It takes a village to raise a tide: nonlinear multiple-mode coupling and mode identification in KOI-54

TL;DR

This study investigates the complex nonlinear pulsations and mode interactions in the eccentric binary KOI-54 using Kepler data, revealing mode identification, amplitude evolution, and nonlinear coupling phenomena.

Contribution

It provides the first detailed analysis of nonlinear mode coupling and mode identification in KOI-54, advancing understanding of stellar pulsations in eccentric binaries.

Findings

Identified over 120 pulsations, including harmonic and anharmonic modes.

Confirmed the onset of traveling wave regime for certain pulsations.

Evidence of nonlinear mode coupling lowering interaction thresholds.

Abstract

We explore the tidal excitation of stellar modes in binary systems using Kepler observations of the remarkable eccentric binary KOI-54 (HD 187091; KIC 8112039), which displays strong ellipsoidal variation as well as a variety of linear and nonlinear pulsations. We report the amplitude and phase of over 120 harmonic and anharmonic pulsations in the system. We use pulsation phases to determine that the two largest-amplitude pulsations, the 90th and 91st harmonics, most likely correspond to axisymmetric m=0 modes in both stars, and thus cannot be responsible for resonance locks as had been recently proposed. We find evidence that the amplitude of at least one of these two pulsations is decreasing with a characteristic timescale of ~100 yr. We also use the pulsations' phases to confirm the onset of the traveling wave regime for harmonic pulsations with frequencies <~50 Omega_orbit, in agreement with theoretical expectations. We present evidence that many pulsations that are not harmonics of the orbital frequency correspond to modes undergoing simultaneous nonlinear coupling to multiple linearly driven parent modes. Since coupling among multiple modes can lower the threshold for nonlinear interactions, nonlinear phenomena may be easier to observe in highly eccentric systems, where broader arrays of driving frequencies are available. This may help to explain why the observed amplitudes of the linear pulsations are much smaller than the theoretical threshold for decay via three-mode coupling.