Oscillations in stellar superflares

• Published in: Monthly notices of the Royal Astronomical Society 2015-06, Vol.450 (1), p.956-966
• Main Authors: Balona, L. A., Broomhall, A.-M., Kosovichev, A., Nakariakov, V. M., Pugh, C. E., Van Doorsselaere, T.
• Format: Article
• Language: English
• Subjects: Acoustics; Amplitudes; Astronomy; Flares; Oscillations; Pulsars; Pulsation; Searching; Stars; Stars & galaxies; Stellar flares; Sun
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stv661

Two different mechanisms may act to induce quasi-periodic pulsations (QPP) in whole-disc observations of stellar flares. One mechanism may be magnetohydromagnetic forces and other processes acting on flare loops as seen in the Sun. The other mechanism may be forced local acoustic oscillations due to the high-energy particle impulse generated by the flare (known as ‘sunquakes’ in the Sun). We analyse short-cadence Kepler data of 257 flares in 75 stars to search for QPP in the flare decay branch or post-flare oscillations which may be attributed to either of these two mechanisms. About 18 per cent of stellar flares show a distinct bump in the flare decay branch of unknown origin. The bump does not seem to be a highly damped global oscillation because the periods of the bumps derived from wavelet analysis do not correlate with any stellar parameter. We detected damped oscillations covering several cycles (QPP), in seven flares on five stars. The periods of these oscillations also do not correlate with any stellar parameter, suggesting that these may be a due to flare loop oscillations. We searched for forced global oscillations which might result after a strong flare. To this end, we investigated the behaviour of the amplitudes of solar-like oscillations in eight stars before and after a flare. However, no clear amplitude change could be detected. We also analysed the amplitudes of the self-excited pulsations in two δ Scuti stars and one γ Doradus star before and after a flare. Again, no clear amplitude changes were found. Our conclusions are that a new process needs to be found to explain the high incidence of bumps in stellar flare light curves, that flare loop oscillations may have been detected in a few stars and that no conclusive evidence exists as yet for flare induced global acoustic oscillations (starquakes).