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 $P=3.34395390\pm 0.00000020$ d 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: $M_A=0.448\pm 0.011$ $M_{\odot }$, $M_B=0.2721_{-0.0042}^{+0.0041}$ $M_{\odot }$, $R_A=0.4548_{-0.0036}^{+0.0035}$ $R_{\odot }$, and $R_B=0.2913_{-0.0024}^{+0.0023}$ $R_{\odot }$. We obtained a FIRE/Magellan near-infrared spectrum of the primary star during a total secondary eclipse, and use this to obtain disentangled spectra of both components. We determine spectral types of ST${}_A=\mathrm{M}3.71\pm 0.69$ and ST${}_B=\mathrm{M}5.01\pm 0.73$, and effective temperatures of T${}_{\mathrm{eff},\mathrm{A}}=3190\pm 110$ K and T${}_{\mathrm{eff},\mathrm{B}}=3100\pm 110$ K, for the primary and secondary star, respectively. We also measure a metallicity of [Fe/H]$=+0.298\pm 0.080$ for the system. We find that the system has a small, but significant, non-zero eccentricity of $0.0136\pm 0.0026$. The K2 light curve shows a coherent variation at a period of $3.41315_{-0.00032}^{+0.00030}$ d, 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 HAT-TR-318-007, that contain M dwarfs with $0.2M_{\odot }<M<0.5M_{\odot }$, 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.

34 pages, 22 figures, 15 tables, accepted for publication in AJ

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