Detection and photometry of hot horizontal branch stars in the core of M32

We present the deepest near-UV image of M32 to date, which for the first ti me resolves hot horizontal branch (HB) stars in an elliptical galaxy. Given the near-solar metallicity of M32, much larger than that of globular clust ers, the existence of an extended horizontal branch is a striking example o f the second parameter effect, and, most importantly, provides direct evide nce that hot PIE stars and their progeny are the major contributors to the UV upturn phenomenon observed in elliptical galaxies. Our image, obtained w ith the Space Telescope Imaging Spectrograph (STIS), detects approximately 8000 stars in a 25 " x 25 " field, centered 7." 7 from the galaxy nucleus. These stars span a range of 21-28 mag in the STMAG system, and in the deepe st parts of the image, our catalog is reasonably complete (>25%) to a magni tude of 27. The hot HE spans a magnitude range of 25-27 mag at effective te mperatures hotter than 8500 K. We interpret this near-UV luminosity functio n with an extensive set of HE and post-MB evolutionary tracks. Although the W-to-optical flux ratio in M32 is weak enough to be explained solely by th e presence of post-asymptotic giant branch (post-AGB) stars, our image conc lusively demonstrates that it arises from a small fraction (less than or si milar to 5%) of the population passing through the hot HE phase. The produc tion of these hot HE stars does not appear to rely upon dynamical mechanism s-mechanisms that map play a role in the HE morphology of globular clusters . The majority of the population presumably evolves through the red HE and subsequent post-AGE phases; however, we see far fewer UV-bright stars than expected from the lifetimes of canonical hydrogen-burning low-mass post-AGE tracks. There are several possible explanations: (1) the transition from A GE to T-eff > 60,000 Ii could be much more rapid than previously thought; ( 2) the vast majority of the post-AGE stars could be evolving along helium-b urning tracks; (3) the post-AGE stars could be surrounded by circumstellar dust during the transition from the AGE to T-eff > 60,000 K.