A Pre-main-sequence γ Dor-δ Sct Hybrid with Extremely Slow Internal Rotation in a Short-period Eclipsing Binary KIC 9850387 Revealed by Asteroseismology

A complete understanding of the formation process of close binaries relies on the reliable determination of their masses and ages, as well as the interior structure of pre-main-sequence (PMS) binaries. PMS eclipsing binaries containing stellar oscillations are therefore benchmarks since the binarity makes it possible to measure the masses and radii independently with great accuracy, while stellar oscillations represent a unique method of probing stellar interiors. Here we report the discovery of KIC 9850387, a short-period PMS eclipsing binary exhibiting hybrid γ Dor-δ Sct pulsations that is also the inner binary of a probable hierarchical triple system. From the light-curve and radial-velocity modeling we determine the masses and radii of the component stars, which suggest the probable PMS nature of the system. Based on four years of Kepler data, the intrinsic oscillations of the star are analyzed. The pulsation spectrum shows 17 low-frequency peaks spaced at exactly equidistant periods that are identified as dipole gravity modes. The practically constant period spacing indicates an extremely slow near-core rotation of the pulsator with a period deduced to be longer than 1 yr. This enables us to fit the pulsation frequencies precisely with non-rotating models. The stellar parameters yielded by asteroseismology modeling are in consistent with the dynamic ones given by the binary model. The results indicate that the pulsator is a young PMS star with an age between 6.4 and 7.1 Myr. KIC 9850387 is therefore the first PMS eclipsing binary identified by asteroseismology.