KOI-54: The Kepler Discovery of Tidally-Excited Pulsations and Brightenings in a Highly Eccentric Binary (1102.1730v2)

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.

KOI-54: Binary Star System with Tidally-Excited Pulsations

The paper by Welsh et al. presents a detailed paper of KOI-54, a binary star system observed by the Kepler Mission, uncovering significant phenomena due to tidal interactions. Contrary to its initial classification as an ordinary A-type star, KOI-54's Kepler observations reveal periodic brightening accompanied by pulsations that bear a distinct phase correlation with the periastron events.

Observational Findings and Analysis

The Kepler light curve of KOI-54 displays sharp brightening events every 41.8 days which coincide with the periastron passage of a highly eccentric binary orbit (e=0.83). Spectroscopic data confirms that KOI-54 comprises two nearly identical A stars on a nearly face-on orbit with an inclination of 5.5 degrees. Kepler's high-precision photometry reveals two dominant oscillation frequencies, which are intriguingly the 91st and 90th harmonics of the orbital frequency, implying resonant tidal interactions.

The presence of numerous pulsation modes is particularly noteworthy, with 30 modes exhibiting a signal-to-noise ratio greater than 7. The frequencies are characterized by integer multiples or tidally-split multiples of the orbital frequency, strongly suggesting tidal forcing of internal stellar modes.

Modeling and Theoretical Implications

The paper employs sophisticated binary modeling, integrating radial velocities and photometric data, to characterize the system. Using a modified ELC modeling code, the tidal effects and irradiation during close passage are simulated, rendering a convincing fit to the observational data. The stellar pulsation patterns are explained as resonances between dynamic tides at periastron and the free oscillation modes of the stars, supported by nonadiabatic non-radial pulsation theories.

The stellar evolutionary models suggest these stars have enhanced metallicity compared to the solar benchmark, although they do not exhibit typical $\delta$ Scuti pulsations. The tidal interaction theories point to possible synchronization against a highly eccentric background, with orbital period harmonics exciting free gravity modes.

Discussion and Systematic Considerations

The KOI-54 system presents intriguing puzzles such as the selective excitation of certain modes and the absence of expected p-mode pulsations. Systematic concerns include potential misalignment of stellar rotational axes and photometric detrending challenges. The precise eccentricity and inclination derived from photometry represent a pioneering effort, marking a significant methodological achievement.

Future Prospects

The extraordinary precision achieved by Kepler encourages further exploration of tidal phenomena in binary systems, offering potential insights into stellar interior dynamics. Prospectively, continued monitoring could refine the pulsation frequency resolution, enabling an unprecedented mapping of internal stellar rotation profiles. KOI-54 serves as an exemplary case expanding the understanding of tidal interactions in stellar evolution.

In summary, the investigation into KOI-54 enriches the knowledge of tidal excitation in binaries, marking an important contribution to stellar astrophysics through meticulous observational and computational techniques. This system, with its rich array of tidally-excited modes, underscores the transformative potential of meticulous celestial monitoring enabled by missions like Kepler.