Hubble Space Telescope Observations of M32: The Color-Magnitude Diagram

Abstract

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. There is little evidence for an extended or even a red horizontal branch, but we find a strong clump on the giant branch itself. 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.Comment: 22 pages, AASTeX, aaspp4 and flushrt style files included, 11 postscript figures, figures 1,2,5,7, and 8 available at ftp://bb3.jpl.nasa.gov/pub/m32 . Submitted to the Astronomical Journa

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WeCAPP -Wendelstein Calar Alto pixellensing project I

“…7 through 16 indicate a typical scatter which is in good agreement with the above estimate. This means that a red clump giant with a brightness of M I = 0 (Grillmair et al 1996, Fig. 7) and a colour of (R − I) = 0.5 (Lejeune et al 1998) has to be amplified by a factor of 10 to be detected with a peak signal-to-noise ratio of (S/N ) = 3 in our survey.…”

The Upper Asymptotic Giant Branch of the Elliptical Galaxy Maffei 1 and Comparisons with M32 and NGC 5128

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