Testing Models of Stellar Evolution for Metal-poor Stars: An Interferometric-spectroscopic Orbit for the Binary HD 195987

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Abstract
We report spectroscopic and interferometric observations of the moderately metal-poor double-lined binary system HD 195987, with an orbital period of 57.3 days. By combining our radial-velocity and visibility measurements, we determine the orbital elements and derive absolute masses for the components of MA = 0.844 ± 0.018 M⊙ and MB = 0.6650 ± 0.0079 M⊙, with relative errors of 2% and 1%, respectively. We also determine the orbital parallax, πorb = 46.08 ± 0.27 mas, corresponding to a distance of 21.70 ± 0.13 pc. The parallax and the measured brightness difference between the stars in V, H, and K yield the component absolute magnitudes in those bands. We also estimate the effective temperatures of the stars as T = 5200 ± 100 K and T = 4200 ± 200 K. Together with detailed chemical abundance analyses from the literature giving [Fe/H] ≈ -0.5 (corrected for binarity) and [α/Fe] = +0.36, we use these physical properties to test current models of stellar evolution for metal-poor stars. Among the four that we considered, we find that no single model fits all observed properties at the measured composition, although we identify the assumptions in each one that account for the discrepancy, and we conclude that a model with the proper combination of assumptions should be able to reproduce all the radiative properties. The indications from the isochrone fits and the pattern of enhancement of the metals in HD 195987 are consistent with this being a thick disk object, with an age of 10–12 Gyr.