Astronomy and Astrophysics – Astrophysics
Scientific paper
May 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993a%26a...272..255j&link_type=abstract
Astronomy and Astrophysics, Vol.272, NO. 1/MAYI, P. 255, 1993
Astronomy and Astrophysics
Astrophysics
15
Stellar Evolution, Mass Loss, Horizontal-Branch Stars, Stars: Fundamental Parameters, Globular Clusters: General, Globular Clusters: M92, 47 Tuc
Scientific paper
In this paper we discuss the effects on the horizontal branch morphology of possible star-to-star variations in various parameters of the stars in globular clusters. These parameters include the mass loss efficiency, the mass of the core at the helium core flash, the metallicity, and the initial mass. We have developed a method that allows us to compute many evolutionary tracks quickly and accurately, and we are therefore able to study the effects of a wide range of changes in the parameter space. Our method relies on observed relations between luminosity and effective temperature, and on existing model computations of the core mass and luminosity at the helium core flash.
The mass, MRGB, of the stars at the red giant branch is determined from a method that is to a large extent independent of computed models. With MRGB known, we can determine the value of the mixing length parameter, , that is required (for a given grid of models) in order to fit observed colour-magnitude diagrams. By applying this method to observations of globular clusters (GCs) of a wide range of metallicities, we find that α (on the RGB) is independent of metallicity (but its value is dependent on the code used). For the extensive grid of Sweigart & Gross (1978) we find its value to be α = 1.4±0.1.
The variation in colour of the HB stars in a given GC can be well explained as a spread in the ratio of total mass to core mass. It is demonstrated that the mass differences between RGB and HB stars, as well as the mass spread on the HB, can be explained as a consequence of normal Reimers-type mass loss. We show that star-to-star variations of the mass loss parameter, η, between η=0.0 and η=0.7 can explain the HB morphology in typical GCs while at the same time being fully consistent with observations of field stars (e.g., those on which Reimers mass loss law is based), as well as with the RGB morphology of the GCs. We predict that the variation in Teff near the red giant branch tip (RGT) for stars with different η will be nearly unobservable and stars with different mass loss efficiency will reach approximately the same luminosity at the RGT.
If, as an alternative, star-to-star variations in the mass, McRGT, of the core at the helium core flash should explain the HB morphology (e.g., due to variations in the core rotation velocity), this variation must be ±0.05 Msun in order to explain the typical spread in mass observed in GCs. Such variations in McRGT are only consistent with the observed RGB in GCs if a few of the stars that are usually believed to be AGB stars are in fact RGB stars with "delayed" core flash.
Other scenarios that could explain the average mass difference between stars at the HB and the RGB are either unable to explain the spread in HB mass, or lead to contradictions with observations of the RGB stars. Such scenarios include star-to- star variations in the mixing length parameter, the value of Z, and the initial mass on the RGB, as well as variations in possible mass loss caused by the He core flash.
Due to the high flexibility of our method to continuous variations in a wide range of parameters, it is also suitable for estimating for example the likelihood of forming hot subdwarfs in single-star evolution, the possible contribution of HB-manque' star to the UV light in galaxies of various metallicities, and the conditions for forming precursors of white dwarfs.
Jorgensen Uffae G.
Thejll Peter
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