Asteroseismic Determination of Stellar Rotation: On Synchronization in the Close Eclipsing Binaries AB Cas and OO Dra

Abstract

A star's rotation rate is difficult to estimate without surface inhomogeneities such as dark or bright spots. This paper presents asteroseismic results to determine the rotation rates of $δ$ Sct-type pulsating primary stars in two eclipsing binary systems, AB Cas and OO Dra. After removing the binarity-induced light variations from the archival TESS data and carefully examining the combination frequencies, we identified 12 independent frequencies for AB Cas and 11 frequencies for OO Dra, with amplitudes higher than $\sim$0.3 mmag, as $δ$ Sct-type pulsation frequencies excited in each primary star. The theoretical frequencies for seismic analysis were obtained by fully considering the rotation effects. Grid fitting for various stellar properties, such as mass, radius, metallicity, and rotation rate, yielded the best solution for which theoretical frequencies and stellar parameters agreed well with the observations. The rotation rate of the AB Cas primary was tightly constrained to 0.81 $\pm$ 0.01 day$^{-1}$ ($f_{\rm rot} / f_{\rm orb}$ = 1.11$^{+0.01}_{-0.02}$), which is slightly faster than the synchronized rotation. In contrast, the rotation rate of 0.63 $\pm$ 0.01 day$^{-1}$ for the OO Dra primary is lower than the synchronous value of approximately 0.81 day$^{-1}$. Subsynchronous rotation is uncommon in short-period binaries, and its physical mechanism is not yet well understood. Our results show that asteroseismology can be used to precisely measure the rotation rate of fast-rotating $δ$ Sct stars and thus provide a valuable constraint on rotation-orbit synchronization in close binary systems.