Two serendipitous low-mass LMC clusters discovered with HST

Abstract

We present V and I photometry of two open clusters in the LMC down to V ~ 26. The clusters were imaged with the Wide Field and Planetary Camera - 2 on board of the Hubble Space Telescope, as part of the Medium Deep Survey Key-Project. Both are low luminosity (Mv ~ -3.5), low mass systems (M ~ 10^3 Msolar). The chance discovery of these two clusters in two parallel WFPC2 fields suggests a significant incompleteness in the LMC cluster census near the bar. One of the clusters is roughly elliptical and compact, with a steep light profile, a central surface brightness Mu_v(0) 20.2 mag/arcsec2, half-light radius Rhl ~ 0.9 pc (total visual major diameter D ~ 3 pc) and an estimated mass M ~ 1500 Msolar. From the colour-magnitude diagram and isochrone fits we estimate its age as t ~ 2-5 10^8 years. Its mass function has a fitted slope of Gamma = \Delta log \phi (M) / \Delta log M = -1.8 +/- 0.7 in the range probed (0.9 < M/Msolar < 4.5). The other cluster is more irregular and sparser, having shallower density and surface brightness profiles. We obtain Gamma = -1.2 +/- 0.4, and estimate its mass as M ~ 400 Msolar. A derived upper limit for its age is t < 5 10^8 years. Both clusters have mass functions with slopes similar to that of R136, a massive LMC cluster, for which HST results indicate Gamma ~ -1.2. They also seem to be relaxed in their cores and well contained in their tidal radii.