# Spitzer Microlensing of MOA-2016-BLG-231L : A Counter-Rotating Brown Dwarf Binary in the Galactic Disk

## TL;DR

This paper reports the discovery and analysis of a binary brown dwarf system in the Galactic disk through microlensing observations from ground and Spitzer, highlighting Spitzer's role in characterizing such low-mass binaries.

## Contribution

It presents the fifth binary brown dwarf discovered via microlensing and demonstrates how Spitzer data helps determine the physical properties of short-timescale brown dwarf binaries.

## Key findings

- The binary brown dwarf has masses of approximately 21 and 9 Jupiter masses.
- The system is located about 2.85 kpc in the Galactic disk.
- Spitzer observations are crucial for constraining the properties of short-duration microlensing events.

## Abstract

We analyze the binary microlensing event MOA-2016-BLG-231, which was observed from the ground and from Spitzer. The lens is composed of very low-mass brown dwarfs (BDs) with $M_1 = 21^{+12}_{-5} \ M_J$ and $M_2 = 9^{+5}_{-2}\ M_J$, and it is located in the Galactic disk $D_{\rm L} = 2.85^{+0.88}_{-0.50}\ {\rm kpc}$. This is the fifth binary brown dwarf discovered by microlensing, and the BD binary is moving counter to the orbital motion of disk stars. Constraints on the lens physical properties come from late time, non-caustic-crossing features of the Spitzer light curve. Thus, MOA-2016-BLG-231 shows how Spitzer plays a crucial role in resolving the nature of BDs in binary BD events with short timescale ($\lesssim 10$ days).