scholarly journals The CNO-Cycle Elements in Atmospheres of B-Type Main Sequence Stars

1991 ◽  
Vol 145 ◽  
pp. 125-135
Author(s):  
L.S. Lyubimkov
Keyword(s):  
Main Sequence ◽  
Short Review ◽  
Stellar Atmospheres ◽  
Main Sequence Stars ◽  
Oxygen Abundance ◽  
Cno Cycle ◽  
C And N ◽  
Stellar Interiors ◽  
Principle Source

The aim of this short review is to pay attention to some problems connected with the He, C and N abundances in atmospheres of B-type main sequence stars. These elements participate in CNO-cycle which is the principle source of energy in such stars. As known, the He, C and N abundances in stellar interiors are considerably changed owing to CNO-cycle (oxygen abundance alter insignificantly). There are some variations of the He, C and N abundances in stellar atmospheres, too, and our task is to discuss probable causes of such variations. It is necessary to emphasize that only normal B-type stars are considered (not He-rich or He-weak, for example).

C and N abundances among 47 Tucanae main-sequence stars

1991 ◽  
Vol 373 ◽  
pp. 482 ◽  
Author(s):  
Michael M. Briley ◽  
James E. Hesser ◽  
R. A. Bell
Keyword(s):  
Main Sequence ◽  
Main Sequence Stars ◽  
C And N

scholarly journals Non-Local Convection Models for Stellar Atmospheres and Envelopes

2003 ◽  
Vol 210 ◽  
pp. 143-156
Author(s):  
F. Kupka
Keyword(s):  
Numerical Simulations ◽  
Main Sequence ◽  
Stellar Atmospheres ◽  
Surface Velocity ◽  
Line Profiles ◽  
Main Sequence Stars ◽  
Solar Type ◽  
Non Local ◽  
Stress Models ◽  
Do So

We present an overview of the concepts underlying advanced non-local Reynolds stress models of turbulent convection and review a comparison of this approach with a series of numerical simulations of fully compressible convection. We then discuss results from applications of the model to complete envelopes of A-type main sequence stars. The non-local model reproduces surface velocities in agreement with the lower limit of observed macro- and microturbulence velocities of A-star photospheres, the asymmetry of the surface velocity field as inferred from spectral line profiles, and the overall structure of the photospheric and subphotospheric convection zones, as predicted by the most recent numerical simulations available for these stars. Traditionally, local models of convection are unable to do so. We conclude with a brief survey of extensions of the model which are interesting for other applications such as atmospheres of solar type stars and overshooting below deep convective envelopes or above the core in massive stars.

scholarly journals Commission 36: Theory of Stellar Atmospheres

1985 ◽  
Vol 19 (1) ◽  
pp. 503-507
Author(s):  
B. Gustafsson ◽  
K. Kodaira ◽  
D. F. Gray ◽  
A. G. Hearn ◽  
W. Kalkofen ◽  
...  
Keyword(s):  
Main Sequence ◽  
Stellar Atmospheres ◽  
Compact Objects ◽  
Radiation Hydrodynamics ◽  
Main Sequence Stars

Commission 36 acts as a sponsor or co-sponsor at the following symposi and colloquia: IAU Colloquium No. 90 “Upper Main Sequence Stars with Anomalous Abundances”, Crima, USSR (May 1985), IAU Colloquium No. 89 “Radiation Hydrodynamics in Stars and Compact Objects”, Copenhagen, Denmark (June 1985), IAU Symposium No. 120 “Astrochemistry”, Goa, India (December 1985), IAU Colloquium No. 87 “Hydrogen Deficient Stars and Related Objects”, Bangalore, India (December 1985).

scholarly journals Winds

1993 ◽  
Vol 137 ◽  
pp. 620-633
Author(s):  
K.B. MacGregor
Keyword(s):  
Mass Loss ◽  
Internal Structure ◽  
Physical State ◽  
Main Sequence ◽  
Stellar Atmosphere ◽  
Present Review ◽  
Main Sequence Stars ◽  
Dynamical State ◽  
Stellar Interiors

