HST / WFPC 2 Color - magnitude Diagrams for Globular Clusters in M

We report new photometry for 10 globular clusters in M31, observed to a uniform depth of four orbits in F5555W(V) and F814W(I) using WFPC2 on board HST. Additionally we have reanalyzed HST archival data of comparable quality, for 2 more clusters. A special feature of our analysis is the extraordinary care taken to account for the effects of blended stellar images and 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. – 2 – We present the color-magnitude diagrams (CMDs) and discuss their main features also in comparison with the properties of the Galactic globular clusters. This analysis is augmented with CMDs previously obtained and discussed by Fusi Pecci et al. (1996) on 8 other M31 clusters. We report the following significant results: 1. The locus of the red giant branches give reliable photometric metallicity deter-minations which compare generally very well with ground-based integrated spectro-scopic 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 2nd-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 the Milky Way 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 Milky Way counterparts by 1-2 Gyr; further study is required. 3. The M V (HB) versus [Fe/H] relationship has been re-determined 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 (M V = 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.