scholarly journals New models for the convective flux in stellar atmospheres

1996 ◽  
Vol 176 ◽  
pp. 557-564 ◽  
Author(s):  
F. Kupka
Keyword(s):  
Stellar Evolution ◽  
Stellar Atmosphere ◽  
Stellar Atmospheres ◽  
Turbulence Theory ◽  
Mixing Length ◽  
Convective Flux ◽  
The Past ◽  
New Models ◽  
F Stars ◽  
Mixing Length Theory

Over the past decades various forms of the mixing length theory (MLT) have been used to describe convection in stellar atmospheres. Recent advances in turbulence theory now allow for major improvements in modelling thermal convection. We review several models for convection which have been derived from turbulence theory, and describe one of them, the “CM model”, in detail. The CM model has been used in several stellar evolution and helioseismology codes during the last four years and has now been applied to model atmospheres. An overwiew comparing stellar atmosphere models based on the CM formulation with its MLT predecessors indicates improvements on model atmospheres for A and F stars.

scholarly journals New models for the rapid evolution of the central star of the Stingray Nebula

2021 ◽  
Author(s):  
T M Lawlor
Keyword(s):  
Mass Loss ◽  
Stellar Evolution ◽  
Planetary Nebula ◽  
Rapid Evolution ◽  
Central Star ◽  
Asymptotic Giant Branch ◽  
Initial Mass ◽  
Thermal Pulse ◽  
The Past ◽  
New Models

Abstract We present stellar evolution calculations from the Asymptotic Giant Branch (AGB) to the Planetary Nebula (PN) phase for models of initial mass 1.2 M⊙ and 2.0 M⊙ that experience a Late Thermal Pulse (LTP), a helium shell flash that occurs following the AGB and causes a rapid looping evolution between the AGB and PN phase. We use these models to make comparisons to the central star of the Stingray Nebula, V839 Ara (SAO 244567). The central star has been observed to be rapidly evolving (heating) over the last 50 to 60 years and rapidly dimming over the past 20–30 years. It has been reported to belong to the youngest known planetary nebula, now rapidly fading in brightness. In this paper we show that the observed timescales, sudden dimming, and increasing Log(g), can all be explained by LTP models of a specific variety. We provide a possible explanation for the nebular ionization, the 1980’s sudden mass loss episode, the sudden decline in mass loss, and the nebular recombination and fading.

scholarly journals Turbulent Convection: A New Model

1991 ◽  
Vol 130 ◽  
pp. 27-32
Author(s):  
V. M. Canuto
Keyword(s):  
Observational Data ◽  
Turbulent Convection ◽  
Mixing Length ◽  
Convective Flux ◽  
New Model ◽  
Mixing Length Theory

AbstractWe use the latest models of turbulence to compute a new expression for the turbulent convective flux, Fc. The new values of Fc are up to ten times larger than those given by the mixing length theory, MLT. Astrophysical considerations indicate that the new model fares better with observational data than the MLT.

scholarly journals Presentation and Interpretation of High Resolution Infrared Spectra of Late-Type Stars

1974 ◽  
Vol 3 ◽  
pp. 255-268 ◽  
Author(s):  
R. I. Thompson
Keyword(s):  
Stellar Evolution ◽  
Carbon Star ◽  
Stellar Atmosphere ◽  
Stellar Atmospheres ◽  
Stellar Spectra ◽  
Cool Star ◽  
Convective Mixing ◽  
Processed Material ◽  
Carbon Compounds ◽  
Red Giant

Current interest in stellar evolution is concentrated on the life of a star after it has left the main sequence. Of particular interest are the red giant or supergiant periods during the hydrogen and helium shell burning phases. Convective mixing during these stages can mix nuclear processed material to the surface where it may be viewed by spectroscopic methods. It is imperative that this rare chance to view processed material be exploited fully to increase our knowledge of stellar evolution.The observation and interpretation of cool star spectra has its own particular set of problems and advantages. A particular difficulty is the formation of molecules at the low temperatures which occur in the atmospheres of late stars. Not only must the particularly complex spectra of molecules be dealt with but the problem of chemical equilibrium in the atmosphere must be solved accurately before quantitative analysis may be performed. The formation of molecules, however, has one advantage in that it very dramatically separates those stars with carbon to oxygen ratios greater than one from those with ratios less than one. It is the very high dissociation energy of 11.1 eV for the CO molecule which performs this separation. If carbon is less abundant than oxygen all of the carbon is tied up in CO and only oxides are formed in the stellar atmosphere which produce typical M star spectra. If, however, carbon is more abundant than oxygen then carbon compounds such as C2 are formed in place of the oxides and a carbon star spectrum is formed. One of the great advantages of infrared stellar spectra is that it is the only ground based technique for observing CO in stellar atmospheres.