AbstractIt is by now well known that most main sequence stars continuously lose mass as a consequence of the winds they emit. In addition to affecting the thermal and dynamical state of the stellar atmosphere, such mass loss can also induce changes in the interiors of stars. In the present review, we consider a few of the ways in which sustained, wind-like mass loss can alter the physical state of main sequence stellar interiors by examining the differences in internal structure, composition, and rotation between mass-losing and conservatively evolving stars.

scholarly journals The Occurrence of Lithium in Stars

1968 ◽  
Vol 1 ◽  
pp. 230-232
Author(s):  
G. H. Herbig
Keyword(s):  
Main Sequence ◽  
Solar Nebula ◽  
T Tauri Stars ◽  
Stellar Atmospheres ◽  
Limited Time ◽  
Main Sequence Stars ◽  
Solar Abundance ◽  
T Tauri ◽  
Observational Knowledge ◽  
Li Content

My task is to outline our present observational knowledge of the occurrence of lithium in stellar atmospheres. On account of the limited time, I shall not attempt to include a description of the situation in post-main sequence stars. Also for shortness of time, forgive me if I do not stop at each point to give due credit to the astronomers who have contributed to that topic.The youngest stars we know which are cool enough to exhibit neutral Li are the T Tauri stars, which are in the early stages of contraction toward the main sequence. All these objects that have been adequately observed are very abundant in Li: the range is between 50 and 400 times the so-called ‘solar abundance’, a convenient unit which corresponds to a H/Li ratio by number of atoms of about 1011. It is significant that the average Li content of chondritic meteorites, which are often considered to be samples of the non-volatile, unprocessed material of the original solar nebula, is about 150 on this system, within the range observed in the TTauri stars.

scholarly journals Vibrational instability of Population III very massive main-sequence stars due to the -mechanism

2012 ◽  
Vol 421 (1) ◽  
pp. L34-L38 ◽  
Author(s):  
Takafumi Sonoi ◽  
Hideyuki Umeda
Keyword(s):  
Nuclear Energy ◽  
Main Sequence ◽  
Early Stage ◽  
Cooling Process ◽  
Main Sequence Stars ◽  
Hydrogen Burning ◽  
Cno Cycle ◽  
The Universe ◽  
Vibrational Instability ◽  
Population Iii

ABSTRACT Very massive stars are thought to be formed in the early Universe because of a lack of cooling process by heavy elements, and might have been responsible for the later evolution of the Universe. We had an interest in the vibrational stability of their evolution and have carried out a linear non-adiabatic analysis of radial and non-radial oscillations for Population III very massive main-sequence stars with . We found that only the radial fundamental mode becomes unstable due to the -mechanism for these stars. The instability appears just after the CNO cycle is activated and the nuclear energy generation rate becomes large enough to stop the pre-main-sequence contraction, and continues during the early stage of core hydrogen burning. Also, we have roughly estimated the amount of mass loss due to the instability to evaluate its significance.

scholarly journals On the Nitrogen Abundance - Age Relation For Main Sequence Early B-Stars

1993 ◽  
Vol 137 ◽  
pp. 183-186
Author(s):  
Leonid S. Lyubimkov
Keyword(s):  
Main Sequence ◽  
Massive Stars ◽  
Mixing Process ◽  
B Stars ◽  
Cno Cycle ◽  
Nitrogen Abundance ◽  
Age Relation ◽  
N Enrichment ◽  
Stellar Interiors

