scholarly journals Particle Transport in Non-Magnetic Stars

1986 ◽  
Vol 90 ◽  
pp. 459-472
Author(s):  
G. Michaud
Keyword(s):  
Simple Model ◽  
Mass Loss ◽  
Large Fraction ◽  
Arbitrary Parameter ◽  
Horizontal Branch ◽  
Horizontal Branch Stars ◽  
Free Model ◽  
Magnetic Stars ◽  
Abundance Anomalies ◽  
F Stars

AbstractThe observations of AmFm, λ Booti, HgMn and He rich stars that are explained without any arbitrary parameter by diffusion are briefly reviewed, followed by those observations that are not explained by this simple model. Mass loss is then shown to explain a large fraction of the observations that are not explained in the parameter free model. It seems to play a role in the λ Booti, AmFm, He rich and the hot horizontal branch stars. It is only of about 10−15 to 10−13 Mo/yr. Abundance anomalies then help to determine stellar hydrodynamics. It is finally suggested that recent observations of Li underabundances in F stars of the Hyades represent an extension of the AmFm star phenomenon.

scholarly journals Diffusion, Meridional Circulation and Mass Loss in Main Sequence and Horizontal Branch Stars

1991 ◽  
Vol 145 ◽  
pp. 111-124
Author(s):  
G. Michaud
Keyword(s):  
Diffusion Coefficient ◽  
Mass Loss ◽  
Main Sequence ◽  
Meridional Circulation ◽  
Circulation Model ◽  
Atomic Diffusion ◽  
Horizontal Branch ◽  
Halo Stars ◽  
Horizontal Branch Stars ◽  
F Stars

Photospheric abundances are used to determine the importance of atomic diffusion, meridional circulation, mass loss and turbulence in main sequence and horizontal branch stars. Atomic diffusion leads to the appearance of the Li gap and the AmFm and HgMn phenomena at approximately the Teff at which they are observed. It leads to a 12 to 25% reduction in the age of halo stars. The Li abundance in Halo stars is probably 50% of the original abundance. Atomic diffusion competes with meridional circulation. The V sin i at which the HgMn and AmFm phenomena disappear give an argument in favour of the meridional circulation model of Tassoul and Tassoul. Mass loss is probably present in AmFm stars and cooler F stars but only at the rate of 10-15 M⊙ yr-1. In many objects, the turbulent particle diffusion coefficient is at most 10 times larger than the atomic diffusion coefficient.

scholarly journals Non-variable horizontal-branch stars

1997 ◽  
Vol 189 ◽  
pp. 363-368
Author(s):  
Robert T. Rood
Keyword(s):  
Mass Loss ◽  
Physical Basis ◽  
Horizontal Branch ◽  
Loss Process ◽  
Horizontal Branch Stars ◽  
Theoretical Results ◽  
Branch Morphology

For 25 years our ignorance of the physical basis of this mass loss process has been the barrier to progress in understanding horizontal branch morphology. I review some recent observational and theoretical results which may be giving us clues about the nature of the mass loss process.

scholarly journals On the Interpretation of RR Lyrae Properties in Globular Clusters and in Other Population II Systems

1973 ◽  
Vol 21 ◽  
pp. 197-206
Author(s):  
V. Castellani ◽  
P. Giannone ◽  
A. Renzini
Keyword(s):  
Mass Loss ◽  
Globular Clusters ◽  
Horizontal Branch ◽  
Rr Lyrae ◽  
Horizontal Branch Stars

AbstractThe differences in observational parameters of the RR Lyrae variables and horizontal branch stars of globular clusters and other population II systems are considered. A discontinuous behaviour of some parameters is outlined. The Oosterhoff dichotomy and the HB morphology are discussed with regard to a conjecture of mass loss in the pre-HB phase.

scholarly journals “Hot Helium Flashers” – The Road to Extreme Horizontal Branch Stars

