An HST/WFPC2 survey of bright young clusters in M 31. IV. Age and mass estimates
Aims. We present the main results of an imaging survey of possible young massive clusters (YMC) in M 31 performed with the Wide Field and Planetary Camera 2 (WFPC2) on the Hubble Space Telescope (HST), with the aim of estimating their age and their mass. We obtained shallow (to B ~ 25) photometry of individual stars in 19 clusters (of the 20 targets of the survey). We present the images and color magnitude diagrams (CMDs) of all of our targets. Methods. Point spread function fitting photometry of individual stars was obtained for all the WFPC2 images of the target clusters, and the completeness of the final samples was estimated using extensive sets of artificial stars experiments. The reddening, age, and metallicity of the clusters were estimated by comparing the observed CMDs and luminosity functions (LFs) with theoretical models. Stellar masses were estimated by comparison with theoretical models in the (Age) vs. absolute integrated magnitude plane, using ages estimated from our CMDs and integrated J, H, K magnitudes from 2MASS-6X. Results. Nineteen of the twenty surveyed candidates were confirmed to be real star clusters, while one turned out to be a bright star. 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 assembled a sample of 25 clusters younger than 1 Gyr, with mass ranging from 0.6 × 10^4 M_☉ to 6 × 10^4 M_☉, with an average of ~3 × 10^4 M_☉. Our estimates of ages and masses well agree with recent independent studies based on integrated spectra. Conclusions. The clusters considered here are confirmed to have masses significantly higher than Galactic open clusters (OC) in the same age range. Our analysis indicates that YMCs are relatively common in all the largest star-forming galaxies of the Local Group, while the lack of known YMC older than 20 Myr in the Milky Way may stem from selection effects.
Additional Information© 2010 ESO. Received: 13 October 2009; accepted: 5 November 2009. We are grateful to an anonymous referee for a constructive report and for useful suggestions that improved the quality of this paper. S.P. and M.B. acknowledge the financial support of INAF through the PRIN 2007 grant CRA 1.06.10.04 "The local route to galaxy formation...". P.B. acknowledges research support through a Discovery Grant from the Natural Sciences and Engineering Research Council of Canada. J.G.C. is grateful for partial support through grant HST-GO-10818.01-A from the STcI. T.H.P. gratefully acknowledges support in form of a Plaskett Fellowship at the Herzberg institute of Astrophysics in Victoria, BC. J.S. was supported by NASA through an Hubble Fellowship, administered by STScI. We are grateful to S. van den Bergh for having pointed out some errors in the historical reconstruction of the discovery of VdB0 that were reported in a previous version of the paper. We are grateful to M. Gieles, V.D. Ivanov, N. Caldwell, and, in particular, to M. Messineo for useful discussions and suggestions.
Published - Perina2010p7478Astron_Astrophys.pdf
Accepted Version - 0911.3166.pdf