scholarly journals On the corpuscular emission theory of stellar evolution

1959 ◽  
Vol 10 ◽  
pp. 115-119
Author(s):  
V. G. Fessenkov ◽  
G. M. Idlis
Keyword(s):  
Molecular Weight ◽  
Chemical Composition ◽  
Stellar Evolution ◽  
Main Sequence ◽  
Heavy Elements ◽  
Main Sequence Stars ◽  
Evolutionary Path ◽  
The Mean ◽  
Image Position ◽  
Mean Molecular Weight

Considerations regarding the evolutionary path of the main sequence stars depend essentially on the theorem of Vogt-Russell. According to this theorem the structure of stars with thermonuclear sources of energy is determined uniquely by their masses and chemical composition, as characterised by the mean molecular weight X, Y, Z being the relative amounts of hydrogen, helium and of the mixture of the heavy elements.

scholarly journals Observational Evidence for Atmospheric Chemical Composition Peculiarities Relevant to Stellar Evolution

1977 ◽  
Vol 4 (2) ◽  
pp. 119-135
Author(s):  
B. E. J. Pagel
Keyword(s):  
Chemical Composition ◽  
Stellar Evolution ◽  
Supernova Remnant ◽  
Main Sequence ◽  
Observational Evidence ◽  
Galactic Evolution ◽  
Red Giants ◽  
Main Sequence Stars ◽  
Separation Processes ◽  
Cassiopeia A

Abundance peculiarities in successive stages of stellar evolution are reviewed. Main-sequence stars show anomalies in lithium and, on the upper main sequence, the Am, Ap and Bp effects, which may be largely due to separation processes, and helium and CNO anomalies to which nuclear evolution and mixing could have contributed. Red giants of both stellar Populations commonly show more or less extreme variations among the C, N, 0 isotopes, sometimes accompanied by s-process enhancement, due to mixing out in various evolutionary stages. Detailed anomalies expected from galactic evolution are also briefly considered. Novae show strong effects in C, N, 0 and synthesis of heavier elements is displayed by the supernova remnant Cassiopeia A.

scholarly journals Why Do Low-Mass Stars Become Red Giants?

2009 ◽  
Vol 26 (3) ◽  
pp. 203-208 ◽  
Author(s):  
Richard J. Stancliffe ◽  
Alessandro Chieffi ◽  
John C. Lattanzio ◽  
Ross P. Church
Keyword(s):  
Molecular Weight ◽  
Stellar Evolution ◽  
Energy Generation ◽  
Red Giants ◽  
Low Mass Stars ◽  
Change In The Mean ◽  
Low Mass ◽  
The Mean ◽  
Red Giant ◽  
Mean Molecular Weight

AbstractWe revisit the problem of why stars become red giants. We modify the physics of a standard stellar evolution code in order to determine what does and what does not contribute to a star becoming a red giant. In particular, we have run tests to try to separate the effects of changes in the mean molecular weight and in the energy generation. The implications for why stars become red giants are discussed. We find that while a change in the mean molecular weight is necessary (but not sufficient) for a 1-M⊙ star to become a red giant, this is not the case in a star of 5 M⊙. It therefore seems that there may be more than one way to make a giant.

scholarly journals Stellar evolution with turbulent diffusion mixing in low mass stars and 12C/13C ratio in giants of the first ascending branch

1984 ◽  
Vol 105 ◽  
pp. 525-527
Author(s):  
O. Bienaymé ◽  
A. Maeder ◽  
E. Schatzman
Keyword(s):  
Molecular Weight ◽  
Stellar Evolution ◽  
Main Sequence ◽  
Turbulent Transport ◽  
Chemical Species ◽  
Life Time ◽  
Low Mass Stars ◽  
Low Mass ◽  
Red Giant ◽  
Mean Molecular Weight

We consider stellar evolution in low mass stars (1–3 Mo) near the main sequence with the hypothesis that mild turbulence is present within the all star. Turbulent transport of the elements is modeled by diffusion equations where the diffusion coefficient is chosen to be D = R✶eν where ν is the kinematical viscosity and R✶e is a Reynolds number. We consider the effects of the growth of the gradient of the mean molecular weight on turbulence. The main consequences of diffusion on stellar evolution are (1) an increase of the life time near the main sequence and (2) a change of the radial distributions of chemical species (12C, 13C, 14N, 160) (figure 1). The inhibition of the turbulence, when the gradient of mean molecular weight reaches a certain critical value, allows the evolution towards the red giant branch. When stars evolve towards the giant branch, chemical species are dredged up to the surface. At this stage models with and without diffusion, predict substantially different surface abundances (in particular the 12C/13C and C/N ratios). Comparison between models and the available data on giants during the first dredge-up show that abundance anomalies can be explained if turbulent mixing is present during the main sequence phase (figure 2).

