Horizontal Branch Stellar Evolution

I review aspects of the evolution of horizontal branch (HB) stars. The defining characteristic of this, the core He-burning phase of evolution for M < Mflash ∼ 2.2M⊙, is a spread in observed colour amongst HB stars within the (almost) chemically homogeneous, isochronous populations found in Galactic globular clusters. The inference is that the stars within a given cluster must have undergone varying degrees of mass loss during the earlier, red giant (RGB) phase. I start by reviewing current topics in the study of HB stellar evolution, including a brief review of the main determinants of the structure of low-mass core helium burning stars and of HB morphology. I describe the main concerns addressed by earlier studies of HB morphology, viz. the ‘first’ and ‘second’ parameter effects on HB morphology, and attempt to summarize the current state of affairs and how future investigations might improve our findings in this area. I also briefly discuss what appears to be the topic that has generated the most theoretical attention over the last 5 years, that of the hot end of HB sequences. The stars that have undergone the most mass loss — termed extreme HB stars (EHB) — form a group that is often separated observationally from the ‘blue’ HB stars seen in metal-poor globular clusters. I then discuss the current state of HB theory, reviewing the effect of new physics on the models, and the special considerations (partial mixing) that arise in core helium burning models. Assuming only that the gas cannot, on average, be strongly superadiabatic anywhere, one finds that the convective core must be surrounded by a partially mixed semiconvective region. This result holds when the radiation pressure is small and the opacity has a significant Kramers-type component. I also review the controversy surrounding the behaviour of the core in the late phases of evolution. Finally, I give two further examples of questions that are still outstanding. After noting that the mass loss mechanism is not at all understood, I review detailed work by several different groups concerning the age, abundance spread, and rotational properties of RGB and HB stars in the ‘second parameter’ clusters M3 and M13. I conclude by discussing the HB luminosity-metallicity relation. The discrepancy between the slope of this relation from different lines of argument appears largely to have been resolved. However, the observational data and theoretical sequences (if Y ∼> 0.23) are still in conflict as far as the HB luminosity zero-point is concerned. “Stellar Evolution : What Should Be Done”; 32st Liège Int. Astroph. Coll., 1995