The Benchmark Ultracool Subdwarf Hd 114762b: A Test of Low-Metallicity Atmospheric and Evolutionary Models

We present a near-infrared spectroscopic study of HD 114762B, the latest-type metal-poor companion discovered to date and the only ultracool subdwarf with a known metallicity, inferred from the primary star to be [Fe/H] = −0.7. We obtained a medium-resolution ( R ∼ 3800) Keck/OSIRIS 1.18–1.40 μm spectrum and a low-resolution ( R ∼ 150) Infrared Telescope Facility/SpeX 0.8–2.4 μm spectrum of HD 114762B to test atmospheric and evolutionary models for the first time in this mass–metallicity regime. HD 114762B exhibits spectral features common to both late-type dwarfs and subdwarfs, and we assign it a spectral type of d/sdM9 ± 1. We use a Monte Carlo technique to fit PHOENIX/ GAIA synthetic spectra to the observations, accounting for the coarsely gridded nature of the models. Fits to the entire OSIRIS J -band and to the metal-sensitive J -band atomic absorption features (Fe i , K i , and Al i lines) yield model parameters that are most consistent with the metallicity of the primary star and the high surface gravity expected of old late-type objects. The effective temperatures and radii inferred from the model atmosphere fitting broadly agree with those predicted by the evolutionary models of Chabrier & Baraffe, and the model color–absolute magnitude relations accurately predict the metallicity of HD 114762B. We conclude that current low-mass, mildly metal-poor atmospheric and evolutionary models are mutually consistent for spectral fits to medium-resolution J -band spectra of HD 114762B, but are inconsistent for fits to low-resolution near-infrared spectra of mild subdwarfs. Finally, we develop a technique for estimating distances to ultracool subdwarfs based on a single near-infrared spectrum. We show that this "spectroscopic parallax" method enables distance estimates accurate to ≲10% of parallactic distances for ultracool subdwarfs near the hydrogen burning minimum mass.