Asteroseismology revealing trapped modes in KIC 10553698A^{⋆,⋆⋆,⋆⋆⋆}

¹ Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
e-mail: roy@ster.kuleuven.be
² Nordic Optical Telescope, Rambla José Ana Fernández Pérez 7, 38711 Breña Baja, Spain
³ Department of Physics, Astronomy, and Materials Science, Missouri State University, Springfield, MO 65804, USA
⁴ Uniwersytet Pedagogiczny w Krakowie, ul. Podchora¸żych 2, 30-084 Kraków, Poland
⁵ Dr. Karl Remeis-Observatory & ECAP, Astronomisches Inst., FAU Erlangen-Nuremberg, 96049 Bamberg, Germany

Received: 10 February 2014
Accepted: 10 June 2014

Abstract

The subdwarf-B pulsator, KIC 10553698A, is one of 16 such objects observed with a one-minute sampling rate for most of the duration of the Kepler mission. Like most of these stars, it displays a rich g-mode pulsation spectrum with several clear multiplets that maintain regular frequency splitting. We identify these pulsation modes as components of rotationally split multiplets in a star rotating with a period of ~41 d. From 162 clearly significant periodicities, we are able to identify 156 as likely components of ℓ = 1 or ℓ = 2 multiplets. For the first time we are able to detect 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 signal is also present in the Kepler photometry at 3.387 d. Spectroscopic observations reveal a radial-velocity amplitude of 64.8 km s^-1. We find that the radial-velocity variations and the photometric signal have phase and amplitude that are perfectly consistent with a Doppler-beaming effect and conclude that the unseen companion, KIC 10553698B, must be a white dwarf most likely with a mass close to 0.6 M_⊙.