The results of the He, N and C abundance determinations for MS early B-stars, summarized in the review [1], lead to the inference that there are evolutionary variations in atmospheric contents of these elements. The conclusion was made that CNO-cycle products emerge on a surface of B-stars during their MS phase yet and such appearance can be considered as a probable manifestation of some mixing process between outer layers and stellar interiors. In particular the N enrichment with the age was found, which reachs about 1.0 dex for most massive B-stars (see [1,2]). However Gies and Lambert [3] (further GL) investigated recently the C, N and O abundances in a sample of early B-stars and did not confirm such great N enrichment. Since the problem of early mixing is very important for the understanding of first evolutionary stages of massive stars, the GL data need further and thorough analysis.

scholarly journals Estimating Stellar Parameters and Interstellar Extinction from Evolutionary Tracks

2016 ◽  
Vol 25 (1) ◽  
Author(s):  
S. Sichevsky ◽  
O. Malkov
Keyword(s):  
Main Sequence ◽  
Formation Rate ◽  
Interstellar Extinction ◽  
Mass Function ◽  
Initial Mass Function ◽  
Stellar Atmospheres ◽  
Red Giants ◽  
Main Sequence Stars ◽  
Stellar Radius ◽  
Multicolor Photometry

AbstractDeveloping methods for analyzing and extracting information from modern sky surveys is a challenging task in astrophysical studies. We study possibilities of parameterizing stars and interstellar medium from multicolor photometry performed in three modern photometric surveys: GALEX, SDSS, and 2MASS. For this purpose, we have developed a method to estimate stellar radius from effective temperature and gravity with the help of evolutionary tracks and model stellar atmospheres. In accordance with the evolution rate at every point of the evolutionary track, star formation rate, and initial mass function, a weight is assigned to the resulting value of radius that allows us to estimate the radius more accurately. The method is verified for the most populated areas of the Hertzsprung-Russell diagram: main-sequence stars and red giants, and it was found to be rather precise (for main-sequence stars, the average relative error of radius and its standard deviation are 0.03% and 3.87%, respectively).

scholarly journals Lithium and Beryllium in Main Sequence Stars

1991 ◽  
Vol 145 ◽  
pp. 85-97 ◽  
Author(s):  
R. Rebolo
Keyword(s):  
Present State ◽  
Main Sequence ◽  
Open Cluster ◽  
Transport Processes ◽  
Stellar Atmospheres ◽  
Stellar Rotation ◽  
Main Sequence Stars ◽  
Halo Stars

Li and Be abundances in Main Sequence stars later than F0 are reviewed. Observations of Li in open cluster stars and metal-poor dwarf halo stars has promoted the development of a great variety of models invoking transport processes in the outermost layers of these stars. Although measurements in their present state do not allow to establish definitive conclusions on which process is the most important for each range of masses, they do manifest that metallicity and stellar rotation play an important role in controlling the processes which trigger disappearance of these elements from stellar atmospheres.

scholarly journals Rotating Stellar Interiors (Review Paper)

1987 ◽  
Vol 92 ◽  
pp. 123-146
Author(s):  
Robert Connon Smith
Keyword(s):  
Angular Momentum ◽  
Large Scale ◽  
Main Sequence ◽  
Unsolved Problem ◽  
Mechanical Energy ◽  
Main Sequence Stars ◽  
Hot Stars ◽  
Total Luminosity ◽  
Internal Distribution ◽  
Stellar Interiors

AbstractRotation has two main qualitative effects on the structure of a star: it distorts the shape and it reduces the total luminosity. The luminosity depends only on the total angular momentum but the shape, and so the observable properties, depends on the internal distribution of angular momentum. The expected distribution, even for main-sequence stars, is an unsolved problem. Redistribution of angular momentum can occur as a result of large-scale circulation currents, instabilities and magnetic stresses and there may be no steady state. If shear instabilities are dominant, then stars may rotate nearly uniformly. Studies of non-radial oscillations might perhaps eventually lead to a direct probing of the internal angular momentum distribution by the methods now being used to study the internal rotation of the Sun. Differential rotation near the surface would probably give rise to turbulent motions, which could generate the mechanical energy flux which seems to be a necessary input to the winds in hot stars.

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