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 center of M32 based on Hubble Space Telescope WFPC2 images. The broad color-luminosity distribution of red giants shows that the stellar population comprises stars with a wide range in metallicity. This distribution cannot be explained by a spread in age. The blue side of the giant branch rises to M_I_ ~ -4.0 and can be fitted with isochrones having [Fe/H] ~ - 1.5. The red side consists of a heavily populated and dominant sequence that tops out at M_I_~ -3.2, and extends beyond V-I=4. This sequence can be fitted with isochrones 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 branch stars seen in previous ground- based work and argue that the majority of them were artifacts of crowding. Our results are consistent with the presence of the infrared- luminous giants found in ground-based studies, 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 sparsely sampled by this data set to provide a precise distance estimate. At fainter magnitudes, the rising giant branch is significantly wider (FWHM_V-I_ ~0.6 mag down to M_I_ ~ - 1.0) than can be accounted for by photometric uncertainties, again due to a metallicity spread. There is little evidence for an extended or even a red horizontal 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 chemical evolution, and also appears somewhat narrower than that of the solar neighborhood. Overall, the M32 HST color-magnitude diagram is consistent with the average luminosity-weighted age of 8.5 Gyr and [Fe/H] ~-0.25 inferred from integrated spectral indices, extrapolated to the same radius and analyzed with the same population models.