Brown Dwarfs and the Low-Mass Initial Mass Function in Young Clusters

Summary

We have conducted an extensive program of optical and IR imaging and spectroscopy targeted at the low-mass populations of nearby (≤ 300 pc) young (∼ 1-10 Myr old) clusters: L1495E, IC 348, and ρ Oph. By combining the spectroscopic data with IR luminosity function modeling, we arrive at mass functions which are roughly flat or slowly declining in logarithmic mass units below ∼0.4 M_⊙ into the substellar regime. With the discovery of several likely brown dwarfs, we demonstrate the potential of young clusters in studying the formation and mass functions of substellar objects.

Introduction

Young, nearby (< 500 pc) clusters offer unique advantages in the search for brown dwarfs and the study of the low-mass initial mass function (IMF). Young (< 10 Myr) low-mass stars and brown dwarfs are quite luminous relative to evolved (> 1 Gyr) objects found in the field. Because young clusters often occupy small regions on the sky (D ∼ 10′), many low-mass candidates can be identified in only a limited amount of imaging. In addition, the mass function can be studied in the context of a compact, well-defined region of star formation where the stars have a common history and origin. Compared to open cluster studies, contamination by background stars is reduced significantly by extinction of the natal molecular cloud and the compact nature of the cluster. These factors also facilitate completeness estimates, which can be highly problematic in studies of low-mass objects in the field.