Pulsating subdwarf B stars observed with K2 during Campaign 7 and an examination of seismic group properties

Abstract

ABSTRACT We report the discovery of four new pulsating subdwarf B (sdBV) stars from Campaign 7 of the Kepler spacecraft’s K2 mission. EPIC 215776487, EPIC 217280630, EPIC 218366972, and EPIC 218717602 are all gravity (g)-mode pulsators and we also detect two pressure (p)-mode pulsations in EPIC 218717602. We detect asymptotic $\ell \, =\, 1$ sequences in all four stars, allowing us to identify nearly all of the g modes. We detect evenly spaced frequency multiplets in EPIC 218717602 from which we determine a rotation period near 7 d. Spectroscopic observations determine that EPIC 218366972 is in a 5.92 d binary with most likely a white dwarf companion of canonical mass while the others have no detected companions. As we detect no multiplets in EPIC 218366972, it is added to the growing list of subsynchronously rotating stars. With 40 Kepler-detected sdBV stars and a growing number of Transiting Exoplanet Survey Satellite (TESS) publications, we update an examination of the group properties to provide direction for models. We notice a correlation between effective temperature and period of maximum pulsation amplitude, at least for g-mode pulsations, and update the previously observed effective temperature–rotation period relation.