AN ESTIMATE OF THE RELATIVE HELIUM CONTENT OF THE EARLY-TYPE STARS

1951 ◽  
Vol 29 (5) ◽  
pp. 447-457 ◽  
Author(s):  
Anne B. Underhill
Keyword(s):  
Relative Abundance ◽  
Equivalent Width ◽  
Model Atmosphere ◽  
Stellar Atmosphere ◽  
Abundance Ratio ◽  
Equal Weight ◽  
Pure Hydrogen ◽  
Good Representation ◽  
Fair Representation ◽  
Early Type Stars

The relative abundance of hydrogen to helium in O-type atmospheres is estimated by a comparison of the computed strength of the HeII line λ4541 with observed values. The line profile of λ4541 in an O9.5 V and in an O5 V model atmosphere is computed when the ratio H: He = 5.67 and when it is 13.33, and the equivalent width of the line derived. Observations of the strength of λ4541 in the spectra of the O9 V star, 10 Lacertae, and the O9.5 star, σ Orionis, are given, and the observed equivalent width of λ4541 in a real stellar atmosphere of spectral type corresponding to the O9.5 V model atmosphere is estimated. A comparison of the observed strength of HeII λ4541 with the computations indicates that the abundance ratio H: He lies in the range 20 to 25, that is, the relative abundance of helium is reduced by a factor of 1/3 to 1/2 from the amounts estimated by several authors by other spectroscopic methods. It is shown that the O9.5 V model atmosphere gives a fair representation of a late O-type main-sequence atmosphere, and that the computations can best be reconciled with observation by reducing the relative abundance of helium to about 4 or 5% by number, the rest of the stellar atmosphere being effectively pure hydrogen. The computations for the O5 model atmosphere are not in agreement with this result, but reasons are given for believing that the O5 model atmosphere does not give a good representation of an early O-type stellar atmosphere, and that consequently the results obtained from this model atmosphere are not of equal weight to those obtained from the O9.5 model atmosphere.

scholarly journals The Use of Model Atmospheres for Temperature-Gravity Calibration

1973 ◽  
Vol 54 ◽  
pp. 173-221
Author(s):  
J. C. Pecker
Keyword(s):  
Model Building ◽  
Stellar Atmosphere ◽  
Line Formation ◽  
Early Type ◽  
New Concepts ◽  
Total Luminosity ◽  
Classical Models ◽  
Early Type Stars ◽  
Two Parameters

Regardless of the degree of elaboration of series of models, just how can they be used for calibration purposes? And how much is this calibration sensitive to the quality of the model theory? These two questions are the basis of our discussion, which covers : I – The general principles of the use of model atmospheres in stellar calibration (1 – The two dimensional classifications; 2 – The use of the total luminosity; 3 – The cases of Vega and Sirius; 4 – The calibration of ST – Teff relation); II – The failures of the two parameters model atmospheres (1 – The observational need for more-than-two-parameters classification; 2 – The abundance of elements, the line formation, and the model atmospheres; 3 – Various sources of unadequacy of models; 4 – Envelopes or shell features; their influence on model-building; 5 – The case of HD 45677. Diagnostic of early-type stars; 6 – Various unexplained spectral features); III – The present state of the model factory (1 – The classical models; 2 – New concepts in the description of a stellar atmosphere; 3 – New approaches in model making; 4 – Conclusions).

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 Photospheric and “Chromospheric” CIV Lines in B-Main-Sequence Stars

1986 ◽  
Vol 116 ◽  
pp. 113-116
Author(s):  
Fiorella Castelli ◽  
Carlo Morossi ◽  
Roberto Stalio
Keyword(s):  
High Temperature ◽  
Stellar Wind ◽  
High Velocity ◽  
Main Sequence ◽  
Stellar Atmosphere ◽  
Uv Spectra ◽  
Spectral Lines ◽  
Early Type ◽  
Main Sequence Stars ◽  
Early Type Stars

The presence in the far-UV spectra of early-type stars of spectral lines of superionized atoms is argument of controversial debate among astronomers. Presently there is agreement on the non-radiative origin of these ions but not on the proposed mechanisms for their production nor on the proposed locations in the stellar atmosphere where they are abundant. Cassinelli et al. (1978) suggest that the Auger mechanism is operative in a cool wind blowing above a narrow corona to produce these ions; Lucy and White (1980) introduce radiative instabilities growing into hot blobs distributed across the stellar wind; Doazan and Thomas (1982) make these ions to be formed in both pre- and post-coronal, high temperature regions at low and high velocity respectively.

