Deep HST V- and I-Band Observations of the Halo of M31: Evidence for   Multiple Stellar Populations
  We present deep V & I WFPC2 photometry in two fields in the M31 halo 32' and 50' from the center of M31 along the SE minor axis at the locations of the M31 GCs G302 and G312 respectively. The M31 halo LFs are not consistent with a single high-metallicity population but are consistent with a mix of 50% to 75% metal-rich stars and 25% to 50% metal-poor stars. This agrees with the RGB morphology, the HB luminosity, and the RR Lyrae stars in the M31 halo. The RGB morphology indicates a metallicity spread of -2 < [m/H] < -0.2 with most stars having [m/H] = -0.6, making the M31 halo significantly more metal- rich than either the Galactic halo or the M31 GC system. The HB is dominated by a red clump similar to the 47 Tuc HB but a small number of blue HB stars are visible, supporting the conclusion that there is a metal-poor component to the M31 halo. The number of HB stars is smaller than would be expected from the observed metallicity distribution but it is not clear if this is due to the photometric limits of our data or a 2nd parameter effect. We find a He abundance of Y >~ 0.20 to 0.27, comparable with the Galaxy. LFs show weak evidence that the R = 50' field contains a higher fraction of metal-poor stars than the R = 32' field but the metallicity distributions of the RGB stars strongly suggest that both fields have the same mix of stellar populations. 
Stephen Holland, Gregory G. Fahlman, and Harvey B. Richer