KIC?7668647: a 14 day beaming sdB+WD binary with a pulsating subdwarf

摘要

The recently discovered subdwarf B (sdB) pulsator KIC?7668647 is one of the 18 pulsating sdB stars detected in the Kepler field. It features a rich g-mode frequency spectrum, with a few low-amplitude p-modes at short periods. This makes it a promising target for a seismic study aiming to constrain the internal structure of this star, and of sdB stars in general. We use new ground-based low-resolution spectroscopy, and the near-continuous 2.88?year Kepler light curve, to reveal that KIC?7668647 consists of a subdwarf B star with an unseen white-dwarf companion with an orbital period of 14.2?d. An orbit with a radial-velocity amplitude of 39?km?s-1 is consistently determined from the spectra, from the orbital Doppler beaming seen by Kepler at 163?ppm, and from measuring the orbital light-travel delay of 27?s by timing of the many pulsations seen in the Kepler light curve. The white dwarf has a minimum mass of 0.40?M⊙. We use our high signal-to-noise average spectra to study the atmospheric parameters of the sdB star, and find that nitrogen and iron have abundances close to solar values, while helium, carbon, oxygen and silicon are underabundant relative to the solar mixture. We use the full Kepler Q06–Q17 light curve to extract 132 significant pulsation frequencies. Period-spacing relations and multiplet splittings allow us to identify the modal degree ? for the majority of the modes. Using theg-mode multiplet splittings we constrain the internal rotation period at the base of the envelope to 46?48?d as a first seismic result for this star. The few p-mode splittings may point at a slightly longer rotation period further out in the envelope of the star. From mode-visibility considerations we derive that the inclination of the rotation axis of the sdB in KIC?7668647 must be around ~60°. Furthermore, we find strong evidence for a few multiplets indicative of degree 3 ≤ ? ≤ 8, which is another novelty in sdB-star observations made possible by Kepler.