On the Effective Temperatures of Extended Photospheres

1935 ◽  
Vol 31 (3) ◽  
pp. 390-393 ◽  
Author(s):  
S. Chandrasekhar
Keyword(s):  
Optical Depth ◽  
Effective Temperature ◽  
Boltzmann Constant ◽  
Thermodynamical Equilibrium ◽  
Effective Temperatures ◽  
Image Position ◽  
Local Thermodynamical Equilibrium

For material stratified in parallel planes in local thermodynamical equilibrium we have Milne's well-known result thatwhere πF is the constant net integrated flux of radiation, τ is the optical depth and B is the “ergiebigkeit” which is related to the temperature T by the relationσ being the Stefan-Boltzmann constant. If we define the effective temperature by the relationwe have from (1)where T1 is the temperature ar τ = 1. In this note we establish a similar result for extended photospheres where the curvature of the outer layers is properly taken into account.

scholarly journals Pulsating low-mass white dwarfs in the frame of new evolutionary sequences

2018 ◽  
Vol 620 ◽  
pp. A196 ◽  
Author(s):  
Leila M. Calcaferro ◽  
Alejandro H. Córsico ◽  
Leandro G. Althaus ◽  
Alejandra D. Romero ◽  
S. O. Kepler
Keyword(s):  
Stellar Evolution ◽  
Effective Temperature ◽  
Evolutionary Models ◽  
Long Period ◽  
Low Mass ◽  
Complete Set ◽  
Effective Temperatures ◽  
Binary Evolution ◽  
Stellar Masses ◽  
Gravity Mode

Context. Some low-mass white-dwarf (WD) stars with H atmospheres currently being detected in our galaxy, show long-period g(gravity)-mode pulsations, and comprise the class of pulsating WDs called extremely low-mass variable (ELMV) stars. At present, it is generally believed that these stars have thick H envelopes. However, from stellar evolution considerations, the existence of low-mass WDs with thin H envelopes is also possible. Aims. We present a thorough asteroseismological analysis of ELMV stars on the basis of a complete set of fully evolutionary models that represents low-mass He-core WD stars harboring a range of H envelope thicknesses. Although there are currently nine ELMVs, here we only focus on those that exhibit more than three periods and whose periods do not show significant uncertainties. Methods. We considered g-mode adiabatic pulsation periods for low-mass He-core WD models with stellar masses in the range [0.1554–0.4352] M⊙, effective temperatures in the range [6000–10 000] K, and H envelope thicknesses in the interval −5.8 ≲ log(MH/M⋆)≲ −1.7. We explore the effects of employing different H-envelope thicknesses on the adiabatic pulsation properties of low-mass He-core WD models, and perform period-to-period fits to ELMV stars to search for a representative asteroseismological model. Results. We found that the mode-trapping effects of g modes depend sensitively on the value of MH, with the trapping cycle and trapping amplitude larger for thinner H envelopes. We also found that the asymptotic period spacing, ΔΠa, is longer for thinner H envelopes. Finally, we found asteroseismological models (when possible) for the stars under analysis, characterized by canonical (thick) and by thin H envelope. The effective temperature and stellar mass of these models are in agreement with the spectroscopic determinations. Conclusions. The fact that we have found asteroseismological solutions with H envelopes thinner than canonical gives a suggestion of the possible scenario of formation of these stars. Indeed, in the light of our results, some of these stars could have been formed by binary evolution through unstable mass loss.

scholarly journals Concerning the Temperatures of Central Stars of Planetary Nebulae

1983 ◽  
Vol 103 ◽  
pp. 534-535
Author(s):  
L. Kohoutek ◽  
W. Martin
Keyword(s):  
Effective Temperature ◽  
Planetary Nebulae ◽  
Photoelectric Photometry ◽  
Effective Temperatures ◽  
Central Stars

Recently Pottasch (1981, Astron. Astrophys. 94, L13) published extremely high effective temperatures of some central stars of planetary nebulae (> 200 000 K). Our study of planetary nebulae based on photoelectric photometry does not confirm his results. A histogram of Tz(HI) and Tz(HeII) shows smooth distribution of Tz with the maximum of about 48 000°K (HI) and 90 000°K (HeII), respectively; the effective temperature of none of the 62 planetary nuclei exceeds 120 000°K.

scholarly journals Effective Temperatures of Stars with “Standard” Angular Diameters

