Joanna Lisa Levine

A Study of the Luminosity and Mass Functions of the Young IC 348 Cluster Using FLAMINGOS Wide-Field Near-Infrared Images

We present wide-field near-infrared (JHK) images of the young, τ = 2 Myr IC 348 cluster taken with FLAMINGOS. We use these new data to construct an infrared census of sources, which is sensitive enough to detect a 10 Jup brown dwarf seen through an extinction of AV ~ 7. We examine the clusters structure and relationship to the molecular cloud and to construct the clusters K-band luminosity function. Using our model luminosity function algorithm, we derive the clusters initial mass function (IMF) throughout the stellar and substellar regimes and find that the IC 348 IMF is very similar to that found for the Trapezium cluster, with both cluster IMFs having a mode between 0.2–0.08 ⊙. In particular, we find that, similar to our results for the Trapezium, brown dwarfs constitute only one in four of the sources in the IC 348 cluster. We show that a modest secondary peak forms in the substellar IC 348 K-band luminosity function (KLF), corresponding to the same mass range responsible for a similar KLF peak found in the Trapezium. We interpret this KLF peak as either evidence for a corresponding secondary IMF peak at the deuterium burning limit or as arising from a feature in the substellar mass-luminosity relation that is not predicted by current theoretical models. Finally, we find that IC 348 displays radial variations of its subsolar (0.5–0.08 ⊙) IMF on a parsec scale. Whatever mechanism that is breaking the universality of the IMF on small spatial scales in IC 348 does not appear to be acting on the brown dwarf population, whose relative size does not vary with distance from the cluster center.

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.