Vol. 568
In section 7. Stellar structure and evolution

Asteroseismology revealing trapped modes in KIC 10553698A

by R. H. Ostensen, J. H. Telting, M. D. Reed, A. S. Baran, P. Nemeth, and F. Kiaeerad A&A 568, A15


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.