Microlensing Discovery of a Population of Very Tight, Very Low-mass Binary Brown Dwarfs

TL;DR

This paper reports the discovery of the lowest-mass, tightest binary brown dwarf systems via microlensing, providing new insights into brown dwarf formation mechanisms and their binary properties.

Contribution

It presents the first direct measurements of very low-mass, tight binary brown dwarfs, expanding understanding of their formation and properties.

Findings

Discovered binary brown dwarfs with masses around 0.02-0.03 Msun

Measured the tightest separations for field brown dwarf binaries

Indicates brown dwarf binaries can form at very low masses

Abstract

Although many models have been proposed, the physical mechanisms responsible for the formation of low-mass brown dwarfs are poorly understood. The multiplicity properties and minimum mass of the brown-dwarf mass function provide critical empirical diagnostics of these mechanisms. We present the discovery via gravitational microlensing of two very low-mass, very tight binary systems. These binaries have directly and precisely measured total system masses of 0.025 Msun and 0.034 Msun, and projected separations of 0.31 AU and 0.19 AU, making them the lowest-mass and tightest field brown-dwarf binaries known. The discovery of a population of such binaries indicates that brown dwarf binaries can robustly form at least down to masses of ~0.02 Msun. Future microlensing surveys will measure a mass-selected sample of brown-dwarf binary systems, which can then be directly compared to similar samples of stellar binaries.