Hubble Space Telescope WFPC2 Color-Magnitude Diagrams for Globular Clusters in M31

We report new photometry for 10 globular clusters in M31, observed to a uniform depth of four orbits in F555W (V) and F814W (I) using WFPC2 on board the Hubble Space Telescope (HST). In addition, we have reanalyzed HST archival data of comparable quality for two more clusters. A special feature of our analysis is the extraordinary care taken to account for the effects of blended stellar images and the required subtraction of contamination from the field stellar populations in M31 in which the clusters are embedded. We thus reach 1 mag fainter than the horizontal branch (HB), even in unfavorable cases. We also show that an apparent peculiar steep slope of the HB for those clusters with blue HB stars is actually due to blends between blue HB stars and red giants. We present the color-magnitude diagrams (CMDs) and discuss their main features in comparison with the properties of the Galactic globular clusters. This analysis is augmented with CMDs previously obtained and discussed on eight other M31 clusters. We report the following significant results: (1) The loci of the red giant branches give reliable photometric metallicity determinations that generally compare very well with ground-based integrated spectroscopic and photometric measures, as well as giving good reddening estimates. (2) The HB morphologies follow the same behavior with metallicity as the Galactic globular clusters, with indications that the second-parameter effect can be present in some clusters of our sample. However, at [Fe/H] =∼ -1.7 we observe a number of clusters with red HB morphology such that the HB type versus [Fe/H] relationship is offset from that of the Milky Way (MW) and resembles that of the Fornax dwarf spheroidal galaxy. One explanation for the offset is that the most metal-poor M31 globular clusters are younger than their MW counterparts by 1–2 Gyr; further study is required. (3) The MV(HB) versus [Fe/H] relationship has been redetermined, and the slope (∼0.20) is very similar to the values derived from RR Lyrae stars in the MW and the LMC. The zero point of this relation (MV = 0.51 at [Fe/H] = -1.5) is based on the assumed distance modulus (m - M)0(M31) = 24.47 ± 0.03, and is consistent with the distance scale that places the LMC at (m - M)0(LMC) = 18.55.