Brown dwarfs are substellar objects that span the gap
between the most massive planets and the least massive stars. These objects are
believed to form in the same way stars do, and in their infancy, can possess disks
of material from which planets might form. Hannah Broekhoven-Fiene et al.
(2014) report on the discovery of a circumsubstellar disk of material around a
young brown dwarf identified as KPNO Tau 3. The discovery was based on submillimeter
observations of KPNO Tau 3 using the Submillimetre Common-User Bolometer Array
(SCUBA) on the James Clerk Maxwell Telescope (JCMT). Submillimeter astronomy is
a branch of observational astronomy that involves the part of the
electromagnetic spectrum between the far-infrared and microwave wavebands.
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Artist’s impression of how a planetary system around a brown
dwarf might look like. Image credit: Drew Taylor.
KPNO Tau 3 is situated in the relatively nearby Taurus
star-forming region, ~450 light-years away. The circumsubstellar disk detected
around KPNO Tau 3 is estimated to contain ~130 Earth-masses worth of material, assuming
a gas to dust ratio of 100:1. A planetary system consisting of a few sub-Earth-mass
or Earth-mass planets might eventually coalesce out from this circumsubstellar
disk. Furthermore, the detection of cold, ~20 K dust grains implies that a
significant fraction of dust in the circumsubstellar disk is at a large enough distance
from KPNO Tau 3 where the radiated energy from the young brown dwarf is too
feeble to have sufficiently warmth the dust grains by much.
The presence of cold dust in the circumsubstellar disk
around KPNO Tau 3 is consistent with the belief that brown dwarfs, at least
some fraction of them, form in the same manner as low-mass stars. An alternate
brown dwarf formation mechanism involves the ejection of a stellar embryo from
its place of birth. The ejection process ‘starves’ the stellar embryo such that
is no long able to accrete enough matter to form a full-fledge star and instead
settles as a brown dwarf. A formation scenario like this would truncated the
brown dwarf’s circumsubstellar disk and result in the absence of cold dust grains
far from the brown dwarf.
Reference:
Hannah Broekhoven-Fiene et al. (2014), “The Disk around the
Brown Dwarf KPNO Tau 3”, arXiv:1407.0700 [astro-ph.SR]