Probing the age and structure of the nearby very young open clusters NGC 2244 and NGC 2239

Abstract

The very young open cluster (OC) NGC 2244 in the Rosette Nebula was studied with field-star-decontaminated 2MASS photometry, which shows the main-sequence (MS) stars and an abundant pre-MS (PMS) population. Fundamental and structural parameters were derived with colour-magnitude diagrams (CMDs), stellar radial density profiles (RDPs) and mass functions (MFs). Most previous studies centred NGC 2244 close to the bright K0V star 12 Monocerotis, which is not a cluster member. Instead, the near-IR RDP indicates a pronounced core near the O5 star HD 46150. We derive an age within 1--6 Myr, an absorption $\aV=1.7\pm0.2$, a distance from the Sun $\ds=1.6\pm0.2$ kpc ($\approx1.5$ kpc outside the Solar circle), an MF slope $\chi=0.91\pm0.13$ and a total (MS+PMS) stellar mass of $\sim625 \ms$. Its RDP is characterised by the core and cluster radii $\rc\approx5.6\arcmin$ ($\approx2.6$ pc) and $\rl\approx10\arcmin$ ($\approx4.7$ pc), respectively. Departure from dynamical equilibrium is suggested by the abnormally large core radius and the marked central stellar excess. We also investigate the elusive neighbouring OC NGC 2239, which is low-mass ($m_{MS+PMS}\approx301 \ms$), young ($5\pm4$ Myr) rather absorbed ($\aV=3.4\pm0.2$), and located in the background of NGC 2244 at $\ds=3.9\pm0.4$ kpc. Its RDP follows a King-like function of $\rc\approx0.5\arcmin\approx0.5$ pc and $\rl\approx5.0\arcmin\approx5.6$ pc. The MF slope, $\chi=1.24\pm0.06$, is essentially Salpeter's IMF. NGC 2244 is probably doomed to dissolution in a few $10^7$ yr. Wide-field extractions and field-star decontamination increase the stellar statistics and enhance both CMDs and RDPs, which is essential for faint and bright star clusters.