The effect of tides on near-core rotation: analysis of 35 Kepler $\gamma$ Doradus stars in eclipsing and spectroscopic binaries

TL;DR

This study analyzes how tidal forces influence the near-core rotation of gamma Doradus stars in binary systems, revealing tidal locking in short-period binaries and proposing a new 'inverse tides' mechanism for extremely slow rotators.

Contribution

It provides the first systematic analysis of tidal effects on near-core rotation in gamma Doradus stars within eclipsing and spectroscopic binaries, introducing the concept of 'inverse tides' for slow rotators.

Findings

Many stars are tidally locked if orbital periods are under 10 days.

Gamma Dor stars in binaries tend to rotate slower than single stars.

Identification of three stars with extremely slow near-core rotation rates.

Abstract

We systematically searched for gravity- and Rossby-mode period spacing patterns in Kepler eclipsing binaries with $\gamma$ Doradus pulsators. These stars provide an excellent opportunity to test the theory of tidal synchronisation and angular momentum transport in F- and A-type stars. We discovered 35 systems that show clear patterns, including the spectroscopic binary KIC 10080943. Combined with 45 non-eclipsing binaries with $\gamma$ Dor components that have been found using pulsation timing, we measured their near-core rotation rates and asymptotic period spacings. We find that many stars are tidally locked if the orbital periods are shorter than 10 days, in which the near-core rotation periods given by the traditional approximation of rotation (TAR) are consistent with the orbital period. Compared to the single stars, $\gamma$ Dor stars in binaries tend to have slower near-core rotation rates, likely a consequence of tidal spin-down. We also find three stars that have extremely slow near-core rotation rates. To explain these, we hypothesise that unstable tidally excited oscillations can transfer angular momentum from the star to the orbit, and slow the star below synchronism, a process we refer to as `inverse tides'.