1985 ◽  
Vol 111 ◽  
pp. 465-467
Author(s):  
I. N. Glushneva
Keyword(s):  
Effective Temperature ◽  
Astronomical Institute ◽  
Joint Determination ◽  
Sternberg Astronomical Institute ◽  
Effective Temperatures ◽  
Angular Diameters

For 12 stars from the list of stars with “standard” angular diameters (Fracassini et al. 1983), effective temperatures, bolometric corrections, radii and luminosities were determined. These stars are included in the stellar spectrophotometric catalog of the Sternberg Astronomical Institute and three of them were used as spectrophotometric standards. A comparison was made of Teff obtained directly using angular diameters from the list of Fracassini et al. (1983) and by means of joint determination of Teff and θ (Blackwell and Shallis 1977). For 7 stars the differences in Teff values don't exceed 1–1.5% and the maximum discrepancies are about 6% for BS 2294, 2943 and 4% for the spectrophotometric standard α Aql (BS 7557). Effective temperature values of α Lyr obtained by these two methods are in the agreement within 0.5%.

scholarly journals On the Pulsation Properties of Red Supergiant Variables

2000 ◽  
Vol 176 ◽  
pp. 105-108
Author(s):  
Giuseppe Bono ◽  
Nino Panagia
Keyword(s):  
Chemical Composition ◽  
Theoretical Investigation ◽  
Effective Temperature ◽  
Main Parameter ◽  
Preliminary Results ◽  
Blue Edge ◽  
Radial Displacements ◽  
Effective Temperatures ◽  
Stellar Masses

AbstractWe present the preliminary results of a detailed theoretical investigation on the pulsation properties and modal stability of red supergiant (RSG) stars at solar chemical composition and for stellar masses ranging from 10 to 20 M⊙. We find that the main parameter governing their pulsation behavior is the effective temperature, and indeed when moving from the blue edge toward lower effective temperatures the models show an increase in the pulsation amplitudes. For temperatures cooler than 3800 K the pulsation amplitudes are still large but the radial displacements become aperiodic.

Quenching rates and critical densities of c-C3H2

2019 ◽  
Vol 15 (S350) ◽  
pp. 148-151
Author(s):  
Malek Ben Khalifa ◽  
Emna Sahnoun ◽  
Silvia Spezzano ◽  
Laurent Wiesenfeld ◽  
Kamel Hammami ◽  
...  
Keyword(s):  
Potential Energy Surfaces ◽  
Full Potential ◽  
Thermodynamical Equilibrium ◽  
Medium Temperature ◽  
Permanent Electric Dipole Moment ◽  
Temperature Regimes ◽  
Wide Range ◽  
History Of ◽  
Energy Surfaces ◽  
Local Thermodynamical Equilibrium

AbstractCyclopropenylidene,, is a simple hydrocarbon, ubiquitous in astrophysical gases, and possessing a permanent electric dipole moment. Its readily observed multifrequency rotational transitions make it an excellent probe for the physics and history of interstellar matter. The collisional properties of with He are presented here. We computed the full Potential Energy Surfaces, and we perform quantum scattering in order to provide rates of quenching and excitation for low to medium temperature regimes. We discuss issues with the validity of the usual Local Thermodynamical Equilibrium assumption, and also the intricacies of the spectroscopy of an asymmetric top. We present the wide range of actual critical densities, as recently observed.

scholarly journals The effective temperatures of the O stars

1970 ◽  
Vol 36 ◽  
pp. 59-63
Author(s):  
Donald C. Morton
Keyword(s):  
Spectral Type ◽  
Effective Temperature ◽  
Theoretical Relation ◽  
Accurate Model ◽  
Average Temperature ◽  
Lyman Continuum ◽  
Effective Temperatures

Effective temperatures of O-type stars imbedded in diffuse nebulae are derived from measurements of Hα and radio fluxes from the nebulae and the apparent magnitudes of the stars. Accurate model atmospheres, with ultraviolet line blanketing where appropriate, are used for the theoretical relation between effective temperature and the ratio of Lyman continuum to visual stellar fluxes. Although there is considerable scatter in the results, an average temperature of 48000 K is found for spectral type O5, 40000 K for O6, and 35000 K for O7.

scholarly journals About the existence of warm H-rich pulsating white dwarfs

2019 ◽  
Vol 633 ◽  
pp. A20 ◽  
Author(s):  
Leandro G. Althaus ◽  
Alejandro H. Córsico ◽  
Murat Uzundag ◽  
Maja Vučković ◽  
Andrzej S. Baran ◽  
...  
Keyword(s):  
Transition Zone ◽  
Effective Temperature ◽  
Observational Evidence ◽  
Time Dependent ◽  
Theoretical Studies ◽  
Ionization Zone ◽  
Transition Zones ◽  
Element Diffusion ◽  
Effective Temperatures ◽  
Thermal Pulses

Context. The possible existence of warm (Teff ∼ 19 000 K) pulsating DA white dwarf (WD) stars, hotter than ZZ Ceti stars, was predicted in theoretical studies more than 30 yr ago. These studies reported the occurrence of g-mode pulsational instabilities due to the κ mechanism acting in the partial ionization zone of He below the H envelope in models of DA WDs with very thin H envelopes (MH/M⋆ ≲ 10−10). However, to date, no pulsating warm DA WD has been discovered, despite the varied theoretical and observational evidence suggesting that a fraction of WDs should be formed with a range of very low H content. Aims. We re-examine the pulsational predictions for such WDs on the basis of new full evolutionary sequences. We analyze all the warm DAs observed by the TESS satellite up to Sector 9 in order to search for the possible pulsational signal. Methods. We computed WD evolutionary sequences of masses 0.58 and 0.80 M⊙ with H content in the range −14.5 ≲ log(MH/M⋆)≲ − 10, appropriate for the study of pulsational instability of warm DA WDs. Initial models were extracted from progenitors that were evolved through very late thermal pulses on the early cooling branch. We use LPCODE stellar code into which we have incorporated a new full-implicit treatment of time-dependent element diffusion to precisely model the H–He transition zone in evolving WD models with very low H content. The nonadiabatic pulsations of our warm DA WD models were computed in the effective temperature range of 30 000 − 10 000 K, focusing on ℓ = 1 g modes with periods in the range 50 − 1500 s. Results. We find that traces of H surviving the very late thermal pulse float to the surface, eventually forming thin, growing pure H envelopes and rather extended H–He transition zones. We find that such extended transition zones inhibit the excitation of g modes due to partial ionization of He below the H envelope. Only in the cases where the H–He transition is assumed much more abrupt than predicted by diffusion do models exhibit pulsational instability. In this case, instabilities are found only in WD models with H envelopes in the range of −14.5 ≲ log(MH/M⋆)≲ − 10 and at effective temperatures higher than those typical for ZZ Ceti stars, in agreement with previous studies. None of the 36 warm DAs observed so far by TESS satellite are found to pulsate. Conclusions. Our study suggests that the nondetection of pulsating warm DAs, if WDs with very thin H envelopes do exist, could be attributed to the presence of a smooth and extended H–He transition zone. This could be considered as indirect proof that element diffusion indeed operates in the interior of WDs.

scholarly journals A detailed study of lithium in 107 CHEPS dwarf stars

2018 ◽  
Vol 611 ◽  
pp. A27 ◽  
Author(s):  
Ya. V. Pavlenko ◽  
J. S. Jenkins ◽  
O. M. Ivanyuk ◽  
H. R. A. Jones ◽  
B. M. Kaminsky ◽  
...  
Keyword(s):  
Isotopic Ratio ◽  
Small Sample Size ◽  
Small Sample ◽  
Thermodynamical Equilibrium ◽  
Lithium Abundance ◽  
Dwarf Stars ◽  
Upper Limits ◽  
Resonance Doublet ◽  
Non Local ◽  
Local Thermodynamical Equilibrium

