The Extreme Microlensing Event OGLE-2007-BLG-224: Terrestrial Parallax
  Observation of a Thick-Disk Brown Dwarf
  Parallax is the most fundamental technique to measure distances to
astronomical objects. Although terrestrial parallax was pioneered over 2000
years ago by Hipparchus (ca. 140 BCE) to measure the distance to the Moon, the
baseline of the Earth is so small that terrestrial parallax can generally only
be applied to objects in the Solar System. However, there exists a class of
extreme gravitational microlensing events in which the effects of terrestrial
parallax can be readily detected and so permit the measurement of the distance,
mass, and transverse velocity of the lens. Here we report observations of the
first such extreme microlensing event OGLE-2007-BLG-224, from which we infer
that the lens is a brown dwarf of mass M=0.056 +- 0.004 Msun, with a distance
of 525 +- 40 pc and a transverse velocity of 113 +- 21 km/s. The velocity
places the lens in the thick disk, making this the lowest-mass thick-disk brown
dwarf detected so far. Follow-up observations may allow one to observe the
light from the brown dwarf itself, thus serving as an important constraint for
evolutionary models of these objects and potentially opening a new window on
sub-stellar objects. The low a priori probability of detecting a thick-disk
brown dwarf in this event, when combined with additional evidence from other
observations, suggests that old substellar objects may be more common than
previously assumed.
