and with line profiles that are highly reminiscent of observed spectra are understood to be a result of the reconnection of emerging flux with itself in a long-lasting current sheet that extends over several scale heights through the chromosphere. Synthetic spectra in the Hα and Si IV 139.376 nm lines both show characteristics that are typical of the observations. These synthetic diagnostics suggest that there are no compelling reasons to assume that UV bursts occur in the photosphere. Instead, EBs and UV bursts are occasionally formed at opposite ends of a long current sheet that resides in an extended bubble of cool gas.

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