KOI-54: The Kepler Discovery of Tidally-Excited Pulsations and Brightenings in a Highly Eccentric Binary

TL;DR

KOI-54 is a binary star system with highly eccentric orbit exhibiting tidally-excited pulsations and brightenings, providing insights into stellar oscillations driven by tidal interactions during close periastron passages.

Contribution

This study presents the first detailed modeling of tidally-excited pulsations in KOI-54, linking observed oscillations to dynamic tidal resonances in a highly eccentric binary.

Findings

Brightening events caused by tidal distortion and irradiation during periastron.

Detection of 30 pulsation modes resonant with orbital harmonics.

Identification of tidal resonance as the mechanism for pulsation excitation.

Abstract

Kepler observations of the star HD 187091 (KID 8112039, hereafter KOI-54) revealed a remarkable light curve exhibiting sharp periodic brightening events every 41.8 days with a superimposed set of oscillations forming a beating pattern in phase with the brightenings. Spectroscopic observations revealed that this is a binary star with a highly eccentric orbit, e=0.83. We are able to match the Kepler light curve and radial velocities with a nearly face-on (i=5.5 degree) binary star model in which the brightening events are caused by tidal distortion and irradiation of nearly identical A stars during their close periastron passage. The two dominant oscillations in the light curve, responsible for the beating pattern, have frequencies that are the 91st and 90th harmonic of the orbital frequency. The power spectrum of the light curve, after removing the binary star brightening component, reveals a large number of pulsations, 30 of which have a signal-to-noise ratio > 7. Nearly all of these pulsations have frequencies that are either integer multiples of the orbital frequency or are tidally-split multiples of the orbital frequency. This pattern of frequencies unambiguously establishes the pulsations as resonances between the dynamic tides at periastron and the free oscillation modes of one of the stars. KOI-54 is only the 4th star to show such a phenomenon, and is by far the richest in terms of excited modes.