A Brown Dwarf 0.6 AU from a Red Dwarf Star

OGLE-2014-BLG-0257 is a gravitational microlensing event involving a binary composed of a brown dwarf and a low-mass red dwarf star crossing the line-of-sight to a background star. Analysis of the gravitational microlensing lightcurve indicates that the brown dwarf has 37.7 ± 5.2 times the mass of Jupiter and the low-mass red dwarf star has 0.19 ± 0.02 times the mass of the Sun. Assuming the binary is located 4.08 ± 0.42 light years away, the projected separation between the binary components is 0.61 ± 0.07 AU. If the mass of the low-mass red dwarf star is scaled up to the mass of the Sun, and if the projected separation between the brown dwarf and the low-mass red dwarf star is scaled up by the same factor, then the separation between the binary components becomes ~3.2 AU per solar mass. Such a separation places the brown dwarf in the “brown dwarf desert”. Basically, the “brown dwarf desert” is a region of space within ~5 AU of Sun-like stars where it is exceedingly rare to find brown dwarfs residing there.

Reference:
Han et al. (2016), “OGLE-2014-BLG-0257L: A Microlensing Brown Dwarf Orbiting a Low-mass M Dwarf”, arXiv:1603.04567 [astro-ph.SR]