Formation of Planetary-Mass Objects by Protostellar Collapse and Fragmentation

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Abstract
Searches for very low mass objects in young star clusters have uncovered evidence for free-floating objects with inferred masses possibly as low as 5-15 Jupiter masses (MJup), similar to the masses of several extrasolar planets. We show here that the process that forms single and multiple protostars, namely, collapse and fragmentation of molecular clouds, might be able to produce self-gravitating objects with initial masses less than ~1 MJup, provided that magnetic field tension effects are important and can be represented approximately by diluting the gravitational field. If such fragments can be ejected from an unstable quadruple protostar system, prior to gaining significantly more mass, protostellar collapse might then be able to explain the formation of free-floating objects with masses below 13 MJup. These objects might then be best termed "sub-brown dwarf stars."