An Extreme-mass Ratio, Short-period Eclipsing Binary Consisting of a B Dwarf Primary and a Pre-main Sequence M Star Companion Discovered by KELT

TL;DR

This paper reports the discovery and detailed analysis of an extreme-mass ratio, short-period eclipsing binary with a B dwarf primary and a pre-main sequence M star companion, providing insights into binary star formation and stellar property variations.

Contribution

It presents the first detailed characterization of a very low mass ratio eclipsing binary with a B star primary, including its light curves, radial velocities, and spectral energy distribution.

Findings

Primary star is a late-B type, about 11,960 K.

Companion is a fully convective M star, about 0.22 solar masses.

Companion's radius is inflated by 26% compared to models.

Abstract

We present the discovery of \thisstar\ (HD 58730), a very low mass ratio ($q \equiv M_2/M_1 \approx 0.07$) eclipsing binary (EB) identified by the Kilodegree Extremely Little Telescope (KELT) survey. We present the discovery light curve and perform a global analysis of four high-precision ground-based light curves, the Transiting Exoplanets Survey Satellite (TESS) light curve, radial velocity (RV) measurements, Doppler Tomography (DT) measurements, and the broad-band spectral energy distribution (SED). Results from the global analysis are consistent with a fully convective ($M_2 = 0.22 \pm 0.02\ M_{\odot})$ M star transiting a late-B primary ($M_1 = 3.34^{+0.07}_{-0.09}\ M_{\odot};\ T_{\rm eff,1} = 11960^{+430}_{-520}\ {\rm K}$). We infer that the primary star is $183_{-30}^{+33}$ Myr old and that the companion star's radius is inflated by $26 \pm 8\%$ relative to the predicted value from a low-mass isochrone of similar age. We separately and analytically fit for the variability in the out-of-eclipse TESS phase curve, finding good agreement between the resulting stellar parameters and those from the global fit. Such systems are valuable for testing theories of binary star formation and understanding how the environment of a star in a close-but-detached binary affects its physical properties. In particular, we examine how a star's properties in such a binary might differ from the properties it would have in isolation.