Two serendipitous low-mass LMC clusters discovered with HST

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 (M_v ∼ - 3.5), low-mass (M ∼ 10³ M_⊙) 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 μ_v(0)∼20.2 mag arcsec^-2, a half-light radius r_hl∼0.9 pc (total visual major diameter D ∼ 3 pc) and an estimated mass M ∼ 1500 M_⊙. From the colour-magnitude diagram and isochrone fits we estimate its age as τ ∼(2-5) × 10⁸ yr. Its mass function has a fitted slope of Γ = Δ log φ(M )/ΔlogM= - 1.8±0.7 in the range probed (0.9≲M/M_⊙≲4.5). The other cluster is more irregular and sparse, having shallower density and surface brightness profiles. We obtain Γ= -1.2±0.4, and estimate its mass as M∼400 M_⊙. A derived upper limit for its age is τ≲5 × 10⁸ yr. Both clusters have mass functions with slopes similar to that of R136, a massive LMC cluster, for which HST results indicate Γ ∼ - 1.2. They also seem to be relaxed in their cores and well contained in their tidal radii.