Stochastic pulsations in the subdwarf-B star KIC 2991276

¹ Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
e-mail: roy@ster.kuleuven.be
² Department of Physics, Astronomy, and Materials Science, Missouri State University, Springfield MO 65804, USA
³ Uniwersytet Pedagogiczny w Krakowie, ul. Podchorążych 2, 30-084 Kraków, Poland
⁴ Nordic Optical Telescope, Rambla José Ana Fernández Pérez 7, 38711 Breña Baja, Spain

Received: 28 February 2014
Accepted: 25 March 2014

Abstract

The subdwarf-B star KIC 2991276 was monitored with the Kepler spacecraft for nearly three years. Two pulsation modes with periods of 122 and 132 s are clearly detected in the Fourier spectrum, as well as a few weaker modes with periods ranging from 118 to 216 s. Unlike the other subdwarf-B pulsators with similar high-quality Kepler lightcurves, the modes in KIC 2991276 do not display long-term coherency. Rather, their pulsation amplitudes vary substantially in amplitude and phase on timescales of about a month, sometimes disappearing completely. Thus, while the pulsations are seen to have amplitudes of up to 1.4% in individual months, the amplitude spectrum of the full lightcurve shows a broad, messy peak with an amplitude of only 0.23%. Such stochastic oscillations are normal in the Sun and other cool stars with solar-like pulsations and have been suspected for V361-Hya pulsators, but thanks to the exceptional coverage of Kepler data, this is the first unambiguous case established for a hot subdwarf.