scholarly journals A Model for Stellar Convection and Spectral Line Asymmetries

1990 ◽  
Vol 138 ◽  
pp. 417-420
Author(s):  
H. M. Antia
Keyword(s):  
Spectral Line ◽  
Stellar Disk ◽  
Doppler Velocity ◽  
Mixing Length ◽  
Linear Superposition ◽  
Convective Flux ◽  
Stellar Convection ◽  
Stellar Velocity ◽  
Intensity Fluctuations ◽  
Mixing Length Theory

A model for stellar convection zones based on linear convective modes using a nonlocal mixing length theory is developed to study the spectral line asymmetries and the line shifts resulting from convective motions in the stellar photospheric region. The amplitudes of these linear convective modes is estimated by requiring the convective flux due to a linear superposition of such modes to reproduce the convective flux in the mixing length model. To study the spectral line asymmetries the convective mode with the largest amplitude in the photospheric line formation region is chosen to represent the stellar velocity field and the accompanying intensity fluctuations. Synthetic spectral line profiles are obtained by summing locally symmetric profiles over the stellar disk according to the local Doppler velocity and intensity fluctuations. The resulting line bisector shapes and the line shifts are compared with observations for α-Cen B. It is found that while the simple model proposed here can explain either the line shifts or the line bisector shape reasonably well, it fails to explain both these characteristics simultaneously.

scholarly journals Past and Future Evolution of Sakurai's Object

2003 ◽  
Vol 209 ◽  
pp. 111-112 ◽  
Author(s):  
Falk Herwig
Keyword(s):  
Stellar Evolution ◽  
Convection Zone ◽  
Central Star ◽  
Evolutionary Time ◽  
Future Evolution ◽  
The Past ◽  
Envelope Material ◽  
Mixing Length Theory ◽  
Born Again ◽  
Model Sequence

We present a stellar evolution model sequence of the past and future evolution of the post-AGB born again star Sakurai's object (V 4334 Sgr). In order to match the short evolutionary time scale we have to assume that the convective ingestion of hydrogen-rich envelope material into the He-flash convection zone proceeds slower than predicted by the mixing length theory. For the future we predict a swift second evolution through the central star region before a second born-again evolution occurs.

Impact of the convective mixing-length parameter α on stellar metallicity

2020 ◽  
Vol 635 ◽  
pp. A176 ◽  
Author(s):  
N. Song ◽  
S. Alexeeva ◽  
T. Sitnova ◽  
L. Wang ◽  
F. Grupp ◽  
...  
Keyword(s):  
Open Cluster ◽  
Synthesis Method ◽  
Stellar Atmospheres ◽  
High Signal ◽  
Mixing Length ◽  
Dwarf Stars ◽  
Giant Stars ◽  
Spectrum Synthesis ◽  
Mixing Length Theory ◽  
The Impact

Context. Mixing-length theory is used to treat stellar convection. As a simulation in one-dimensional stellar atmospheres models, the mixing-length parameter α is calibrated from the Sun and then applied to other stars. However, there is no strong evidence to suggest that α should be the same for stars of different evolutionary stages. Aims. We evaluate the impact of the α value on the metallicity of different types of stars and investigate the correlation between the metallicity discrepancy (Δ[Fe∕H]) and stellar parameters (Teff, log g). Methods. We selected ten well-studied field stars and one open cluster of nine members for which high-resolution and high signal-to-noise spectra are available. The model atmospheres were calculated with the code MAFAGS-OS. We derived iron abundances from Fe I and Fe II lines both under local thermodynamic equilibrium and non-LTE conditions using a spectrum synthesis method. After deriving [Fe/H] for each line, we calculated Δ[Fe∕H] with two different α values, fixed solar-calibrated α, and α obtained for each star individually. Finally, we investigated the correlation between Δ[Fe∕H] caused by revised α with stellar parameters. Results. For FGK dwarf stars, the Δ[Fe∕H] caused by the α correction is less than 0.02 dex, while for turn-off and giant stars, the Δ[Fe∕H] values are no more than 0.03 dex, which are lower than typical uncertainties in metallicity. For main-sequence stars, Δ[Fe∕H] versus Teff and Δ[Fe∕H] versus log g are well fit by linear relations.

