Thursday, January 9, 2014

Birth of a Brown Dwarf

Brown dwarfs are sub-stellar objects that are not massive enough to fuse hydrogen in their interiors and shine as full-fledged stars. Nevertheless, brown dwarfs are thought to form in the same way as stars do - from collapsing clouds of gas and dust. A study by Lee et al. (2013) of an isolated dense molecular cloud core, L328, shows that it contains three sub-cores. One of which, identified as L328-IRS, is a Very Low Luminosity Object (VeLLO) that is believed to be in the process of collapsing to form a brown dwarf.

Artist’s impression of a young brown dwarf that is in the process of accreting matter. A pair of bipolar jets can be seen stemming from it. Credit: ESO.

Observations of carbon monoxide as a tracer for the motion of matter reveal a bipolar outflow stemming from L328-IRS. By analysing the outflow, the accretion rate of the proto-brown dwarf is found to be an order of magnitude less than the accretion rate for standard star formation, consistant with the formation of a brown dwarf. Based on the accretion rate, L328-IRS is expected to grow to no more than ~0.05 solar mass. However, the accretion rate may be uncertain due to several unknown factors of the outflow itself.

Nonetheless, L328-IRS has a small total envelop mass of ~0.09 solar mass and ~100 percent star formation efficiency is also unlikely. As a result, L328-IRS is expected to be a proto-brown dwarf since it is unlikely to accrete more than ~0.08 solar mass, which is the minimum mass necessary to become a full-fledged star. The three sub-cores in L328 are though to have formed concurrently in a gravitational fragmentation process. In one of the sub-cores, global contraction of the gaseous envelop is underway to form the proto-brown dwarf L328-IRS. All these indicate that the formation of L328-IRS is consistant with the idea that brown dwarfs form like normal stars.

Chang Won Lee et al., “Early Star-forming Processes in Dense Molecular Cloud L328; Identification of L328-IRS as a Proto-brown Dwarf”, ApJ, 777:50 (15pp), 2013 November 1