A Prediction of Brown Dwarfs in Ultracold Molecular Gas

A recent model for the stellar initial mass function (IMF), in which thestellar masses are randomly sampled down to the thermal Jeans mass fromhierarchically structured pre-stellar clouds, predicts that regions ofultra-cold CO gas, such as those recently found in nearby galaxies by Allen andcollaborators, should make an abundance of Brown Dwarfs with relatively fewnormal stars. This result comes from the low value of the thermal Jeans mass,considering that the hierarchical cloud model always gives the Salpeter IMFslope above this lower mass limit. The ultracold CO clouds in the inner disk ofM31 have T~3K and pressures that are probably 10 times higher than in the solarneighborhood. This gives a mass at the peak of the IMF equal to 0.01 Msun, wellbelow the Brown Dwarf limit of 0.08 Msun. Using a functional approximation tothe IMF, the ultracold clouds would have 50% of the star-like mass and 90% ofthe objects below the Brown Dwarf limit. The brightest of the Brown Dwarfs inM31 should have an apparent, extinction-corrected K-band magnitude of ~21 magin their pre-main sequence phase.Comment: 13 pages, 2 figures, to be published in Astrophysical Journal, Vol 522, September 10, 199