Tidal Flows from Asynchronous Rotation in Binaries

TL;DR

This paper models tidal flows caused by asynchronous rotation in binary stars, showing how these flows affect surface velocities and energy dissipation, which may influence asymmetric, variable mass loss in massive binaries.

Contribution

It introduces a simplified one-layer stellar model to simulate tidal flows and links these flows to observable spectral features and energy dissipation rates in eccentric binary systems.

Findings

Tidal flows perturb surface velocity fields from uniform rotation.

Energy dissipation peaks near periastron and shifts with orbital phase.

Implications for asymmetric and variable mass loss in massive binaries.

Abstract

Asynchronous rotation in binary stars produces non-radial oscillations that are known to cause observable variability on orbital timescales. The horizontal perturbations of the surface velocity fields are referred to as "tidal flows". In this paper we illustrate the manner in which tidal flows perturb the surface velocity field from that of uniform rotation, using a one-layer stellar model for the calculations. We justify the validity of this simplified model by the striking similarity between the photospheric absorption line-profiles it predicts and observational data of the binary system alpha Virginis. The velocity perturbations are used to compute the mechanical energy dissipation rates, dot-E, due to the shearing flows for the case of a massive (50+28 Mo) binary system having a moderately eccentric (e=0.3) orbit. The largest value of dot-E around periastron phases is found on the hemisphere facing the companion. However, at other orbital phases the maximum dot-E may migrate towards the poles. Assuming that dot-E plays a role in the mass-loss characteristics of massive binary systems, this suggests that peculiar binaries such as HD 5980 and eta Carinae may have a highly non-spherically symmetric mass-loss distribution which, in addition, is time-variable.