scholarly journals Revisiting the Hunter diagram with the Geneva Stellar Evolution Code

2014 ◽  
Vol 9 (S307) ◽  
pp. 142-143
Author(s):  
R. Simoniello ◽  
G. Meynet ◽  
S. Ekström ◽  
C. Georgy ◽  
A. Granada
Keyword(s):  
Chemical Composition ◽  
Stellar Evolution ◽  
Initial Velocity ◽  
Main Sequence ◽  
Large Magellanic Cloud ◽  
Magellanic Cloud ◽  
Main Sequence Stars ◽  
Star Evolution ◽  
Initial Chemical Composition ◽  
Stellar Masses

AbstractWe produced a model grid of rotating main and post-main sequence stars with the Geneva Stellar Evolution Code (GENEC). The initial chemical composition is tailored to compare with observations of early OB type stars in the Large Magellanic Cloud (LMC) and the grid covers stellar masses in the range of 7 ≤ M/M⊙ ≤ 15 and initial velocity between 0 km s−1 ≤ v sin(i) ≤ 300 km s−1. The model grid has been used to determine the changes in the surface Nitrogen abundances during the star evolution and the results have been compared with observations.

scholarly journals Helium, [Fe/H] Abundances and the HR (Log TEFF, MBol) Diagram With Hipparcos Data of The Four Nearest Open Clusters: Hyades, Coma Berenices, The Pleiades and Praesepe

1998 ◽  
Vol 11 (1) ◽  
pp. 565-565
Author(s):  
G. Cayrel de Strobel ◽  
R. Cayrel ◽  
Y. Lebreton
Keyword(s):  
Main Sequence ◽  
Open Clusters ◽  
Main Sequence Stars ◽  
Spectroscopic Analyses ◽  
Solar Metallicity ◽  
Metal Abundances ◽  
The Mean ◽  
The Moment ◽  
The One ◽  
Best Fit

After having studied in great detail the observational HR diagram (log Teff, Mbol) composed by 40 main sequence stars of the Hyades (Perryman et al.,1997, A&A., in press), we have tried to apply the same method to the observational main sequences of the three next nearest open clusters: Coma Berenices, the Pleiades, and Praesepe. This method consists in comparing the observational main sequence of the clusters with a grid of theoretical ZAMSs. The stars composing the observational main sequences had to have reliable absolute bolometric magnitudes, coming all from individual Hipparcos parallaxes, precise bolometric corrections, effective temperatures and metal abundances from high resolution detailed spectroscopic analyses. If we assume, following the work by Fernandez et al. (1996, A&A,311,127), that the mixing-lenth parameter is solar, the position of a theoretical ZAMS, in the (log Teff, Mbol) plane, computed with given input physics, only depends on two free parameters: the He content Y by mass, and the metallicity Z by mass. If effective temperature and metallicity of the constituting stars of the 4 clusters are previously known by means of detailed analyses, one can deduce their helium abundances by means of an appropriate grid of theoretical ZAMS’s. The comparison between the empirical (log Teff, Mbol) main sequence of the Hyades and the computed ZAMS corresponding to the observed metallicity Z of the Hyades (Z= 0.0240 ± 0.0085) gives a He abundance for the Hyades, Y= 0.26 ± 0.02. Our interpretation, concerning the observational position of the main sequence of the three nearest clusters after the Hyades, is still under way and appears to be greatly more difficult than for the Hyades. For the moment we can say that: ‒ The 15 dwarfs analysed in detailed in Coma have a solar metallicity: [Fe/H] = -0.05 ± 0.06. However, their observational main sequence fit better with the Hyades ZAMS. ‒ The mean metallicity of 13 Pleiades dwarfs analysed in detail is solar. A metal deficient and He normal ZAMS would fit better. But, a warning for absorption in the Pleiades has to be recalled. ‒ The upper main sequence of Praesepe, (the more distant cluster: 180 pc) composed by 11 stars, analysed in detail, is the one which has the best fit with the Hyades ZAMS. The deduced ‘turnoff age’ of the cluster is slightly higher than that of the Hyades: 0.8 Gyr instead of 0.63 Gyr.

scholarly journals Tracing early stellar evolution with asteroseismology: pre-main sequence stars in NGC 2264

2015 ◽  
Vol 101 ◽  
pp. 01010 ◽  
Author(s):  
Konstanze Zwintz ◽  
Luca Fossati ◽  
Tatiana Ryabchikova ◽  
David Guenther ◽  
Conny Aerts
Keyword(s):  
Stellar Evolution ◽  
Main Sequence ◽  
Main Sequence Stars

scholarly journals The light elements in a helio- and asteroseismic perspective

2009 ◽  
Vol 5 (S268) ◽  
pp. 387-394
Author(s):  
Sylvie Vauclair
Keyword(s):  
Main Sequence ◽  
Light Element ◽  
Heavy Elements ◽  
Light Elements ◽  
Main Sequence Stars ◽  
General Introduction ◽  
Helium Content ◽  
Element Abundances ◽  
The Past

