British astronomers reporting in the journal Monthly Notices of the Royal Astronomical Society (arXiv.org) have found two of the oldest brown dwarfs in our Galaxy.
Brown dwarfs are star-like objects but are much less massive, and do not generate internal heat through nuclear fusion like stars. Because of this brown dwarfs simply cool and fade with time and very old brown dwarfs become very cool indeed.
The two new brown dwarfs, labeled WISE 0013+0634 and WISE 0833+0052, were identified in the survey made by NASA’s Wide-field Infrared Survey Explorer.
WISE 0013+0634 and WISE 0833+0052 lie in the Pisces and Hydra constellations respectively. They are moving at speeds of 100-200 km per second, much faster than normal stars and other brown dwarfs and are thought to have formed when our Galaxy was very young, more than 10 billion years ago.
The astronomers studied the infrared light emitted from these objects, which are unusual compared to typical slower moving brown dwarfs. The spectral signatures of their light reflects their ancient atmospheres, which are almost entirely made up of hydrogen rather than having the more abundant heavier elements seen in younger stars.
“Unlike in other walks of life, the Galaxy’s oldest members move much faster than its younger population,” said lead author Dr David Pinfield from the University of Hertfordshire.
Stars near to the Sun are made up of three overlapping populations – the thin disk, the thick disk and the halo. The thick disk is much older than the thin disk, and its stars move up and down at a higher velocity. Both these disk components sit within the halo that contains the remnants of the first stars that formed in the Galaxy.
Thin disk objects dominate the local volume, with thick disk and halo objects being much rarer. About 97 percent of local stars are thin disk members, while just 3 percent are from the thick-disk or halo.
Brown dwarfs population numbers probably follow those of stars, which explains why these fast-moving thick-disk/halo objects are only now being discovered.
There are thought to be as many as 70 billion brown dwarfs in the Milky Way’s thin disk, and the thick disk and halo occupy much larger Galactic volumes. So even a small local population signifies a huge number of ancient brown dwarfs in our Galaxy.
“These two brown dwarfs may be the tip of an iceberg and are an intriguing piece of astronomical archaeology”, Dr Pinfield concluded.
Bibliographic information: Pinfield DJ 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. Monthly Notices of the Royal Astronomical Society, accepted for publication; arXiv: 1308.0495