An Ancient Brown Dwarf Transiting a Metal-Poor Thick Disk Star

Abstract

We report the discovery of TOI-7019b, the first transiting brown dwarf (BD) known to orbit a star that is part of the Milky Way's ancient thick disk, as defined chemically ([Fe/H] $=-0.79\pm 0.05$ dex, [$\hat{I}\pm$/Fe] $=+0.26\pm 0.05$ dex, [M/H] $=-0.59\pm 0.06$ dex) and kinematically ($v_{\perp }\approx 150\pm 1$ km s${}^{-1}$). We estimate a system age $\ddot{I}„=12\pm 2$ Gyr by fitting the host star's spectrum and spectral energy distribution to alpha-enhanced isochrones, and independently using the age-metallicity relation of the thick disk. This makes TOI-7019 by far the most metal-poor and ancient BD host known to date. We measure a BD mass of $61.3\pm 2.1$ $M_{\mathrm{J}}$ and radius of $0.82\pm 0.02$ $R_{\mathrm{J}}$ from a joint analysis of transit photometry and radial velocity measurements, along with an orbital period of $48.2592\pm 0.0001$ days and an orbital eccentricity of $0.403\pm 0.002$. The measured radius appears $12.3\%\pm 2.8\%$ larger than predicted relative to standard evolutionary models for old, metal-poor brown dwarfs, hinting at missing physics like the magnetic inhibition of convection. TOI-7019b lowers the probed metallicity regime for transiting BDs by over a factor of two, making it a benchmark system to test evolutionary models in the low-metallicity regime. Future measurements of TOI-7019b's atmosphere will test whether a brown dwarf's atmospheric composition tracks its host star's abundances, as expected for binary-like co-formation.

Submitted to ApJL