2008 ◽  
Vol 4 (S252) ◽  
pp. 261-262
Author(s):  
O. Yaron ◽  
A. Kovetz ◽  
D. Prialnik
Keyword(s):  
Mass Loss ◽  
Mass Loss Rate ◽  
Cooling Curve ◽  
Horizontal Branch ◽  
The Road ◽  
Horizontal Branch Stars ◽  
Low Mass ◽  
Loss Efficiency ◽  
Ehb Stars ◽  
The Masses

AbstractObservational and theoretical investigations, performed especially over the last two decades, have strongly attributed the far-UV upturn phenomenon to low-mass, small-envelope, He-burning stars in Extreme Horizontal Branch (EHB) and subsequent evolutionary phases.Using our new stellar evolution code – a code that follows through complete evolutionary tracks, Pre-MS to cooling WD – without any interruption or intervention, we are able to produce a wide array of EHB stars, lying at bluer (Teff ≥ 20,000 K) and less luminous positions on HRD, and also closely examine their post-HB evolution until the final cooling as White Dwarfs.HB morphology is a complex multiple parameter problem. Two leading players, which seem to possess the ability to affect considerably positions of HB, are those of: 1.Helium abundance, and 2.mass-loss efficiency on the first giant branch. We focus here on the latter; thus, EHB stars are produced in our calculations by increasing the mass-loss rate on the RGB, to a state where prior to reaching core He flash conditions, only a very small H-rich envelope remains. The core flash takes place at hotter positions on the HRD, sometimes while already descending on the WD cooling curve. We show preliminary results for a range of initial masses (MZAMS = 0.8 − 1.1 M⊙) and for metallicities covering both populations I and II (Z = 0.01 − 0.001). The [M,Z] combinations have been chosen such that the masses would be above and close to typical MS turnoff masses (e.g. the estimation of MTO ≃ 0.85 for NGC 2808), and also so that the ages at HB are of order of 10 ± 5 Gyr.

Abundance Anomalies in Horizontal Branch Stars and Atomic Diffusion

2008 ◽  
Vol 675 (2) ◽  
pp. 1223-1232 ◽  
Author(s):  
G. Michaud ◽  
J. Richer ◽  
O. Richard
Keyword(s):  
Atomic Diffusion ◽  
Horizontal Branch ◽  
Horizontal Branch Stars ◽  
Abundance Anomalies

scholarly journals Main Sequence Abundances, Mass Loss and Meridional Circulation

1988 ◽  
Vol 108 ◽  
pp. 3-12
Author(s):  
Georges Michaud
Keyword(s):  
Mass Loss ◽  
Loss Rate ◽  
Main Sequence ◽  
Meridional Circulation ◽  
Mass Loss Rate ◽  
Rotational Velocity ◽  
Open Clusters ◽  
Halo Stars ◽  
Abundance Anomalies ◽  
F Stars

AbstractConstraints that abundance anomalies observed on main sequence stars put on turbulence, meridional circulation and mass loss are reviewed. The emphasis is on recent observations of Li abundances.Upper limits to turbulence are obtained from the Be abundance in the Sun and from underabundances of Ca and Sc in FmAm stars. The Li abundance in G type stars suggests the presence of turbulence below convection zones.The abundance anomalies, both over and underabundances, observed in FmAm and λ Booti stars can be explained by diffusion in the presence of mass loss. A mass loss rate of 10−15 Mo yr−1 is required to explain the FmAm stars while a mass loss rate of 10−13 Mo yr−1 is required by the λ Booti stars.The position and width of the Li abundance gap observed in Hyades and other open clusters is explained by diffusion. A detailed reproduction of the Li(Teff) curve seems to require a mass loss rate of slightly more than 10−15 Mo yr−1, of the same order as the mass loss rate required by the FmAm stars. In the presence of such a mass loss only small overabundances of heavy elements are expected. The observed variations in the Li abundance as a function of the age of clusters suggests that the Li abundance observed in old halo stars does not represent the cosmological abundance.Detailed two dimensional calculations of diffusion in presence of meridional circulation for HgMn and FmAm stars lead to a cut-off of about 100 km s−1 for the maximum equatorial rotational velocity at which abundance anomalies are expected in these objects. This agrees with observations. A similar calculation for the F stars of the Hyades where Li underabundances are observed leads to a contradiction, unless meridional circulation patterns are modified by the presence of convection zones once they become as large as in late F stars. There remains a possibility that meridional circulation would be responsible for some of the reduction of the Li abundance as observed in the Hyades and UMa. Further observations are suggested to distinguish the effects of settling and nuclear destruction.