AbstractAsteroseismology is a powerful tool to derive stellar parameters, including the helium content and internal helium gradients, and the macroscopic motions which can lead to lithium, beryllium, and boron abundance variations. Precise determinations of these parameters need deep analyses for each individual stars. After a general introduction on helio and asteroseismology, I first discuss the solar case, the results which have been obtained in the past two decades, and the crisis induced by the new determination of the abundances of heavy elements. Then I discuss asteroseismology in relation with light element abundances, especially for the case of main sequence stars.

scholarly journals On Some Peculiar Features in the Sequences of the Old Open Star Clusters

1974 ◽  
Vol 59 ◽  
pp. 109-111
Author(s):  
A. Maeder
Keyword(s):  
Chemical Composition ◽  
Stellar Evolution ◽  
Main Sequence ◽  
Star Clusters ◽  
Open Star Clusters ◽  
Hydrogen Burning ◽  
The Core ◽  
Open Star ◽  
Solar Type ◽  
Good Agreement

In spite of the rather good agreement between the theory of stellar evolution and the observations, there exist some difficulties when one compares closely the sequences of open star clusters and the theoretical isochrones. Several, if not all, of the old open star clusters seem to be concerned, especially those which are accurately measured, namely Praesepe, NGC 2360, 752, 3680 and M67. The problem concerns the gap occuring in the HR diagram at the end of the phase of hydrogen burning in the core; it corresponds to the phase of hydrogen exhaustion (or of overall contraction). The sequence of M67 has been studied by Racine (1971) and Torres-Peimbert (1971). The well apparent gap is located farther from the zero-age main sequence than indicated by the models and the hook towards a larger Teff predicted during this phase is not observed. Differences in chemical composition may not be held responsible for these anomalies. From Torres-Peimbert's models, it may be assumed that neither solar type, nor super metal rich composition are able to reduce the discrepancies. As a further illustration, let us mention the case of NGC 752. In Table I, the main features related to the gap are examined: the disagreement, like in M67, essentially concern features 1 and 2. The observations are based on a recent study of Grenon and Mermillod (1973) and on Bell's data (1972). Bell has also mentioned the existence of discrepancies. As in M67, the gap is too far from the zero-age main sequence and does not present any sudden turning towards a larger Teff.

scholarly journals Small and Intermediate Mass Stellar Evolution - Main Sequence and Close to It

1993 ◽  
Vol 137 ◽  
pp. 410-425 ◽  
Author(s):  
A. Noels ◽  
N. Grevesse
Keyword(s):  
Mass Loss ◽  
Stellar Evolution ◽  
Turbulent Diffusion ◽  
Main Sequence ◽  
Main Sequence Stars ◽  
Intermediate Mass ◽  
Physical Mechanisms ◽  
Standard Models

AbstractWe present the standard models for small and intermediate main sequence stars and we discuss some of the problems arising with semiconvection and overshooting. The surface abundance of Li serves as a test for other physical mechanisms, including microscopic and turbulent diffusion, rotation and mass loss.

scholarly journals A preliminary color-magnitude diagram for late-type stars in the solar neighborhood

1959 ◽  
Vol 10 ◽  
pp. 39-40
Author(s):  
O. C. Wilson
Keyword(s):  
Main Sequence ◽  
Star Clusters ◽  
Solar Neighborhood ◽  
Late Type ◽  
Main Sequence Stars ◽  
Individual Values ◽  
Magnitude Diagram ◽  
Trigonometric Parallaxes ◽  
Nearby Stars ◽  
The Mean

Modern photoelectric techniques yield magnitudes and colors of stars with accuracies of the order of a few thousandths and a few hundredths of a magnitude respectively. Hence for star clusters it is possible to derive highly accurate color-magnitude arrays since all of the members of a cluster may be considered to be at the same distance from the observer. It is much more difficult to do this for the nearby stars where all of the objects concerned are at different, and often poorly determined, distances. If one depends upon trigonometric parallaxes, the bulk of the reliable individual values will refer to main sequence stars, and while the mean luminosities of brighter stars are given reasonably well by this method, the scatter introduced into a color-magnitude array by using individual trigonometrically determined luminosities could obscure important features. Somewhat similar objections could be raised against the use of the usual spectroscopic parallaxes which also should be quite good for the main sequence but undoubtedly exhibit appreciable scatter for some, at least, of the brighter stars.

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