A Prediction of Brown Dwarfs in Ultracold Molecular Gas

A recent model for the stellar initial mass function (IMF), in which the stellar masses are randomly sampled down to the thermal Jeans mass from hierarchically structured pre-stellar clouds, predicts that regions of ultra-cold CO gas, such as those recently found in nearby galaxies by Allen and collaborators, should make an abundance of Brown Dwarfs with relatively few normal stars. This result comes from the low value of the thermal Jeans mass, which scales as MJ ∝ T /P 1/2 for temperature T and pressure P , considering that the hierarchical cloud model always gives the Salpeter IMF slope above this lower mass limit. The ultracold CO clouds in the inner disk of M31 have T ∼ 3K and pressures that are probably 10× higher than in the solar neighborhood. This gives a mass at the peak of the IMF equal to 0.01 M⊙, well below the Brown Dwarf limit of 0.08 M⊙. Using a functional approximation to the IMF given by (