Tidal Interactions in Post Common-Envelope sdB Binaries

TL;DR

This study models tidal interactions in close sdB binaries to assess synchronization timescales, finding that they generally exceed the sdB lifetime, challenging the assumption of tidal locking in these systems.

Contribution

The paper provides detailed 1D stellar models to calculate tidal dissipation and synchronization timescales, demonstrating that synchronization is unlikely within the sdB lifetime.

Findings

Synchronization timescales are longer than sdB lifetimes.

Tidal locking is unlikely in observed sdB binaries.

Convective processes do not facilitate synchronization.

Abstract

Over half of all observed hot subdwarf B (sdB) stars are found in binaries, and over half of these are found in close configurations with orbital periods of 10$ \,\rm{d}$ or less. In order to estimate the companion masses in these predominantly single-lined systems, tidal locking has frequently been assumed for sdB binaries with periods less than half a day. Observed non-synchronicity of a number of close sdB binaries challenges that assumption and hence provides an ideal testbed for tidal theory. We solve the second-order differential equations for detailed 1D stellar models of sdB stars to obtain the tidal dissipation strength and hence to estimate the tidal synchronization time-scale owing to Zahn's dynamical tide. The results indicate synchronization time-scales longer than the sdB lifetime in all observed cases. Further, we examine the roles of convective overshooting and convective dissipation in the core of sdB stars and find no theoretical framework in which tidally-induced synchronization should occur.