Two serendipitous low-mass lmc dusters 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 (WFPC2) on board the Hubble Space Telescope (HST), as part of the Medium Deep Survey Key Project. Both are low-luminosity (MV~-3.5), low-mass (M~10³Mо) systems. 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 µV(0)~20.2 mag arcsec-², a half-light radius rhl~0.9 pc (total visual major diameter D~3 pc) and an estimated mass M~1500 Mо. From the colour magnitude diagram and isochrone fits we estimate its age as Τ~(2–5)x10 8 yr. Its mass function has a fitted slope of Г = ∆logØ (M)/ ∆logM =-1.8±0.7 in the range probed (0.9<~M/Mо<~4.5). The other cluster is more irregular and sparse, having shallower density and surface brightness profiles. We obtain Г=-1.2±0.4, and estimate its mass as M~400 Mо. A derived upper limit for its age is t<~5x10 8 yr. Both clusters have mass functions with slopes similar to that of R136, a massive LMC cluster, for which HST results indicate Г~-1.2. They also seem to be relaxed in their cores and well contained in their tidal radii.