Low-Mass Stars and Brown Dwarfs in NGC 2024: Constraints on the Substellar Mass Function

We present results from a near-infrared spectroscopic study of candidate brown dwarfs and low-mass stars in the young cluster NGC 2024. Using FLAMINGOS on the KPNO 2.1 m and 4 m telescopes, we have obtained spectra of ~70 new members of the cluster and classified them via the prominent J- and H-band water absorption features. Derived spectral types range from ~M1 to later than M8 with typical classification errors of 0.5-1 subclasses. By combining these spectral types with JHK photometry, we place these objects on the H-R diagram and use pre-main-sequence evolutionary models to infer masses and ages. The mean age for this low-mass population of NGC 2024 is 0.5 Myr, and derived masses range from ~0.7 to 0.02 M☉ with 23 objects falling below the hydrogen-burning limit. The logarithmic mass function rises to a peak at ~0.2 M☉ before turning over and declining into the substellar regime. There is a possible secondary peak at ~0.035 M☉; however, the errors are also consistent with a flat IMF in this region. The ratio of brown dwarfs to stars is similar to that found in the Trapezium but roughly twice the ratio found in IC 348, leading us to conclude that the substellar IMF in young clusters may be dependent on the local star-forming environment.