The low-mass content of the massive young star cluster RCW38

Abstract

RCW38 is a deeply embedded young (~1 Myr), massive star cluster located at a distance of 1.7 kpc. Twice as dense as the Orion nebula cluster, orders of magnitude denser than other nearby star-forming regions and rich in massive stars, RCW38 is an ideal place to look for potential differences in brown dwarf formation efficiency as a function of environment. We present deep, high-resolution adaptive optics data of the central ~0.5 × 0.5 pc2 obtained with NACO at the Very Large Telescope. Through comparison with evolutionary models, we determine masses and extinction for ~480 candidate members, and derive the first initial mass function (IMF) of the cluster extending into the substellar regime. Representing the IMF as a set of power laws in the form dN/dMαM-α, we derive the slope α = 1.60 ± 0.13 for the mass range 0.5-20M⊙,which is shallower than the Salpeter slope, but in agreement with results in several other young massive clusters. At the low-mass side, we find α =0.71±0.11 formasses between 0.02 and 0.5M⊙, or α =0.81±0.08 for masses between 0.02 and 1M⊙. Our result is in agreement with the values found in other young star-forming regions, revealing no evidence that a combination of high stellar densities and the presence of numerous massive stars affects the formation efficiency of brown dwarfs and very-low-mass stars.We estimate that the Milky Way galaxy contains between 25 and 100 billion brown dwarfs (with masses > 0.03M⊙).