HUBBLE-SPACE-TELESCOPE OBSERVATIONS OF M32 - THE COLOR-MAGNITUDE DIAGRAM

We present a V-I color-magnitude diagram for a region 1'-2' from the c enter of M32 based on Hubble Space Telescope WFPC2 images. The broad c olor-luminosity distribution of red giants shows that the stellar popu lation comprises stars with a wide range in metallicity. This distribu tion cannot be explained by a spread in age. The blue side of the gian t branch rises to M(I) approximate to-4.0 and can be fitted with isoch rones having [Fe/H]approximate to-1.5. The red side consists of a heav ily populated and dominant sequence that tops out at M(I) approximate to-3.2, and extends beyond V-I=4. This sequence can be fitted with iso chrones with -0.2<[Fe/H]<+0.1, for ages running from 15 Gyr to 5 Gyr, respectively. We do not find the optically bright asymptotic giant bra nch stars seen in previous ground-based work and argue that the majori ty of them were artifacts of crowding. Our results are consistent with the presence of the infrared-luminous giants found in ground-based st udies, though their existence cannot be directly confirmed by our data . The tip of the metal-poor portion of the giant branch occurs at the luminosity expected if M32 is at the same distance as M31 but is too s parsely sampled by this data set to provide a precise distance estimat e. At fainter magnitudes, the rising giant branch is significantly wid er (FWHM(V-I)similar to 0.6 mag down to M(I) similar to-1.0) than can be accounted for by photometric uncertainties, again due to a metallic ity spread. There is little evidence for an extended or even a red hor izontal branch, but we find a strong clump on the giant branch itself, as expected for the high metallicities inferred from the giant branch . If the age spread is not extreme, the distribution of metallicities in M32 is considerably narrower than that of the closed-box model of c hemical evolution, and also appears somewhat narrower than that of the solar neighborhood. Overall, the M32 HST color-magnitude diagram is c onsistent with the average luminosity-weighted age of 8.5 Gyr and [Fe/ H]approximate to-0.25 inferred from integrated spectral indices, extra polated to the same radius and analyzed with the same population model s. (C) 1996 American Astronomical Society.