HST/ACS colour-magnitude diagrams of M31 globular clusters
  With the aim of increasing the sample of M31 clusters for which a colour
magnitude diagram is available, we searched the HST archive for ACS images
containing objects included in the Revised Bologna Catalogue of M31 globular
clusters. Sixty-three such objects were found. We used the ACS images to
confirm or revise their classification and we obtained useful CMDs for 11 old
globular clusters and 6 luminous young clusters. We obtained simultaneous
estimates of the distance, reddening, and metallicity of old clusters by
comparing their observed field-decontaminated CMDs with a grid of template
clusters of the Milky Way. We estimated the age of the young clusters by
fitting with theoretical isochrones. For the old clusters, we found
metallicities in the range -0.4<=[Fe/H]<=-1.9, that generally agree with
existing spectroscopic extimates. At least four of them display a clear blue
HB, indicating ages >10 Gyr. All six candidate young clusters are found to have
ages <1Gyr. With the present work the total number of M31 GCs with reliable
optical CMD increases from 35 to 44 for the old clusters, and from 7 to 11 for
the young ones. The old clusters show similar characteristics to those of the
MW. We discuss the case of the cluster B407, with a metallicity [Fe/H] ~-0.6
and located at a large projected distance from the centre of M31 and from the
galaxy major axis. Metal-rich globulars at large galactocentric distances are
rare both in M31 and in the MW. B407, in addition, has a velocity in stark
contrast with the rotation pattern shared by the bulk of M31 clusters of
similar metallicity. This, along with other empirical evidence, supports the
hypothesis that the cluster is physically associated with a substructure in the
M31 halo that has been interpreted as the relic of a merging event.
