A Survey of M 31 Globular Clusters using WFPC 2 on board HST

We report new, as yet unpublished HST/WFPC2 V, I pho-tometry of 9 globular clusters in M31. These are part of a total sample of 19 clusters on M31 with well observed CMDs from HST. The clusters have the full range of horizontal branch morphologies from blue HB to red clump, but none are found with an extreme (blue only) horizontal branch. Plots of HB type vs [Fe/H] are similar to those of Galactic clusters , including a hint of second parameter clusters. Sixteen halo fields adjacent to the observed clusters included in our images have been analysed. The M31 halo metallicity distribution peaks at [Fe/H]=–0.7 with a tail toward low metallicity, resembling the halo of NGC 5128; metal rich giants appear to be centrally concentrated. 1. The globular clusters At present, 22 globular clusters have been observed with FOC or WFPC2 on board HST. We report here on the V (F555W) and I (F814W) yet unpublished photometry obtained by Rich and collaborators on the 9 globular clusters most recently observed (GO6671, PI: R.M. Rich). The aim of the project is to compare the stellar populations of the M31 and Milky Way cluster systems, and to measure M HB V vs [Fe/H] for a sample of clusters at the same distance. The data was processed using the HST pipeline, and reduced using the ROMAFOT package which is optimized for accurate photometry in crowded fields. With the exclusion of 3 clusters from the total sample of 22, because they are concentrated in the innermost region of the galaxy and their photometry does not allow the definition of good CMDs, there are now 19 CMDs of sufficiently good quality to afford some general analysis of the characteristics of the globular cluster system in M31: