Tidal Amplitudes in the Magellanic Cloud Population of Heartbeat Stars

Abstract Eccentric ellipsoidal variable (EEV) or “heartbeat” variable stars are stars whose light curves show pulses once per orbit due to ellipsoidal distortions from strong tides at periapse and other proximity effects like irradiation. We analyze the population of EEV stars in the Magellanic Clouds captured by the Optical Gravitational Lensing Experiment (OGLE) survey, and provide broadband spectral energy distribution fitting to estimate physical properties of the EEV stars. The OGLE EEV stars span a wide range of luminosities, radii, and effective temperatures. However, we find that they cluster near loci of strong tidal influence at periapse, indicating that tides are indeed responsible for their photometric variability. OGLE EEV stars are prevalent where stellar evolution is particularly rapid. We examine the distribution of photometric amplitudes, and show that these can be interpreted through a simplified model of radius growth through stellar evolution and orbital circularization through linear tidal dissipation. When we compare rates of tidal dissipation, we find differences between the modeled modified tidal quality factor among hot ( T eff > 6250 K), Q * ′ ≳ 1 0 7 , and cool ( T eff < 6250 K), Q * ′ ∼ 1 0 5 , stars, which is qualitatively consistent with models of efficient tidal dissipation in the convective envelopes of cool stars. We find that this model can reproduce the observed locations and photometric amplitudes of the OGLE EEV stars in the Hertzsprung–Russell diagram. The hot stars, in particular, extend to amplitudes near, but not beyond, the threshold for nonlinear tidal wave breaking on stellar surfaces, suggesting a physical saturation of tidal amplitudes at this threshold.