How many brown dwarfs should we expect in the Milky Way? I can recall estimates that there could be as many brown dwarfs as main sequence stars back when people started speculating about this, but we have to go by the data, and what we have so far tells another tale. The WISE (Wide-field Infrared Survey Explorer) mission can only come up with one brown dwarf for every six stars, leading Davy Kirkpatrick (Caltech), who is part of the WISE science team, to say “Now that we’re finally seeing the solar neighborhood with keener, infrared vision, the little guys aren’t as prevalent as we once thought” (see Brown Dwarfs Sparser than Expected).
Image: Brown dwarfs in relation to the Sun and planets. Credit: NASA/WISE mission.
This is true, at least, in the Sun’s vicinity, where WISE identifies about 200 brown dwarfs, with 33 measured within 26 light years. In the latter volume, some 211 other stars can be found. If we extrapolated this to the entire galaxy, we would get about 33 billion brown dwarfs, assuming a galactic population of 200 billion stars, but extrapolating from our own backyard may be highly unreliable. We just need to keep accumulating data to get an accurate read on these ‘failed stars,’ which are too cool to ignite hydrogen burning in their cores. Extremely low temperature Y-dwarfs are hard to spot, and it’s possible a few more will be teased out in WISE data.
I try to keep up with brown dwarf studies in the probably vain hope that we might find a Y-dwarf close enough to serve as a target for a future probe — by ‘close,’ I mean still undetected and within a few light years, though the hopes for such a find seem remote. Meanwhile, new work out of the University of Hertfordshire has uncovered two of the oldest brown dwarfs yet observed, thought to go back to the early days of the galaxy some ten billion years ago. Old brown dwarfs really up the ante for detection, for since they cannot ignite internal fusion, they fade with time. The new brown dwarfs have temperatures of 250-600 degrees Celsius. You can contrast that with the surface temperature of the Sun, about 5500 degrees Celsius.
A team working under David Pinfield found the objects in the Pisces and Hydra constellations using WISE data, with additional measurements from the Magellan, Gemini, VISTA and UKIRT instruments on the ground. WISE 0013+0634 and WISE 0833+0052 are moving at speeds of between 100 and 200 kilometers per second, a good deal faster than normal stars, and a marker for their age, which is also flagged by ancient atmospheres made up almost entirely of hydrogen.
This Royal Astronomical Society news release goes on to speculate about the implications of the new work for brown dwarf proliferation. Almost all local stars — about 97 percent — are members of the galactic ‘thin disk,’ a grouping much younger than the ‘thick disk’ in which stars move up and down in relation to galactic center at higher velocities. With only 3 percent of stars in our local volume being from the ‘thick disk’ or the ‘halo’ containing remnants of the earliest stars, it’s no surprise that these are the first brown dwarfs we’ve found from that population.
Image: A brown dwarf from the thick-disk or halo is shown. Although astronomers observe these objects as they pass near to the solar system, they spend much of their time away from the busiest part of the Galaxy. The Milky Way’s disk can be seen in the background. Credit: John Pinfield.
Given that the thick disk and halo occupy much larger volumes than the thin disk, finding a small number of brown dwarfs in the local thick disk/halo population implies a high number of brown dwarfs in the galaxy. Says Pinfield: “These two brown dwarfs may be the tip of an iceberg and are an intriguing piece of astronomical archaeology.” True enough, but we’re still working the numbers as we try to find these faint objects against a background of infrared sources ranging from distant galaxies to clouds of gas and dust. More data, and more insight, lie ahead.
The paper is Pinfield et al., “A deep WISE search for very late type objects and the discovery of two halo/thick-disk T dwarfs: WISE 0013+0634 and WISE 0833+0052,” in press at Monthly Notices of the Royal Astronomical Society (abstract / preprint).