The "Mysterious" Origin of Brown Dwarfs

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Abstract
Hundreds of brown dwarfs (BDs) have been discovered in the last few years in stellar clusters and among field stars. BDs are almost as numerous as hydrogen burning stars and so any theory of star formation should also explain their origin. The ``mystery'' of the origin of BDs is that their mass is more than 10 times smaller than the average Jeans' mass in star--forming clouds, and yet they are so common. In this paper we propose that BDs are formed in the same way as more massive hydrogen burning stars, that is by the process of turbulent fragmentation. Supersonic turbulence in molecular clouds generates an approximately Log--Normal probability density distribution (PDF) of gas density. A fraction of the cores formed by the turbulent flow with BD mass are dense enough to be gravitationally unstable. Our model for the initial mass function (IMF), based on scaling properties of turbulent fragmentation, predicts a relative abundance of BDs in agreement with the observational data. The BD abundance is found to depend on the rms turbulent velocity of the star--forming gas, with a larger BD abundance in regions of stronger turbulence, as observed. The BD abundance predicted by our analytical model of the IMF is also confirmed by numerical simulations of supersonic turbulence.

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