Spectroscopic substellar initial mass function of NGC 2244^⋆,⋆⋆

¹ CENTRA, Faculdade de Ciências, Universidade de Lisboa, Ed. C8, Campo Grande, 1749-016 Lisboa, Portugal
e-mail: valmendros@sim.ul.pt
² Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal
³ Laboratoire d’Astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, Allée Geoffroy Saint-Hillaire, 33615 Pessac, France
⁴ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
⁵ Instituto de Física y Astronomía, Universidad de Valparaíso, Valparaíso, Chile
⁶ SUPA, School of Physics & Astronomy, University of St Andrews, North Haugh, St Andrews KY16 9SS, UK
⁷ Centro de Astronomía (CITEVA), Universidad de Antofagasta, Av. Angamos 601, Antofagasta, Chile
⁸ Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁹ Odjel za fiziku, Prirodoslovno-matematički fakultet, University of Split, Rudera Boškovića 33, 21000 Split, Croatia
¹⁰ Department of Astronomy, Cornell University, Ithaca, NY 14853, USA

Received: 24 February 2023
Accepted: 6 May 2023

Abstract

Context. The dominant formation channel of brown dwarfs (BDs) is not well constrained yet and a promising way to discriminate between scenarios is to test the environment-dependent efficiency in forming BDs. So far, the outcome of star formation, studied through the initial mass function, has been found to be very similar in all clusters that have been inspected.

Aims. We aim to characterize the low-mass (sub)stellar population of the central portion (2.4 pc²) of the ∼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 to explore the effect that OB stars may have on the production of BDs.

Methods. We obtained near-infrared HK spectroscopy of 85 faint candidate members of NGC 2244. We derived the spectral type and extinction by comparison with spectral templates. We evaluated cluster membership using three gravity-sensitive spectral indices based on the shape of the H-band. Furthermore, we evaluated the infrared excess from Spitzer of all the candidate members of the cluster. Finally, we estimated the mass of all the candidate members of the cluster and derived the initial mass function, star-to-BD number ratio, and disk fraction.

Results. The initial mass function is well represented by a power law (dN/dM ∝ M^−α) below 0.4 M_⊙, with a slope α = 0.7–1.1 depending on the fitted mass range. We calculated 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 piece of evidence that OB stars affect the formation of BDs. We find a disk fraction of all the members with a spectral type later than K0 of 39 ± 9% which is lower than typical values found in nearby star-forming regions of similar ages.