THE EXTREME MICROLENSING EVENT OGLE-2007-BLG-224: TERRESTRIAL PARALLAX OBSERVATION OF A THICK-DISK BROWN DWARF

Abstract

Parallax is the most fundamental technique to measure distances to\nastronomical objects. Although terrestrial parallax was pioneered over 2000\nyears ago by Hipparchus (ca. 140 BCE) to measure the distance to the Moon, the\nbaseline of the Earth is so small that terrestrial parallax can generally only\nbe applied to objects in the Solar System. However, there exists a class of\nextreme gravitational microlensing events in which the effects of terrestrial\nparallax can be readily detected and so permit the measurement of the distance,\nmass, and transverse velocity of the lens. Here we report observations of the\nfirst such extreme microlensing event OGLE-2007-BLG-224, from which we infer\nthat the lens is a brown dwarf of mass M=0.056 +- 0.004 Msun, with a distance\nof 525 +- 40 pc and a transverse velocity of 113 +- 21 km/s. The velocity\nplaces the lens in the thick disk, making this the lowest-mass thick-disk brown\ndwarf detected so far. Follow-up observations may allow one to observe the\nlight from the brown dwarf itself, thus serving as an important constraint for\nevolutionary models of these objects and potentially opening a new window on\nsub-stellar objects. The low a priori probability of detecting a thick-disk\nbrown dwarf in this event, when combined with additional evidence from other\nobservations, suggests that old substellar objects may be more common than\npreviously assumed.\n