Asteroseismic effects in close binary stars

TL;DR

This paper investigates how close binary star systems affect the propagation and damping of internal acoustic waves, revealing increased damping and unique energy distribution patterns compared to single stars.

Contribution

It introduces a three-dimensional WKB analysis of acoustic wave behavior in close binaries, highlighting effects of variable pressure scale height and resulting wave damping.

Findings

Higher damping rates of acoustic waves in close binaries.

Development of a ring-like energy distribution at the stellar surface.

Dependence of wave behavior on binary separation and frequency.

Abstract

Turbulent processes in the convective envelopes of the sun and stars have been shown to be a source of internal acoustic excitations. In single stars, acoustic waves having frequencies below a certain cutoff frequency propagate nearly adiabatically and are effectively trapped below the photosphere where they are internally reflected. This reflection essentially occurs where the local wavelength becomes comparable to the pressure scale height. In close binary stars, the sound speed is a constant on equipotentials, while the pressure scale height, which depends on the local effective gravity, varies on equipotentials and may be much greater near the inner Lagrangian point (L_1). As a result, waves reaching the vicinity of L_1 may propagate unimpeded into low density regions, where they tend to dissipate quickly due to non-linear and radiative effects. We study the three dimensional propagation and enhanced damping of such waves inside a set of close binary stellar models using a WKB approximation of the acoustic field. We find that these waves can have much higher damping rates in close binaries, compared to their non-binary counterparts. We also find that the relative distribution of acoustic energy density at the visible surface of close binaries develops a ring-like feature at specific acoustic frequencies and binary separations.