BANYAN. VI. Discovery of a companion at the brown dwarf/planet-mass limit to a Tucana-Horologium M dwarf

TL;DR

This paper reports the discovery of a young, low-mass substellar companion near the planet/brown dwarf boundary to a Tucana-Horologium M dwarf, enabling detailed study due to its wide separation and favorable observational conditions.

Contribution

The discovery of a new substellar companion at the brown dwarf/planet boundary around a young M dwarf using seeing-limited imaging, expanding the known sample of such objects.

Findings

Companion has a mass of 12-15 MJup, near the planet/brown dwarf boundary.

Discovered at a wide separation of 160 AU, accessible for detailed study.

System occupies a sparsely populated region in host mass versus mass ratio diagram.

Abstract

We report the discovery of a substellar companion to 2MASS J02192210-3925225, a young M6 $\gamma$ candidate member of the Tucana-Horologium association (30 - 40 Myr). This L4 $\gamma$ companion has been discovered with seeing-limited direct imaging observations; at a 4" separation (160AU) and a modest contrast ratio, it joins the very short list of young low-mass companions amenable to study without the aid of adaptive optics, enabling its characterization with a much wider suite of instruments than is possible for companions uncovered by high-contrast imaging surveys. With a model-dependent mass of 12-15MJup, it straddles the boundary between the planet and brown dwarf mass regimes. We present near-infrared spectroscopy of this companion and compare it to various similar objects uncovered in the last few years. The J0219-3925 system falls in a sparsely populated part of the host mass versus mass ratio diagram for binaries; the dearth of known similar companions may be due to observational biases in previous low-mass companion searches.