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.
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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.
Reference:
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