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 Recent advances in non-LTE stellar atmosphere models

2016 ◽  
Vol 12 (S329) ◽  
pp. 215-222
Author(s):  
Andreas A. C. Sander
Keyword(s):  
Massive Star ◽  
Model Atmosphere ◽  
Quantitative Information ◽  
Stellar Atmosphere ◽  
Stellar Atmospheres ◽  
Stellar Winds ◽  
Next Generation ◽  
Wide Range ◽  
Sophisticated Model ◽  
Current Stage

AbstractIn the last decades, stellar atmosphere models have become a key tool in understanding massive stars. Applied for spectroscopic analysis, these models provide quantitative information on stellar wind properties as well as fundamental stellar parameters. The intricate non-LTE conditions in stellar winds dictate the development of adequate sophisticated model atmosphere codes. The increase in both, the computational power and our understanding of physical processes in stellar atmospheres, led to an increasing complexity in the models. As a result, codes emerged that can tackle a wide range of stellar and wind parameters.After a brief address of the fundamentals of stellar atmosphere modeling, the current stage of clumped and line-blanketed model atmospheres will be discussed. Finally, the path for the next generation of stellar atmosphere models will be outlined. Apart from discussing multi-dimensional approaches, I will emphasize on the coupling of hydrodynamics with a sophisticated treatment of the radiative transfer. This next generation of models will be able to predict wind parameters from first principles, which could open new doors for our understanding of the various facets of massive star physics, evolution, and death.

scholarly journals Modeling the Hα Emission Surrounding Spica Using the Lyman Continuum from a Gravity-darkened Central Star

2021 ◽  
Vol 923 (1) ◽  
pp. 10
Author(s):  
Jason P. Aufdenberg ◽  
Joseph M. Hammill
Keyword(s):  
Surface Brightness ◽  
Central Star ◽  
Model Atmosphere ◽  
Stellar Atmosphere ◽  
Primary Star ◽  
Near Ir ◽  
Lyman Continuum ◽  
Sky Survey ◽  
Hα Emission ◽  
Star Binary

Abstract The large, faint Hα emission surrounding the early B-star binary Spica has been used to constrain the total hydrogen recombination rate of the nebula and indirectly probe the Lyman continuum luminosity of the primary star. Early analysis suggested that a stellar atmosphere model, consistent with Spica A’s spectral type, has a Lyman continuum luminosity about two times lower than required to account for the measured Hα surface brightness within the nebula. To more consistently model both the stellar and nebular emission, we have used a model atmosphere for Spica A that includes the effects of gravity darkening as input to photoionization models to produce synthetic Hα surface brightness distributions for comparison to data from the Southern Hα Sky Survey Atlas. This paper presents a method for the computation of projected surface brightness profiles from 1D volume emissivity models and constrains both stellar and nebular parameters. A mean effective temperature for Spica A of ≃24,800 K is sufficient to match both the observed absolute spectrophotometry, from the far-UV to the near-IR, and radial Hα surface brightness distributions. Model hydrogen densities increase with the distance from the star, more steeply and linearly toward the southeast. The northwest matter-bounded portion of the nebula is predicted to leak ∼17% of Lyman continuum photons. Model H ii region column densities are consistent with archival observations along the line of sight.

scholarly journals Spatial and temporal analysis of leopards (Panthera pardus), their prey and tigers (Panthera tigris) in Huai Kha Khaeng Wildlife Sanctuary, Thailand

2019 ◽  
Vol 46 (2) ◽  
pp. 73-82
Author(s):  
Apinya Saisamorn ◽  
Prateep Duengkae ◽  
Anak Pattanavibool ◽  
Somphot Duangchantrasiri ◽  
Achara Simcharoen ◽  
...  
Keyword(s):  
Southeast Asia ◽  
Wild Boar ◽  
Relative Abundance ◽  
Temporal Analysis ◽  
Abundance Ratio ◽  
Panthera Tigris ◽  
Panthera Pardus ◽  
Fine Grained ◽  
The North ◽  
Temporal Interactions

AbstractDespite their extensive distribution globally, recent reports indicate leopards are declining, especially in Southeast Asia. To support conservation efforts we analyzed the behavioral interactions between leopards (Panthera pardus), their prey, and tigers to determine if leopards fine-tune their activity to maximize contact with four prey species (sambar; wild boar; barking deer; banteng) and avoid tigers and if prey alter their temporal activity in response to variation in their relative abundance ratio with leopards. A lower density of sambar in the northern part of our study area and a lower density of wild boar and a higher density of tigers in the southern part allowed us to examine fine-grained differences in the behavior of leopards and their prey. We used camera trap data to investigate spatial and temporal overlap. Differences in tiger relative abundance did not appear to impact the temporal activity of leopards. Leopards had similar cathemeral activity at all sites with highest activity at dawn and dusk. This behavior appears to be a compromise to provide access to diurnal wild boar and barking deer and nocturnal sambar and banteng. Sambar showed higher temporal avoidance of leopards in the north where its RAI was lowest; in contrast, wild boar had the highest temporal avoidance in the south where its density was lowest. This is the first study in Southeast Asia to quantify spatial and temporal interactions between the leopard, its primary ungulate prey, and the tiger. It provides new insights for conserving this declining subspecies.

