Radial Velocity Monitoring of Kepler Heartbeat Stars*

Abstract

Heartbeat stars (HB stars) are a class of eccentric binary stars with close periastron passages. The characteristic photometric HB signal evident in their light curves is produced by a combination of tidal distortion, heating, and Doppler boosting near orbital periastron. Many HB stars continue to oscillate after periastron and along the entire orbit, indicative of the tidal excitation of oscillation modes within one or both stars. These systems are among the most eccentric binaries known, and they constitute astrophysical laboratories for the study of tidal effects. We have undertaken a radial velocity (RV) monitoring campaign of Kepler HB stars in order to measure their orbits. We present our first results here, including a sample of 22 Kepler HB systems, where for 19 of them we obtained the Keplerian orbit and for 3 other systems we did not detect a statistically significant RV variability. Results presented here are based on 218 spectra obtained with the Keck/HIRES spectrograph during the 2015 Kepler observing season, and they have allowed us to obtain the largest sample of HB stars with orbits measured using a single instrument, which roughly doubles the number of HB stars with an RV measured orbit. The 19 systems measured here have orbital periods from 7 to 90 days and eccentricities from 0.2 to 0.9. We show that HB stars draw the upper envelope of the eccentricity-period distribution. Therefore, HB stars likely represent a population of stars currently undergoing high eccentricity migration via tidal orbital circularization, and they will allow for new tests of high eccentricity migration theories.

The Pseudosynchronization of Binary Stars Undergoing Strong Tidal Interactions

“…We know from the Kepler stellar catalog (Mathur et al 2017) and the spectroscopic follow-up of these stars (Shporer et al 2016) that the primary star is usually classified as an AFG-type star. Since the majority of the light originates from the primary star, our measured rotation periods likely originate from the primary star.…”

“…Because this equation depends so heavily on eccentricity, we only calculate the pseudosynchronization period for those stars with measured eccentricities. Those from radial velocity measurements are used when available because they are more reliable (Smullen & Kobulnicky 2015;Shporer et al 2016). We expand our sample by including the less reliable eccentricities reported from purely photometric fits (Thompson et al 2013) and use a 0.05 error on the eccentricities.…”

“…From these RV measurements, combined with photometric data, a full orbital solution can be obtained, including the stellar mass, eccentricity, and inclination of the system. Recently, radial velocity measurements have been obtained for more than 20 of these binary systems (Smullen & Kobulnicky 2015;Shporer et al 2016), making it possible to model a statistically interesting sample of these tidally active systems.…”

“…We then use eccentricity measurements obtained from radial velocity orbital solutions recently reported by Shporer et al (2016) to determine the pseudosynchronization period of the system. We use the pixel-level Kepler data to rule-out background sources as a likely source of the observed variations.…”

Three Statistically Validated K2 Transiting Warm Jupiter Exoplanets Confirmed as Low-mass Stars

“…Since we can tolerate a medium RV precision of ∼0.1 km s −1 , we used the so-called telluric lines method where the iodine cell is removed from the light path (see, e.g. Shporer et al 2016, their Section 2.2). Briefly, a wavelength solution is obtained through a nightly exposure of a Thorium-Argon lamp and the RVs are derived by measuring the offset in the position of the telluric absorption bands in the target spectra and that of a reference B-type star (Chubak et al 2012).…”