Primeval very low-mass stars and brown dwarfs – VIII. The first age benchmark L subdwarf, a wide companion to a halo white dwarf

Abstract

We report the discovery of five white dwarf + ultracool dwarf systems identified as common proper motion wide binaries in the Gaia Catalogue of Nearby Stars. The discoveries include a white dwarf + L subdwarf binary, VVV 1256-62AB, a gravitationally bound system located 75.6 ¿pc away with a projected separation of 1375 au. The primary is a cool DC white dwarf with a hydrogen dominated atmosphere, and has a total age of ¿Gyr, based on white dwarf model fitting. The secondary is an L subdwarf with a metallicity of [M/H] = (i.e. [Fe/H] = ¿) and = 2298 ¿K based on atmospheric model fitting of its optical to near infrared spectrum, and likely has a mass just above the stellar/substellar boundary. The subsolar metallicity of the L subdwarf and the system’s total space velocity of 406¿km¿s-1 indicates membership in the Galactic halo, and it has a flat eccentric Galactic orbit passing within 1¿kpc of the centre of the Milky Way every 0.4¿Gyr and extending to 15–31¿kpc at apogal. VVV 1256-62B is the first L subdwarf to have a well-constrained age, making it an ideal benchmark of metal-poor ultracool dwarf atmospheres and evolution.