scholarly journals NLTE spectral analysis of the intermediate helium-rich subdwarf B star CPD−20°1123

2020 ◽  
Vol 497 (1) ◽  
pp. 67-80 ◽  
Author(s):  
L Löbling
Keyword(s):  
Optical Spectrum ◽  
Local Thermodynamic Equilibrium ◽  
Model Atmosphere ◽  
Stellar Atmosphere ◽  
Light Metals ◽  
Helium Burning ◽  
Number Ratio ◽  
Metal Abundances ◽  
Non Local ◽  
Intermediate Surface

ABSTRACT Subdwarf B (sdB) stars are core helium-burning stars with stratified atmospheres. Their atmospheres are dominated by hydrogen (H) while the helium (He) and metal abundances are shaped by an interplay of gravitational settling and radiative levitation. However, a small fraction of these show spectra dominated by He i absorption lines. In between these groups of He-deficient and extreme He-rich sdBs, some are found to have intermediate surface He abundances. These objects are proposed to be young ‘normal’ (He-deficient) sdBs for which the dynamical stratification of the atmosphere is still ongoing. We present an analysis of the optical spectrum of such an intermediate He-rich sdB, namely CPD−20°1123, by means of non-local thermodynamic equilibrium (NLTE) stellar atmosphere models. It has a He-to-H number ratio of He/H = 0.13 ± 0.05 and its effective temperature of $\mbox{$T_\mathrm{eff}$} = 25\, 500 \pm 1000 \, \mathrm{K}$ together with a surface gravity of $\log \, (g$ / cm s−2) = 5.3 ± 0.3 places the star close to the high-temperature edge until which it may be justified to use LTE model atmospheres. This work states a test of the Tübingen NLTE Model Atmosphere Package for this temperature regime. We present the first application of revised, elaborated model atoms of low ionization stages of light metals usable with this atmosphere code.

scholarly journals Discovery of weak magnetic fields in early B-type stars

2003 ◽  
Vol 212 ◽  
pp. 202-203 ◽  
Author(s):  
Huib F. Henrichs ◽  
Coralie Neiner ◽  
Vincent C. Geers
Keyword(s):  
Magnetic Properties ◽  
Stellar Wind ◽  
Magnetic Measurements ◽  
Magnetic Phase ◽  
Model Atmosphere ◽  
Early Type ◽  
Chemical Abundance ◽  
Weak Magnetic Fields ◽  
Magnetic Dipole Field ◽  
Early Type Stars

We summarize recent results of magnetic measurements of three bright early B-type stars, β Cep, ζ Cas, and V2052 Oph, which were found to be oblique rotators with a weak magnetic dipole field with typical strength of a few hundred Gauss. From stellar wind studies we could derive their rotational periods very accurately, and match the stellar wind with the magnetic phase. From model atmosphere fits we derive the angles of rotational and magnetic axis. All three stars show some chemical abundance anomaly, presumably associated with the magnetic properties. The stars are also pulsating variables. This is of high asteroseismological interest, since these are the only early-type stars known with observationally determined rotation, pulsation and magnetic properties.

scholarly journals Formation of the Balmer Line in the Optically Thick Nova Envelope

1990 ◽  
Vol 122 ◽  
pp. 301-302
Author(s):  
Yoichi Takeda
Keyword(s):  
Spherical Model ◽  
Model Atmosphere ◽  
Balmer Line ◽  
Expansion Velocity ◽  
Pure Hydrogen ◽  
Coupled Equations ◽  
Frame Method ◽  
Four Levels ◽  
Photospheric Radius ◽  
Model Atom

Non–LTE effect and formation of the Balmer line Hα in the optically thick nova wind expected from the radiation–pressure–driven continuous ejection model (cf. Bath and Shaviv, 1976; Ruggles and Bath, 1979) was investigated by use of the pure–hydrogen spherical model atmosphere, in which constant radial expansion velocity, inverse square law of density distribution, and planar grey temperature distribution were assumed. The coupled equations of radiative transfer and statistical equilibrium on the four levels plus continuum model atom (in which Hα line and Balmer–, Paschen–, and Brackett– continua are explicitly treated) were solved by the complete linearization procedure based on the comoving frame method (cf. Mihalas and Kunasz, 1978). We calculated four models as results of the combination of the surface density ρsurf (at ᵀRos = 10−13) (10−12 g/cm3, 10−13 g/cm3) and the expansion velocity Vexp (0 km/sec, 1000 km/sec). We assumed 1012 cm for the photospheric radius rph (at ᵀRos = 1), 10000 K for the effective temperature appearing in the temperature formula, and 100 km/sec for the microturbulent parameter. The low density models have larger extension (rsurf/rph ≃4.1) than the high density ones (rsurf/rph ≃1.7).

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