T Dwarfs and the Substellar Mass Function. I. Monte Carlo Simulations

Monte Carlo simulations of the field substellar mass function (MF) are presented, based on the latest brown dwarf evolutionary models from Burrows et al. and Baraffe et al. Starting from various representations of the MF below 0.1M and the stellar birthrate, luminosity functions (LFs) and Teff distributions are produced for comparison with observed samples. These distributions exhibit distinct minima in the mid-type L dwarf regime followed by a rise in number density for fainter/cooler brown dwarfs, predicting many more T-type and cooler brown dwarfs in the field even for relatively shallow mass functions. Deuterium-burning brown dwarfs (0:012 M M 0:075 M) dominate field objects with 400 K TeA 2000 K, while nonfusing brown dwarfs make up a substantial pro-portion offield dwarfs with TeA 500 K. The shape of the substellar LF is fairly consistent for various assumptions of the Galactic birthrate, choice of evolutionary model, and adopted age and mass ranges, particularly for field T dwarfs, which as a population provide the best constraints for the field substellar MF. Exceptions include a depletion of objects with 1200 K TeA 2000 K in ‘‘halo’ ’ systems (ages9 Gyr), and a substantial increase in the number of very cool brown dwarfs for lower minimum formation masses. Unresolved multiple systems tend to enhance features in the observed LF and may contribute significantly to the space density of very cool brown dwarfs. However, these effects are small (10 % for TeAk 300 K) for binary fractions typical for brown dwarf