An HST/WFPC2 survey of bright young clusters in M31. IV. Ages and mass
  estimates
  {Aims.} We present the main results of an imaging survey of possible young
massive clusters (YMC) in M31 performed with the Wide Field and Planetary
Camera2 (WFPC2) on the Hubble Space Telescope (HST). We present the images and
color magnitude diagrams (CMDs) of all of our targets. {Methods.} The
reddening, age and, metallicity of the clusters were estimated by comparing the
observed CMDs and luminosity functions with theoretical models. Stellar masses
were estimated by comparison with theoretical models in the log(Age) vs.
absolute integrated magnitude plane. {Results.} Nineteen of the twenty surveyed
candidates were confirmed to be real star clusters. Three of the clusters were
found not to be good YMC candidates from newly available integrated
spectroscopy and were in fact found to be old from their CMD. Of the remaining
sixteen clusters, fourteen have ages between 25 Myr and 280 Myr, two have older
ages than 500 Myr (lower limits). By including ten other YMC with HST
photometry from the literature we have assembled a sample of 25 clusters
younger than 1 Gyr, with mass ranging from 0.6 x 10^4 M_sun to 6 x 10^4 M_sun,
with an average of ~ 3 x 10^4 M_sun. {Conclusions.} The clusters considered
here are confirmed to have masses significantly higher than Galactic open
clusters in the same age range. Our analysis indicates that YMCs are relatively
common in all the largest star-forming galaxies of the Local Group.
