Inverse Tides in Pulsating Binary Stars

TL;DR

This paper explores a novel 'inverse' tidal mechanism in pulsating binary stars where energy transfer from pulsations can increase orbital eccentricity and misalignment, contrasting with traditional tidal damping effects.

Contribution

It introduces the concept of inverse tides driven by unstable pulsation modes, showing how they can alter stellar spins and orbital parameters in close binaries.

Findings

Inverse tides can drive stellar spins away from synchronization.

Stars with inverse tides may have very slow core rotation.

Inverse tides can maintain large spin-orbit misalignments.

Abstract

In close binary stars, the tidal excitation of pulsations typically dissipates energy, causing the system to evolve towards a circular orbit with aligned and synchronized stellar spins. However, for stars with self-excited pulsations, we demonstrate that tidal interaction with unstable pulsation modes can transfer energy in the opposite direction, forcing the spins of the stars away from synchronicity, and potentially pumping the eccentricity and spin-orbit misalignment angle. This "inverse" tidal process only occurs when the tidally forced mode amplitude is comparable to the mode's saturation amplitude, and it is thus most likely to occur in main sequence gravity mode pulsators with orbital periods of a few days. We examine the long-term evolution of inverse tidal action, finding the stellar rotation rate can potentially be driven to a very large or very small value, while maintaining a large spin-orbit misalignment angle. Several recent asteroseismic analyses of pulsating stars in close binaries have revealed extremely slow core rotation periods, which we attribute to the action of inverse tides.