Discovery of a Wide Binary Brown Dwarf Born in Isolation
  During a survey for stars with disks in the Taurus star-forming region using
the Spitzer Space Telescope, we have discovered a pair of young brown dwarfs,
FU Tau A and B, in the Barnard 215 dark cloud. They have a projected angular
separation of 5.7", corresponding to 800 AU at the distance of Taurus. To
assess the nature of these two objects, we have obtained spectra of them and
have constructed their spectral energy distributions. Both sources are young
(~1 Myr) according to their Halpha emission, gravity-sensitive spectral
features, and mid-IR excess emission. The proper motion of FU Tau A provides
additional evidence of its membership in Taurus. We measure spectral types of
M7.25 and M9.25 for FU Tau A and B, respectively, which correspond to masses of
~0.05 and ~0.015 M\cdot according to the evolutionary models of Chabrier and
Baraffe. FU Tau A is significantly overluminous relative to an isochrone
passing through FU Tau B and relative to other members of Taurus near its
spectral type, which may indicate that it is an unresolved binary. FU Tau A and
B are likely to be components of a binary system based on the low probability
(~3x10^-4) that Taurus would produce two unrelated brown dwarfs with a
projected separation of a </- 6". Barnard 215 contains only one other young
star and is in a remote area of Taurus, making FU Tau A and B the first
spectroscopically-confirmed brown dwarfs discovered forming in isolation rather
than in a stellar cluster or aggregate. Because they were born in isolation and
comprise a weakly bound binary, dynamical interactions with stars could not
have played a role in their formation, and thus are not essential for the birth
of brown dwarfs. ERRATUM: The K-band magnitude for FU Tau B in Table 1 is
incorrect and should be 13.33. The bolometric luminosity of FU Tau B in Table 3
and Figure 5 is incorrect because of that mistake and a separate arithmetic
error. The correct value of the luminosity is 0.0039 Lsun. FU Tau A and B
exhibited different isochronal ages in the original Hertzsprung-Russell diagram
in Figure 5, which was unexpected for members of a binary system. This
discrepancy is reduced in the corrected version of Figure 5 since both objects
are now above the isochrone for 1 Myr. Given the large uncertainties in model
isochrones at such young ages, the positions of FU Tau A and B in Figure 5
could be roughly consistent with coevality.
