Thursday, July 17, 2014

Circumsubstellar Disk around a Young Brown Dwarf

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.

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.

Hannah Broekhoven-Fiene et al. (2014), “The Disk around the Brown Dwarf KPNO Tau 3”, arXiv:1407.0700 [astro-ph.SR]