An HST/WFPC2 survey of bright young clusters in M31^*

I. VdB0, a massive star cluster seen at t 25 Myr

¹ INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy e-mail: michele.bellazzini@oabo.inaf.it
² Università di Bologna, Dipartimento di Astronomia, via Ranzani 1, 40127 Bologna, Italy e-mail: sibilla.perina2@unibo.it
³ Department of Physics and Astronomy, University of Western Ontario, London, ON, N6A 3K7, Canada
⁴ UCO/Lick Observatory, University of California, Santa Cruz, CA 95064, USA
⁵ Instituto de Astrofísica de Canarias, La Laguna 38200, Canary Islands, Spain
⁶ Department of Physics and Astronomy, Arizona State University, Tempe, AZ, USA
⁷ Palomar Observatory, Mail Stop 105-24, California Institute of Technology, Pasadena, CA 91125, USA e-mail: jlc@astro.caltech.edu
⁸ Department of Astronomy, University of Washington, Seattle, WA 98195, USA
⁹ Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA
¹⁰ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
¹¹ Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada

Received: 31 July 2008
Accepted: 28 November 2008

Abstract

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 ) 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   , but our best estimates lie in the range   . The CMD of VdB0 is best reproduced by models having solar metallicity and age  25 Myr. Ages less than 12 Myr and greater than 60 Myr are clearly ruled out by the available data. The cluster has a remarkable number of red super giants (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 (1 mag) than Galactic open clusters of similar age. Its present-day mass and half-light radius ( 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.