Primeval very low-mass stars and brown dwarfs - IV. New L subdwarfs, Gaia astrometry, population properties, and a blue brown dwarf binary

TL;DR

This study expands the catalog of L subdwarfs with 27 new discoveries, analyzes their properties using Gaia data, and identifies a rare binary brown dwarf, enhancing understanding of low-mass stellar populations and their boundaries.

Contribution

The paper reports the discovery and classification of new L subdwarfs, analyzes their population properties with Gaia astrometry, and identifies a unique binary brown dwarf system, providing new insights into low-mass stars and substellar objects.

Findings

Approximately one-third of known L subdwarfs are substellar.

Identified new colour spaces for L subdwarf selection.

Discovered a rare binary brown dwarf system.

Abstract

We present 27 new L subdwarfs and classify five of them as esdL and 22 as sdL. Our L subdwarf candidates were selected with the UKIRT Infrared Deep Sky Survey and Sloan Digital Sky Survey. Spectroscopic follow-up was carried out primarily with the OSIRIS spectrograph on the Gran Telescopio Canarias. Some of these new objects were followed up with the X-shooter instrument on the Very Large Telescope. We studied the photometric properties of the population of known L subdwarfs using colour-spectral type diagrams and colour-colour diagrams, by comparison with L dwarfs and main-sequence stars, and identified new colour spaces for L subdwarf selection/study in current and future surveys. We further discussed the brown dwarf transition-zone and the observational stellar/substellar boundary. We found that about one-third of 66 known L subdwarfs are substellar objects, with two-thirds being very low-mass stars. We also present the Hertzsprung-Russell diagrams, spectral type-absolute magnitude corrections, and tangential velocities of 20 known L subdwarfs observed by the Gaia astrometry satellite. One of our L subdwarf candidates, ULAS J233227.03+123452.0, is a mildly metal-poor spectroscopic binary brown dwarf: a ~L6p dwarf and a ~T4p dwarf. This binary is likely a thick disc member according to its kinematics.