Tidal Amplitudes in the Magellanic Cloud Population of Heartbeat Stars

Abstract: Heartbeat (HB) stars exhibit pulses in their light curves once per orbit due to ellipsoidal distortions from strong tides at periapse. We analyze the population of HB stars in the Magellanic Clouds captured by the OGLE survey, and provide broadband spectral energy distribution fitting to estimate physical properties of the HB stars. The HB 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 in nearly all cases, strong tides are indeed responsible for their photometric variability. HB stars tend to populate regions away from the main sequence, where stellar evolution is particularly rapid. We examine the distribution of tidal 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 ($T_{\rm eff}&gt;6250$~K), $Q_\ast&#39; \gtrsim 10<sup>7$, and cool ($T_{\rm eff}&lt;6250$~K), $Q_\ast&#39;\sim 10<sup>5$, 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 observed locations and amplitudes of HB 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.