2009MNRAS.397.1915B

We investigate the nature of four young and low-mass open clusters (OCs) located in the second and third quadrants with near-infrared Two-Micron All Sky Survey (2MASS) photometry (errors <0.1mag). After field decontamination, the colour-magnitude diagrams (CMDs) display similar morphologies: a poorly populated main sequence (MS) and a dominant fraction of pre-MS (PMS) stars somewhat affected by differential reddening. Pismis5, vdB80 and BDSB96 have MS ages within 5±4Myr, while the MS of NGC1931 is 10±3Myr old. However, non-instantaneous star formation is implied by the wider (∼20Myr) PMS age spread. The cluster masses derived from MS+PMS stars are low, within ∼60-180M_☉, with mass functions (MFs) significantly flatter than Salpeter's initial mass function (IMF). Distances from the Sun are within 1.0-2.4kpc, and the visual absorptions are in the range A_V= 1.0-2.0. From the stellar radial density profiles (RDPs), we find that they are small (R_c≲ 0.48pc, R_RDP≲ 5.8pc), especially Pismis5 with R_c~ 0.2pc and R_RDP~ 1.8pc. Except for the irregular and cuspy inner regions of NGC1931 and Pismis5, the stellar RDPs follow a King-like profile. At ∼10Myr, central cusps - which in old clusters appear to be related to advanced dynamical evolution - are probably associated with a star formation and/or molecular cloud fragmentation effect. Despite the flat MFs, vdB80 and BDSB96 appear to be typical young, low-mass OCs. NGC1931 and especially Pismis5, with irregular RDPs, low cluster mass and flat MFs, do not appear to be in dynamical equilibrium. Both may be evolving into OB associations and/or doomed to dissolution in a few 10⁷yr.