Context. We report results from lithium abundance determinations using high resolution spectral analysis of the 107 metal-rich stars from the Calan-Hertfordshire Extrasolar Planet Search programme. Aims. We aim to set out to understand the lithium distribution of the population of stars taken from this survey. Methods. The lithium abundance taking account of non-local thermodynamical equilibrium effects was determined from the fits to the Li I 6708 Å resonance doublet profiles in the observed spectra. Results. We find that a) fast rotators tend to have higher lithium abundances; b) log N(Li) is higher in more massive and hot stars; c) log N(Li) is higher in stars of lower log g; d) stars with the metallicities >0.25 dex do not show the lithium lines in their spectra; e) most of our planet hosts rotate slower; and f) a lower limit of lithium isotopic ratio is 7Li/6Li > 10 in the atmospheres of two stars with planets (SWP) and two non-SWP stars. Conclusions. Measurable lithium abundances were found in the atmospheres of 45 stars located at distances of 20−170 pc from the Sun, for the other 62 stars the upper limits of log N(Li) were computed. We found well defined dependences of lithium abundances on Teff, V sin i, and less pronounced for the log g. In case of V sin i we see two sequences of stars: with measurable lithium and with the upper limit of log N(Li). About 10% of our targets are known to host planets. Only two SWP have notable lithium abundances, so we found a lower proportion of stars with detectable Li among known planet hosts than among stars without planets. However, given the small sample size of our planet-host sample, our analysis does not show any statistically significant differences in the lithium abundance between SWP and stars without known planets.

scholarly journals LS IV — 14°116 : A Time-Resolved Spectroscopic Study

2017 ◽  
Vol 26 (1) ◽  
Author(s):  
Pamela Martin ◽  
C. Simon Jeffery
Keyword(s):  
Radial Velocity ◽  
Spectroscopic Study ◽  
Optical Depth ◽  
Effective Temperature ◽  
Mode Instability ◽  
Instability Strip ◽  
Time Resolved ◽  
Blue Edge ◽  
Chemical Stratification ◽  
Photospheric Motion

AbstractLSIV-14 116 is a very unusual subdwarf B star. It pulsates non-radially with high-order g-modes, these pulsations are unexpected and unexplained, as the effective temperature is 6 000K hotter than the blue edge of the hot subdwarf g-mode instability strip. Its spectrum is enriched in helium which is not seen in either the V361 Hya (p-mode pulsators) or the V1093 Her stars (g-mode pulsators). Even more unusual is the 4 dex overabundance of zirconium, yttrium, and strontium. It is proposed that these over-abundances are a result of extreme chemical stratification driven by radiative levitation. We have over 20hrs of VLT/UVES spectroscopy from which we have obtained radial velocity curves for individual absorption lines. We are currently exploring ways in which to resolve the photospheric motion as a function of optical depth.

Non-local thermodynamical equilibrium abundance analysis of singly ionized praseodymium for the Sun

2020 ◽  
Vol 496 (4) ◽  
pp. 5361-5371
Author(s):  
Abdelrazek M K Shaltout ◽  
Ali G A Abdelkawy ◽  
M M Beheary
Keyword(s):  
Spectral Lines ◽  
Oscillator Strengths ◽  
Atomic Data ◽  
Dimensional Model ◽  
Thermodynamical Equilibrium ◽  
Solar Abundance ◽  
One Dimensional ◽  
Microturbulent Velocity ◽  
Non Local ◽  
Local Thermodynamical Equilibrium

ABSTRACT Determinations of the solar abundance of praseodymium (Pr) depend critically on the local thermodynamical equilibrium (LTE) and non-local thermodynamical equilibrium (NLTE) techniques beyond the capabilities of a classical one-dimensional model atmosphere. Here, in this analysis, we adopt an atomic model atom of Pr consisting of 105 energy levels and 14 bound–bound transitions of singly ionized praseodymium (Pr ii) and the ground state of the Pr iii continuum limit. We briefly analyse the solar abundance of Pr taking the solar model atmospheres of Holweger & Müller (1974, Solar Physics, 39, 19) with the measured equivalent linewidths and invoking a microturbulent velocity treatment. We succeed in accurately selecting nearby clear sections of the spectrum for 14 spectral lines of Pr ii with the improved atomic data of high-quality oscillator strengths available from the laboratory measurements of several possible sources as well as accurate damping constants successfully determined from the literature. We find a Pr abundance revised to be downwards log ϵPr(NLTE) = 0.75 ± 0.09, which is in good agreement with the meteoritic value (log ϵPr = 0.76 ± 0.03). A comparison of the NLTE abundance corrections with the standard LTE analysis, log ϵPr(LTE) = 0.74 ± 0.08, reveals a positive correction of  +0.01 dex, estimated from the selected solar Pr ii lines. The Pr abundance value is clearly superior following the classical one-dimensional model atmospheres of Holweger & Müller, the absolute scales of gf-values, the microturbulent velocity and the adopted equivalent linewidths.

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