Hubble Space TelescopeWFPC2 Color-Magnitude Diagrams for Globular Clusters in M31

We report new HST/WFPC2 photometry for 10 globular clusters (GC) in M31observed in F5555W(V) and F814W(I). Additionally we have reanalyzed HSTarchival data of comparable quality for 2 more GCs. Extraordinary care is takento account for the effects of blended stellar images and required fieldsubtraction. We thus reach 1 mag fainter than the horizontal branch (HB) evenin unfavorable cases. We present the color-magnitude diagrams (CMDs) anddiscuss their main features also in comparison with the properties of theGalactic GCs. This analysis is augmented with CMDs previously obtained anddiscussed by Fusi Pecci et al. (1996) on 8 other M31 clusters. We report thefollowing significant results: 1. The locus of the red giant branches givereliable metallicity determinations which compare generally very well withground-based integrated spectroscopic and photometric measures, as well asgiving good reddening estimates. 2. The HB morphologies show the same behaviorwith metallicity as the Galactic GCs, with indications that the 2nd-parametereffect can be present in some GCs of our sample. However, at [Fe/H] ~ -1.7 weobserve a number of GCs with red HB morphology such that the HB type versus[Fe/H] relation is offset from the MW and resembles that of the Fornax dwarfspheroidal galaxy. One explanation for the offset is that they are younger thantheir MW counterparts by 1-2 Gyr. 3. The Mv(HB)-[Fe/H] relationship has beendetermined and the slope (~0.20) is very similar to the values derived from RRLyrae stars in the MW and the LMC. The zero-point of this relation based on theassumed distance modulus (m-M)o(M31)=24.47+/-0.03 is consistent with(m-M)o(LMC)=18.55.Comment: 50 pages, 12 figures (one in jpeg format), 8 tables, accepted for publication in The A