HAT-TR-318-007: A Double-lined M Dwarf Binary with Total Secondary Eclipses Discovered by HATNet and Observed by K2

Abstract

We report the discovery by the HATNet survey of HAT-TR-318-007, a P =  3.34395390 0.00000020 day period detached double-lined M dwarf binary with total secondary eclipses. We combine radial velocity (RV) measurements from TRES/FLWO 1.5 m and time-series photometry from HATNet, FLWO 1.2 m, BOS 0.8 m, and NASA K2 Campaign 5, to determine the masses and radii of the component stars: MA 0.448 0.011N =  , MB 0.2721 0.0042 = - +0.0041 N , RA 0.4548 0.0036 = - +0.0035 N , and RB 0.2913 0.0024 = - +0.0023 N . We obtained a FIRE/Magellan near-infrared spectrum of the primary star during a total secondary eclipse, and we use this to obtain disentangled spectra of both components. We determine spectral types of ST M3.71 0.69 A =  and ST M5.01 0.73 B =  and effective temperatures of Teff,A =  3190 110 K and Teff,B =  3100 110 K for the primary and secondary star, respectively. We also measure a metallicity of [Fe/H]=+  0.298 0.080 for the system. We find that the system has a small, but significant, nonzero eccentricity of 0.0136 0.0026  . The K2 light curve shows a coherent variation at a period of 3.41315 0.00032 0.00030 - + days, which is slightly longer than the orbital period, and which we demonstrate comes from the primary star. We interpret this as the rotation period of the primary. We perform a quantitative comparison between the Dartmouth stellar evolution models and the seven systems, including HATTR-318-007, that contain M dwarfs with 0.2 0.5     N N < < M , have metallicity measurements, and have masses and radii determined to better than 5% precision. Discrepancies between the predicted and observed masses and radii are found for three of the systems.