Self-gravitating Magnetically Supported Protostellar Disks and the Formation of Substellar Companions

Isolated low-mass stars are formed, in the standard picture, from thecollapse of dense cores condensed out of strongly magnetized molecular clouds.The dynamically collapsing inflow traps nearly half of the critical magneticflux needed for the core support and deposits it in a small region surroundingthe protostar. It has been argued previously that the deposited flux can slowdown the inflow, allowing matter to pile up and settle along field lines into amagnetically supported, circumstellar disk. Here we show that the disktypically contains $\sim 10%$ of the stellar mass, and that it could becomeself-gravitating under plausible conditions during the rapidly accreting,``Class 0'' phase of star formation. Subsequent fragmentation of theself-gravitating, magnetically subcritical disk, driven by magnetic diffusion,could produce fragments of substellar masses, which collapse to form browndwarfs and possibly massive planets. This scenario predicts substellar objectformation at distances of order 100 AU from the central star, although orbitalevolution is possible after formation. It may provide an explanation for thesmall, but growing, number of brown dwarf companions found around nearby starsby direct imaging. The relatively large formation distances make the substellarcompanions vulnerable to dynamic ejection, particularly in binary (multiple)systems and dense clusters. Those ejected may account for, at least in part,the isolated brown dwarfs and perhaps free-floating planetary mass objects.Comment: 10 pages, no figures, accepted to ApJ Letter