An HST/WFPC2 survey of bright young clusters in M31

Aims. We introduce our imaging survey of possible young massive globular clusters in M31 performed with the Wide Field and Planetary Camera 2 (WFPC2) on the Hubble Space Telescope (HST). We obtained shallow (to $B\sim 25$) photometry of individual stars in 20 candidate clusters. We present here details of the data reduction pipeline that is being applied to all the survey data and describe its application to the brightest among our targets, van den Bergh 0 (VdB0), taken as a test case. Methods. Point spread function fitting photometry of individual stars was obtained for all the WFPC2 images of VdB0 and the completeness of the final samples was estimated using an extensive set of artificial stars experiments. The reddening, the age and the metallicity of the cluster were estimated by comparing the observed color magnitude diagram (CMD) with theoretical isochrones. Structural parameters were obtained from model-fitting to the intensity profiles measured within circular apertures on the WFPC2 images. Results. Under the most conservative assumptions, the stellar mass of VdB0 is $M> 2.4$ $\times$ $10^4~M_{\odot}$, but our best estimates lie in the range $\simeq$$4{-}9$ $\times$ $10^4~M_{\odot}$. The CMD of VdB0 is best reproduced by models having solar metallicity and age $\simeq$ 25 Myr. Ages less than $\simeq$12 Myr and greater than $\simeq$60 Myr are clearly ruled out by the available data. The cluster has a remarkable number of red super giants ($\ga$18) and a CMD very similar to Large Magellanic Cloud clusters usually classified as young globulars such as NGC 1850, for example. Conclusions. VdB0 is significantly brighter ($\ga$1 mag) than Galactic open clusters of similar age. Its present-day mass and half-light radius ($r_{\rm h}=7.4$ pc) are more typical of faint globular clusters than of open clusters. However, given its position within the disk of M31, it is expected to be destroyed by dynamical effects, in particular by encounters with giant molecular clouds, within the next ~4 Gyr