Extremely hot horizontal branch ( HB ) stars and their progeny are widely considered to be responsible for the “ ultraviolet upturn ” ( or UVX ) phenomenon observed in elliptical galaxies and the bulges of spirals .
Yet , the precise evolutionary channels that lead to the production of these stars remain the source of much debate .
In this review , we discuss two key physical ingredients that are required in order for reliable quantitative models of the UV output of stellar populations to be computed , namely , the mass loss rates of red giant branch stars and the helium enrichment “ law ” at high metallicities .
In particular , the recent evidence pointing towards a strong enhancement in the abundances of the \alpha -elements in the Galactic bulge ( compared to the disk ) , and also the available indications of a similar overabundance in ( massive ) elliptical galaxies , strongly suggest that the helium abundance Y may be higher in ellipticals and bulges than it is in spiral disks by an amount that may reach up to 0.15 at { [ Fe / H ] } \sim + 0.5 .
If so , this would strongly favor the production of hot HB stars at high metallicity in galactic spheroids .
We also discuss the existence of mass loss recipes beyond the commonly adopted Reimers “ law ” that are not only more consistent with the available empirical data , but also much more favorable to the production of extended HB stars at high metallicity .
Finally , we discuss new empirical evidence that suggests that different evolutionary channels may be responsible for the production of EHB stars in the field and in clusters .