Mass ratio of the 2 pc binary brown dwarf LUH16 and limits on planetary companions from astrometry

TL;DR

This study precisely measures the mass ratio of the nearest binary brown dwarf LUH16 and sets upper limits on the presence of close-in giant planetary companions using high-precision astrometry.

Contribution

First direct measurement of LUH16's mass ratio and constraints on planetary companions from astrometric data.

Findings

Mass ratio of LUH16 is 0.78±0.10.

Parallax updated to 500.51±0.11 mas, placing it just within 2 parsecs.

No planets larger than 2 M_Jup with 20-300 day periods detected.

Abstract

We analyse FORS2/VLT $I$-band imaging data to monitor the motions of both components in the nearest known binary brown dwarf WISE J104915.57-531906.1AB (LUH16) over one year. The astrometry is dominated by parallax and proper motion, but with a precision of $\sim$0.2 milli-arcsecond per epoch we accurately measure the relative position change caused by the orbital motion of the pair. This allows us to directly measure a mass ratio of $q=0.78\pm0.10$ for this system. We also search for the signature of a planetary-mass companion around either of the A and B component and exclude at 3-$\sigma$ the presence of planets with masses larger than $2\,M_\mathrm{Jup}$ and orbital periods of 20--300 d. We update the parallax of LUH16 to $500.51\pm0.11$ mas, i.e. just within 2 pc. This study yields the first direct constraint on the mass ratio of LUH16 and shows that the system does not harbour any close-in giant planets.