2MASS Brown Dwarfs and a First Estimate of the Substellar Mass Function

Abstract

More than 70 L dwarfs have now been found in followup work on a small portion of the sky surveyed by the Two Micron All Sky Survey. It is estimated that the L dwarfs span a temperature range of 1400 to 2000K. Several have trigonometric parallax determinations, and luminosities between 10^(-3.5) and 10^(-4.3) L⊙. These properties, together with the detection of lithium in one third of the objects, are consistent with the majority having substellar masses. Evolutionary models are utilized to derive estimates of the space density and mass function of substellar objects, predicting a majority that are not detectable with 2MASS. In a given volume the brown dwarfs may outnumber the stars, though contributing only modestly to the local disk mass.