Tidal effects and periastron events in binary stars

TL;DR

This study models the surface velocity field of stars in eccentric binary systems to understand how tidal interactions influence energy dissipation and stellar activity around periastron, with implications for stellar eruptions and mass loss.

Contribution

It presents a first-principles calculation of tidal velocity fields and energy dissipation in eccentric binary stars, linking tidal effects to observed stellar activity and eruptions.

Findings

Energy dissipation peaks before or after periastron depending on stellar radius and rotation

Oscillations in energy dissipation correlate with observed light curve features in HD 5980

Tidal flows may drive stars into active states with non-uniform, time-dependent mass loss.

Abstract

Binary stars in eccentric orbits are frequently reported to present increasing levels of activity around periastron passage. In this paper we present results of a calculation from first principles of the velocity field on the surface of a star that is perturbed by a binary companion. This allows us to follow the orbital phase-dependence of the amount of kinetic energy that may be dissipated through the viscous shear, dot-E, driven by tidal interactions. For stars with relatively small stellar radii compared with the orbital separation at periastron (R/r_per< 0.14), a clear maximum occurs before periastron passage for sub-synchronous rotation and after periastron for super-synchronous rotation. For larger stellar radii however, dot-E oscillates over the orbital cycle and periastron passage does not cause a particularly greater enhancement in energy dissipation rates than some of the other orbital phases. Finally, we perform exploratory calculations for a WR/LBV binary system that in 1993-1994 underwent an LBV-like eruption, HD 5980. Our dot-E calculations reproduce the oscillations that appear around periastron passage in HD5980's recent visual light curve. We suggest that the energy dissipation from tidal flows in asynchronously rotating binary stars may provide a mechanism by which evolving stars may be driven into an active state. Given the nature of the tidal perturbations, the resulting mass-loss distribution is expected to be non-uniform over the stellar surface and highly time-dependent.