Gravity Modes Reveal the Internal Rotation of a Post-mass Transfer Gamma Doradus/Delta Scuti Hybrid Pulsator in Kepler Eclipsing Binary KIC 9592855

Abstract

We report the discovery of a post-mass transfer Gamma Doradus/Delta Scuti hybrid pulsator in the eclipsing binary KIC~9592855. This binary has a circular orbit, an orbital period of 1.2 days, and contains two stars of almost identical masses ($M_1=1.72M_{\odot },M_2=1.71M_{\odot }$). However, the cooler secondary star is more evolved ($R_2=1.96R_{\odot }$) while the hotter primary is still on the zero-age-main-sequence ($R_1=1.53R_{\odot }$). Coeval models from single star evolution cannot explain the observed masses and radii, and binary evolution with mass-transfer needs to be invoked. After subtracting the binary light curve, the Fourier spectrum shows low-order pressure-mode pulsations, and more dominantly, a cluster of low-frequency gravity modes at about $2$ day${}^{-1}$. These g-modes are nearly equally-spaced in period, and the period spacing pattern has a negative slope. We identify these g-modes as prograde dipole modes and find that they stem from the secondary star. The frequency range of unstable p-modes also agrees with that of the secondary. We derive the internal rotation rate of the convective core and the asymptotic period spacing from the observed g-modes. The resulting values suggest that the core and envelope rotate nearly uniformly, i.e., their rotation rates are both similar to the orbital frequency of this synchronized binary.

ApJ accepted. We have added the following discussions: (a) the evidence of Am stars in the spectra; (b)two examples of the effect of mass-transfer on stellar oscillations;(c) the evolution of unstable frequency range of p-modes in Figure 7. We have corrected a mistake in the labeling of effective temperatures in Table 1