KIC10553698: An sdBV+WD binary (R. H. Østensen et al.)

Vol. 568
In section 7. Stellar structure and evolution

Asteroseismology revealing trapped modes in KIC 10553698A

Most hot subdwarf-B (sdB) stars belong to the population of extreme-horizontal-branch (EHB) stars. Several binary scenarios have been identified that can produce EHB stars, including either common-envelope ejection or stable Roche-lobe overflow. The sdBs pulsate with short (p modes) and long periods (g modes) because of the iron-group elements opacity bump (kappa mechanism). A key element in driving pulsations in sdBs is the competition between radiative levitation and gravitational settling, which causes a local overabundance of iron in the driving zone. KIC 10553698A is one of 16 sdBs observed with one-minute sampling rate for most of the duration of the Kepler mission. The authors identify the rich g-mode pulsation modes as components of rotationally split multiplets in a star rotating with a period of ~41 d. They are able to detect for the first time l = 1 modes that interpose in the asymptotic period sequences and that provide a clear indication of mode trapping in a stratified envelope, as predicted by theoretical models. A clear periodicity is also present in the Kepler photometry at 3.387 d. Spectroscopic observations reveal a radial-velocity amplitude of 64.8 km/s, indicating that the companion must be a white dwarf with a mass close to 0.6 M_o.