Session 47 - Old Stellar Populations Beyond The Milky Way - II.
Oral session, Wednesday, June 11
North Main Hall A,

## [47.02] The Extreme Horizontal Branch in Globular Clusters and Elliptical Galaxies

R. W. O'Connell (UVa)

Extreme horizontal branch stars, with envelope masses \leq 0.05M_ødot and temperatures above 15,000K, and their progeny can have a dramatic effect on the integrated far-ultraviolet (\lambda < 1800 Ålight of old stellar populations. Large samples of these objects have recently been identified in globular clusters by HST and UIT.

There is now good evidence that the UV-upturn'' in E galaxies and spiral bulges originates from this type of star. Furthermore, the upturn varies more between galaxies and with radius in a given galaxy than any other photometric or spectroscopic index. It is evidently extraordinarily sensitive to the characteristics of its parent population. Once the underlying drivers of the EHB component (age, abundance, mass-loss processes, dynamics) are understood, the UV upturn will become a uniquely delicate probe of old galaxy populations.