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 time resolves hot horizontal branch (HB) stars in an elliptical galaxy. Given the near-solar metallicity of M32, much larger than that of globular clusters, the existence of an extended horizontal branch is a striking example of the second parameter effect, and, most importantly, provides direct evidence that hot PIE stars and their progeny are the major contributors to the UV upturn phenomenon observed in elliptical galaxies. Our image, obtained with 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 deepest parts of the image, our catalog is reasonably complete (>25%) to a magnitude of 27. The hot HE spans a magnitude range of 25-27 mag at effective temperatures hotter than 8500 K. We interpret this near-UV luminosity function 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 the presence of post-asymptotic giant branch (post-AGB) stars, our image conclusively demonstrates that it arises from a small fraction (less than or similar to 5%) of the population passing through the hot HE phase. The production of these hot HE stars does not appear to rely upon dynamical mechanisms-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 AGE 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-burning tracks; (3) the post-AGE stars could be surrounded by circumstellar dust during the transition from the AGE to T-eff > 60,000 K.