Tidal Spin-up of Subdwarf B Stars

TL;DR

This paper models the tidal interactions in subdwarf B stars, calculating gravity wave excitation and spin-orbit evolution, predicting the orbital periods for potential tidal synchronization consistent with observations.

Contribution

It provides the first direct calculations of tidal torque from gravity waves in realistic sdB models and predicts synchronization thresholds based on stellar and orbital parameters.

Findings

Synchronization likely occurs in binaries with orbital periods less than ~0.2 days.

The transition period depends weakly on companion mass and stellar progenitor.

Predictions include rapid rotation of white dwarfs from synchronized sdB binaries.

Abstract

Hot subdwarf B (sdB) stars are stripped helium-burning stars that are often found in close binaries, where they experience strong tidal interactions. The dissipation of tidally excited gravity waves alters their rotational evolution throughout the sdB lifetime. While many sdB binaries have well-measured rotational and orbital frequencies, there have been few theoretical efforts to accurately calculate the tidal torque produced by gravity waves. In this work, we directly calculate the tidal excitation of internal gravity waves in realistic sdB stellar models and integrate the coupled spin-orbit evolution of sdB binaries. We find that for canonical sdB ($M_\mathrm{sdB}=0.47\,M_\odot$) binaries, the transitional orbital period below which they could reach tidal synchronization in the sdB lifetime is $\sim \! 0.2\;\mathrm{days}$, with weak dependence on the companion masses. For low-mass sdBs ($M_\mathrm{sdB}=0.37\,M_\odot$) formed from more massive progenitor stars, the transitional orbital period becomes $\sim \! 0.15\;\mathrm{days}$. These values are very similar to the tidal synchronization boundary ($\sim \! 0.2\;\mathrm{days}$) evident from observations. We discuss the dependence of tidal torques on stellar radii, and we make predictions for the rapidly rotating white dwarfs formed from synchronized sdB binaries.