OGLE-2014-BLG-0257L: A Microlensing Brown Dwarf Orbiting a Low-mass M Dwarf

TL;DR

This paper reports the discovery of a brown dwarf orbiting a low-mass M dwarf through microlensing, providing precise measurements of the system's properties and contributing to understanding brown dwarf distribution.

Contribution

First microlensing detection of a brown dwarf orbiting an M dwarf with precise mass and distance measurements, expanding knowledge of brown dwarf demographics.

Findings

Brown dwarf mass: 0.036 +/- 0.005 Msun

Distance to system: 1.25 +/- 0.13 kpc

Binary separation: 0.61 +/- 0.07 AU

Abstract

In this paper, we report the discovery of a binary composed of a brown dwarf and a low-mass M dwarf from the observation of the microlensing event OGLE-2014-BLG-0257. Resolution of the very short-lasting caustic crossing combined with the detection of subtle continuous deviation in the lensing light curve induced by the Earth's orbital motion enable us to precisely measure both the Einstein radius \theta_E and the lens parallax pi_E, which are the two quantities needed to unambiguously determine the mass and distance to the lens. It is found that the companion is a substellar brown dwarf with a mass 0.036 +/- 0.005 Msun (37.7 +/- 5.2\ M_J) and it is orbiting an M dwarf with a mass 0.19 +/- 0.02 Msun. The binary is located at a distance 1.25 +/- 0.13 kpc toward the Galactic bulge and the projected separation between the binary components is 0.61 +/- 0.07 AU. The separation scaled by the mass of the host is 3.2 AU/Msun. Under the assumption that separations scale with masses, then, the discovered brown dwarf is located in the zone of the brown dwarf desert. With the increasing sample of brown dwarfs existing in various environments, microlensing will provide a powerful probe of brown dwarfs in the Galaxy.