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

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 ⁺ ₋₁ ¹ ₈ ⁹ pc away with a projected separation of 1375 ⁺ ₋₃₃ ³⁵ au. The primary is a cool DC white dwarf with a hydrogen dominated atmosphere, and has a total age of 10.5 ⁺ ₋₂ ³ ₁ ³ Gyr, based on white dwarf model fitting. The secondary is an L subdwarf with a metallicity of [M/H] = −0.72 ⁺ ₋₀ ⁰ ₁₀ ⁰⁸ (i.e. [Fe/H] = −0.81 ± 0.10) and T _eff = 2298 ⁺ ₋₄₃ ⁴⁵ 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.