Looking deep into the Rosette Nebula's heart : the (sub)stellar content of the massive young cluster NGC 2244

Abstract

As part of the ongoing effort to characterize the low-mass (sub)stellar population in a sample of massive young clusters, we have targeted the ~2 Myr old cluster NGC 2244. The distance to NGC 2244 from Gaia DR2 parallaxes is 1.59 kpc, with errors of 1% (statistical) and 11% (systematic). We used the Flamingos-2 near-infrared camera at the Gemini-South telescope for deep multi-band imaging of the central portion of the cluster (~2.4 pc2). We determined membership in a statistical manner, through a comparison of the cluster's color–magnitude diagram to that of a control field. Masses and extinctions of the candidate members are then calculated with the help of evolutionary models, leading to the first initial mass function (IMF) of the cluster extending into the substellar regime, with the 90% completeness limit around 0.02 M ⊙. The IMF is well represented by a broken power law (dN/dM ∝ M −α ) with a break at ~0.4 M ⊙. The slope on the high-mass side (0.4–7 M ⊙) is α = 2.12 ± 0.08, close to the standard Salpeter slope. In the low-mass range (0.02–0.4 M ⊙), we find a slope α = 1.03 ± 0.02, which is at the high end of the typical values obtained in nearby star-forming regions (α = 0.5–1.0), but still in agreement within the uncertainties. Our results reveal no clear evidence for variations in the formation efficiency of brown dwarfs (BDs) and very low-mass stars due to the presence of OB stars, or for a change in stellar densities. Our finding rules out photoevaporation and fragmentation of infalling filaments as substantial pathways for BD formation.