New ultra-cool and brown dwarf candidates in Gaia DR2

TL;DR

This paper utilizes Gaia DR2 data to identify and characterize ultra-cool and brown dwarf candidates, significantly expanding the known sample and highlighting the incompleteness of the local low-mass object census.

Contribution

It introduces a new method to select ultra-cool and brown dwarf candidates from Gaia DR2 and reports the discovery of thousands of new candidates, improving the low-mass object catalog.

Findings

Recovered 62% of spectroscopically confirmed objects

Identified ~13,000 M7 or later objects and 679 new L candidates

Indicated the local low-mass object census is still incomplete

Abstract

Context. The second Gaia data release (Gaia DR2) contains high precision positions, parallaxes and proper motions for 1.3 billion sources. The resulting Hertzsprung-Russel diagram reveals fine structures all over the mass range. Aims. This paper aims to investigate the content of Gaia DR2 at the low-mass end and to characterize ultra-cool and brown dwarfs. Methods. We first retrieve the sample of spectroscopically confirmed ultra-cool and brown dwarfs in Gaia DR2. We use their locus in the precise Hertzsprung-Russel diagram to select new candidates, and to investigate their properties. Results. The number of spectroscopically confirmed objects recovered in Gaia DR2 corresponds to 62 %, and 71 %, of the expected number of objects with and estimated Gaia magnitude Gest $\leq$ 21.5, and 20.3, respectively, significantly filling the gap with Gaia DR1. Furthermore Gaia DR2 contains ~13 000 $\geq$M7 and 679 new L candidates. A tentative classification suggests that a few hundreds are young or subdwarf candidates. Their distance distribution shows that the solar neighborhood census is still incomplete. Conclusions. Gaia DR2 offers a great wealth of information on the low-mass objects. It provide a homogeneous and precise catalog of candidates worthwhile to be further characterized with spectroscopic observations.