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.
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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.
References:
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]