Friday, August 30, 2013

Uncovering Nearby Brown Dwarfs

The shift in all-sky surveys to longer wavelengths, from the optical, to the near-infrared and mid-infrared, have led to significant progress in the discovery of brown dwarfs with ever cooler temperatures. Brown dwarfs fall in four spectral classes (M, L, T and Y) with spectral class Y being the coolest. Since brown dwarfs cool as they age, the majority of brown dwarfs in the Sun’s neighbourhood with typical ages of several billion years are observed to fall under the T-type and Y-type spectral classes.

Artist’s rendition of a T6.5-type spectral class brown dwarf called 2MASSJ22282889-431026. Credit: NASA/JPL-Caltech

Nevertheless, a complete census of brown dwarfs in the Sun’s neighbourhood is far from complete. Kirkpatrick et al. (2012) found that there are currently about six times more stars than brown dwarfs within 8 pc of the Sun. The known proportion of brown dwarfs to stars is expected to increase with time as new discoveries are catalogued. In fact, the discovery of a pair of brown dwarfs located at just 2 pc from the Sun by Luhman (2013) shows that such an expectation is justifiable.

A recent search by Bihain et al. (2013) using data collected by NASA’s Wide-field Infrared Survey Explorer (WISE) discovered 3 new brown dwarfs in the Sun’s neighbourhood. One brown dwarf in particular, designated as WISE J0521+1025, is a T7.5-type spectral class brown dwarf located at a distance of just 5 pc away. The other 2 brown dwarfs are located slightly beyond 10 pc away. These nearby brown dwarfs were found by looking for high proper motion objects in the WISE dataset. This is because objects nearer to the Sun tend to have higher proper motions, just like how trees along the side of a road appear to move faster than distant mountains.

1. Kirkpatrick et al. (2012), “Further Defining Spectral Type “Y” and Exploring the Low-mass End of the Field Brown Dwarf Mass Function”, arXiv:1205.2122 [astro-ph.SR]
2. Luhman (2013), “Discovery of a Binary Brown Dwarf at 2 Parsecs from the Sun”, arXiv:1303.2401 [astro-ph.GA]
3. Bihain et al. (2013), “An overlooked brown dwarf neighbour (T7.5 at d~5pc) of the Sun and two additional T dwarfs at about 10pc”, arXiv:1307.2722 [astro-ph.SR]