Tidally distorted stars are triaxial pulsators

TL;DR

This paper models how tidal forces in close binary stars distort their pulsation modes, leading to observable amplitude modulations and mode alignments that can aid in detailed asteroseismic analysis.

Contribution

It introduces a perturbation theory approach to compute mode frequencies and geometries of tidally distorted stars, revealing the phenomenon of triaxial pulsations and their observational signatures.

Findings

Tidally coupled dipole modes are dominated by tidal effects, resulting in triaxial pulsations.

Amplitude modulations produce doublets in the power spectrum spaced by twice the orbital frequency.

Mode alignment and modulation patterns can help identify modes and analyze stellar interiors.

Abstract

Stars in close binaries are tidally distorted, and this has a strong effect on their pulsation modes. We compute the mode frequencies and geometries of tidally distorted stars using perturbation theory, accounting for the effects of the Coriolis force and the coupling between different azimuthal orders $m$ of a multiplet induced by the tidal distortion. For tidally coupled dipole pressure modes, the tidal coupling dominates over the Coriolis force and the resulting pulsations are ``triaxial", with each of the three modes in a multiplet ``tidally tilted" to be aligned with the one of the three principal axes of the star. The observed amplitudes and phases of the dipole modes aligned orthogonal to the spin axis are modulated throughout the orbit, producing doublets in the power spectrum that are spaced by exactly twice the orbital frequency. Quadrupole modes have similar but slightly more complex behavior. This amplitude modulation allows for mode identification which can potentially enable detailed asteroseismic analyses of tidally tilted pulsators. Pressure modes should exhibit this behavior in stellar binaries close enough to be tidally synchronized, while gravity modes should remain aligned with the star's spin axis. We discuss applications to various types of pulsating stars, and the relationship between tidal tilting of pulsations and the ``single-sided" pulsations sometimes observed in very tidally distorted stars.