The Galactic Disk Mass Budget. II. Brown Dwarf Mass Function and Density

In this paper, we extend the calculations conducted previously in the stellarregime to determine the brown dwarf IMF in the Galactic disk. We perform MonteCarlo calculations taking into account the brown dwarf formation rate, spatialdistribution and binary fraction. Comparison with existing surveys seems toexclude a power-law MF as steep as the one determined in the stellar regimebelow 1 $\msol$ and tends to favor a more flatish behaviour. Comparison withmethane-dwarf detections tends to favor an eventually decreasing form like thelognormal or the more general exponential distributions determined in theprevious paper. We calculate predicting brown dwarf counts in near-infraredcolor diagrams and brown dwarf discovery functions. These calculations yieldthe presently most accurate determination of the brown dwarf census in theGalactic disk. The brown dwarf number density is comparable to the stellar one,$n_{BD}\simeq n_\star\simeq 0.1$ pc$^{-3}$. The corresponding brown dwarf massdensity, however, represents only about 10% of the stellar contribution, i.e.$\rho_{BD}\simle 5.0\times 10^{-3} \mvol$. Adding up the local stellar densitydetermined previously yields the density of star-like objects, stars and browndwarfs, in the solar neighborhood $\rho_\odot \approx 5.0\times 10^{-2} \mvol$.Comment: 39 pages, Latex file, uses aasms4.sty, to be published in ApJ, corrected version with correct figure