Tidally Induced Oscillations and Orbital Decay in Compact Triple-Star Systems

TL;DR

This paper explores how tidal forces in compact triple-star systems excite high-frequency stellar oscillations, leading to orbital decay, with theoretical predictions aligning well with observations of the HD 181068 system.

Contribution

It provides a detailed theoretical framework for three-body tidal forcing in hierarchical triples and compares predictions with Kepler observations, highlighting the role of resonances in orbital evolution.

Findings

Tidal forcing can excite high-frequency p-modes in triple-star systems.

Tidal effects can cause significant orbital decay over time.

Observations of HD 181068 match the theoretical predictions of tidal oscillations.

Abstract

We investigate the nature of tidal effects in compact triple-star systems. The hierarchical structure of a triple system produces tidal forcing at high frequencies unobtainable in binary systems, allowing for the tidal excitation of high frequency p-modes in the stellar components. The tidal forcing exists even for circular, aligned, and synchronized systems. We calculate the magnitude and frequencies of three-body tidal forcing on the central primary star for circular and coplanar orbits, and we estimate the amplitude of the tidally excited oscillation modes. We also calculate the secular orbital changes induced by the tidally excited modes, and show that they can cause significant orbital decay. During certain phases of stellar evolution, the tidal dissipation may be greatly enhanced by resonance locking. We then compare our theory to observations of HD 181068, which is a hierarchical triply eclipsing star system in the Kepler field of view. The observed oscillation frequencies in HD 181068 can be naturally explained by three-body tidal effects. We then compare the observed oscillation amplitudes and phases in HD 181068 to our predictions, finding mostly good agreement. Finally, we discuss the past and future evolution of compact triple systems like HD 181068.