Ellipsoidal variables
Ellipsoidal variables are non-eclipsing, close binary systems in which one or both components take on an ellipsoidal (or tear-drop) shape due to the strong gravitational pull from the companion. While the orbital inclinations of these systems are too small to produce eclipses, their light variations result from changes in the cross-sectional areas that the distorted stars present to the observer at different orbital phases. Consequently, their light curves display two maxima and two minima per orbital cycle. The minima are typically unequal due to gravity darkening, which is more pronounced at the pointed end of the elongated star.
Eccentric ellipsoidal variables
All the light curves shown above were produced by binary systems with nearly circular orbits. But how do the light curves of ellipsoidal variables with eccentric orbits appear? It turns out that, due to the changing distance between the stars (which affects their mutual tidal deformation) and the varying orbital velocities of the components (as described by Kepler's second law), the light curves of ellipsoidal variables with eccentric orbits have significantly different shapes compared to those of systems with circular orbits. Even with a relatively small orbital eccentricity, the maxima of the light curve become uneven, with noticeable differences in their duration. As the eccentricity increases, the light curves become highly asymmetric, showing rapid changes during the periastron passage and slower, less pronounced changes near apastron. Interestingly, the same binary system can produce different light curve depending on the angle of observation.
When we detected such light curves in the OGLE data in 2004 (Soszyński et al. 2004), we did not know how to interpret them. Professor Bohdan Paczyński came to our aid, suggesting that these were ellipsoidal binary systems with eccentric orbits. In 2012, Thompson et al. (2012) named this type of variables "heartbeat stars" because their light curves sometimes resemble an electrocardiogram. A catalog of nearly 1,000 heartbeat stars found in the OGLE databases was published by Wrona et al. (2022).
|
