Stochastic pulsations in the subdwarf-B star KIC 2991276

The subdwarf-B star KIC 2991276 was monitored with the Kepler spacecraft for nearly three years. Two pulsation modes with periods of 122 and 132s 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 KCI 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 has e 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 0.230. 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.