Spectroscopic substellar initial mass function of NGC 2244

Abstract

We aim at characterizing the low-mass (sub)stellar population of the central portion (2.4 pc${}^2$) of the $\sim$2 Myr old cluster NGC 2244 using near infrared spectroscopy. By studying this cluster, characterized by a low stellar density and numerous OB stars, we aim at exploring the effect that OB stars may have on the production of BDs. We obtain near infrared HK spectroscopy of 85 faint candidate members of NGC 2244. We derive the spectral type and extinction by comparison with spectral templates. We evaluate cluster membership using three gravity-sensitive spectral indices based on the shape of the $H$-band. Furthermore, we evaluate the infrared excess from Spitzer of all the candidate members of the cluster. Finally, we estimate the mass of all the candidate members of the cluster and derive the initial mass function, star-to-BD number ratio and disk fraction. The initial mass function is well represented by a power law ($dN/dM\propto M^{-\alpha }$) below 0.4 $M_{\odot }$, with a slope $\alpha$ = 0.7-1.1 depending on the fitted mass range. We calculate a star-to-BD number ratio of 2.2-2.8. We find the low-mass population of NGC 2244 to be consistent with nearby star-forming regions, although it is at the high-end of BD production. We find BDs in NGC 2244 to be on average closer to OB stars than to low-mass stars, which could potentially be the first evidence of OB stars affecting the formation of BDs. We find a disk fraction of all the members with spectral type later than K0 of 39$\pm$9% which is lower than typical values found in nearby star-forming regions of similar ages.

Accepted by A&A, KMOS spectroscopic data will be made public on Vizier upon publication

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