The Extreme Microlensing Event OGLE-2007-BLG-224: Terrestrial Parallax Observation of a Thick-Disk Brown Dwarf

Parallax is the most fundamental technique for measuring distances to
astronomical objects. Although terrestrial parallax was pioneered over
2000 years ago by Hipparchus (ca. 140 B.C.E.) 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 M
_sun, with a distance of 525 ± 40 pc and a transverse
velocity of 113 ± 21 km s^–1. 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 substellar 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.