scholarly journals Conclusion

1977 ◽  
Vol 4 (2) ◽  
pp. 217-218
Author(s):  
R. Cayrel
Keyword(s):  
Chemical Composition ◽  
Basic Problem ◽  
Stellar Atmosphere ◽  
Stellar Atmospheres ◽  
Classification Systems ◽  
Spectral Classification ◽  
Surface Layers ◽  
Fundamental Parameters ◽  
The Past ◽  
Main Basis

This joint discussion has amply demonstrated that stellar atmospheres are not passive and dumb bodies just sitting there at the surface of a star.First of all the surface “feel” fundamental parameters of the star as a whole, specifically the ratio L/R2 and m/R2. As these fundamental parameters are affected by stellar evolution, the surface layers reflect to a large extent the degree of evolution of the star. This is the main basis for photometric and spectral classification systems. One could even claim that the observation of the stellar atmosphere would univocally determine the state of evolution of the star if i) it was always possible to determine the atmospheric abundances of the key elements to internal structure and ii) if the chemical composition of the atmosphere was always representative of the chemical composition of the whole star. Unfortunately a serious problem occurs for i) with the abundance of helium which element has no photospheric absorption line for all spectral types later than B and also with ii) in a more limited portion of the HR diagram, mainly in late B and A slow rotators. But whereas point i) is a mere problem of contingency, point ii) is a basic problem, greatly overlooked in the past, and to which one fourth of the joint discussion was devoted.

scholarly journals On supersonic convection in stellar atmospheres

2006 ◽  
Vol 2 (S239) ◽  
pp. 266-273
Author(s):  
D. R. Xiong ◽  
L. Deng
Keyword(s):  
Energy Transport ◽  
Physical Reality ◽  
Stellar Atmospheres ◽  
Red Giants ◽  
Mixing Length ◽  
Hydrodynamic Simulation ◽  
Mixing Length Theory

AbstractIt follows from the local mixing length theory that the convection in the atmospheres of yellow and red giants and super-giants become supersonic. In this work we studied the physical reality of such phenomenon and its possible consequences on the structure and evolution of stars involving such situations. The main conclusion is that the supersonic nature of convection as predicted by the local mixing length theory has been overestimated. If supersonic convection is not an artifact in all situations, it is the case at least for all yellow giants and super-giants. We believe that such an artifact is due to the imperfect of the mixing length theory. Owing to the fact that convective energy transport in the atmospheres of yellow giants and super-giants is quite negligible, these artifacts have very limited consequences on the structure and evolution of these stars. However, it is not the case for red giants and super-giants whose properties can be seriously affected by this overestimation. Our investigation shows that the theoretical red phases of stars under consideration are somewhat too blue as predicted with the usual mixing length theory. To this aim, full hydrodynamic simulation is needed in order to clear the doubts on the existence of supersonic convection in these red objects.

Stellar Evolution

1962 ◽  
Vol 11 (02) ◽  
pp. 137-143
Author(s):  
M. Schwarzschild
Keyword(s):  
Solar System ◽  
Stellar Evolution ◽  
Space Craft ◽  
New Techniques ◽  
Substantial Body ◽  
Individual Stars ◽  
The Past ◽  
Evolution Of Galaxies ◽  
History Of ◽  
Fast Flow

It is perhaps one of the most important characteristics of the past decade in astronomy that the evolution of some major classes of astronomical objects has become accessible to detailed research. The theory of the evolution of individual stars has developed into a substantial body of quantitative investigations. The evolution of galaxies, particularly of our own, has clearly become a subject for serious research. Even the history of the solar system, this close-by intriguing puzzle, may soon make the transition from being a subject of speculation to being a subject of detailed study in view of the fast flow of new data obtained with new techniques, including space-craft.

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