Astrometric planet search around southern ultracool dwarfs III. Discovery of a brown dwarf in a 3-year orbit around DE0630-18

TL;DR

This study reports the discovery and detailed orbit characterization of a new ultracool binary system, including a brown dwarf companion, using high-precision astrometry and spectroscopy around nearby ultracool dwarfs.

Contribution

First detailed orbit and physical parameter estimation of a brown dwarf companion around an ultracool dwarf using astrometric and spectroscopic data.

Findings

Discovered a brown dwarf in a 3-year orbit around DE0630-18.

Orbit is nearly edge-on with moderate eccentricity (e=0.23).

System's primary is near the stellar-substellar boundary.

Abstract

Using astrometric measurements obtained with the FORS2/VLT camera, we are searching for low-mass companions around 20 nearby ultracool dwarfs. With a single-measurement precision of 0.1 milli-arcseconds, our survey is sensitive to a wide range of companion masses from planetary companions to binary systems. Here, we report the discovery and orbit characterisation of a new ultracool binary at a distance of 19.5 pc from Earth that is composed of the M8.5-dwarf primary DE0630-18 and a substellar companion. The nearly edge-on orbit is moderately eccentric (e=0.23) with an orbital period of 1120 d, which corresponds to a relative separation in semimajor axis of approximately 1.1 AU. We obtained a high-resolution optical spectrum with UVES/VLT and measured the system's heliocentric radial velocity. The spectrum does not exhibit lithium absorption at 670.8 nm, indicating that the system is not extremely young. A preliminary estimate of the binary's physical parameters tells us that it is composed of a primary at the stellar-substellar limit and a massive brown-dwarf companion. DE0630-18 is a new very low-mass binary system with a well-characterised orbit.