scholarly journals Striking Photospheric Abundance Anomalies in Blue Horizontal-Branch Stars in Globular Cluster M13

1999 ◽  
Vol 517 (2) ◽  
pp. L135-L138 ◽  
Author(s):  
Bradford B. Behr ◽  
Judith G. Cohen ◽  
James K. McCarthy ◽  
S. George Djorgovski
Keyword(s):  
Globular Cluster ◽  
Horizontal Branch ◽  
Horizontal Branch Stars ◽  
Abundance Anomalies

scholarly journals Diffusion and He Overabundances: Hydrodynamical Implications

1985 ◽  
Vol 87 ◽  
pp. 453-469
Author(s):  
G. Michaud
Keyword(s):  
Magnetic Fields ◽  
Mass Loss ◽  
White Dwarfs ◽  
Loss Rate ◽  
Main Sequence ◽  
Mass Loss Rate ◽  
Horizontal Branch ◽  
Main Sequence Stars ◽  
Horizontal Branch Stars ◽  
Cno Abundances

AbstractIn the absence of mass loss, diffusion leads to underabundances of He in main sequence stars. Because of a very strong observational link with Ap and He weak stars, it has however been suggested that diffusion is the explanation for the He rich stars of the upper main sequence. This requires a mass loss rate of 10−12 Mo yr−1 or slightly lower. The mass loss rate must decrease as Teff increases. Magnetic fields must apparently be involved to reduce the mass loss rate. Since this model predicts that the CNO abundances should be normal in the cooler He rich stars, it leads to a clear observational test. Detailed calculations should be made to confirm the importance of this test. The effects of separation in the wind, the atmosphere and the envelope are discussed to conclude that separation in the atmosphere is likely to be most important. The importance of diffusion for He rich white dwarfs and horizontal branch stars are briefly discussed.

scholarly journals Hot horizontal branch stars: Predictions for mass loss

2002 ◽  
Vol 392 (2) ◽  
pp. 553-562 ◽  
Author(s):  
Jorick S. Vink ◽  
Santi Cassisi
Keyword(s):  
Mass Loss ◽  
Horizontal Branch ◽  
Horizontal Branch Stars

scholarly journals Mass loss of different stellar populations in globular clusters

2019 ◽  
Vol 14 (S351) ◽  
pp. 341-345
Author(s):  
Marco Tailo
Keyword(s):  
Mass Loss ◽  
Globular Clusters ◽  
State Of The Art ◽  
Stellar Populations ◽  
Horizontal Branch ◽  
Horizontal Branch Stars ◽  
Colour Distribution ◽  
Chromosome Maps ◽  
First Time

AbstractOnce the age and metallicity are fixed, the colour distribution of horizontal branch stars in a globular cluster depends on few parameters: the helium abundance of the population and the mass lost during the pre-HB stages. These parameters are usually derived from the HB itself, hence they are degenerate. Breaking this degeneracy and understanding their role is a tricky and challenging problem that no study has solved yet. Combining the information obtained from the chromosome maps and the analysis of multi-band photometry with state of the art stellar evolution models, we can obtain a solid estimate of Y for the various stellar populations in a GC. We will then have, for the first time, the possibility to break the parameters’ degeneracy on the HB, understand the role of the mass loss, and lay the foundation to build another piece of the multiple populations mosaic.

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