A subsynchronously rotating pulsating subdwarf B star in a short-period binary with a white dwarf companion

¹ Uniwersytet Pedagogiczny, Obserwatorium na Suhorze, ul. Podchorążych 2, 30-084 Kraków Poland
e-mail: sfbaran@cyf-kr.edu.pl
² Nordic Optical Telescope, Rambla José Ana Fernández Pérez 7, 38711 Breña Baja, Spain
³ Dr. Remeis–Sternwarte, Institute for Astronomy, University, Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
⁴ Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁵ Department of Physics, Astronomy, and Materials Science, Missouri State University, Springfield, MO 65804, USA

Received: 13 August 2015
Accepted: 2 October 2015

Abstract

We present our analysis of KIC 7664467, an sdB pulsator that we have found to be residing in a 1.56-day binary system with a white-dwarf companion. This system was observed photometrically with the Kepler spacecraft and spectroscopically with ground-based telescopes. We analyzed the amplitude spectra detecting 61 periods, rotationally split multiplets, and an equally spaced sequence in period. These two features helped with the mode identification. We derived both the binary and rotation periods showing that this is another binary system with a subsynchronous sdB star. From our spectroscopy of the sdB star, we determined T_eff = 27440 ± 120 K, log g = 5.38 ± 0.02 dex. The abundance pattern follows the general trend observed in sdB stars, where light metals are subsolar, while the Fe abundance is very close to the solar value. We found the N enrichment and low abundances of C and O that resemble the equilibrium abundances of the CNO cycle. We could also measure the Mg and Si abundances. Using the radial velocity amplitude K₁ = 57(3) km s^-1 and the Doppler boosting-dominated photometric signal at the orbital period, we constrained the companion to be a compact object, almost certainly a white dwarf.