Kepler's first view of O-star variability: K2 data of five O stars in Campaign 0 as a proof-of-concept for O-star asteroseismology

Buysschaert, B.; Aerts, C.; Bloemen, S.; Debosscher, J.; Neiner, C.; Briquet, M.; Vos, J.; Papics, P.; Manick, R.; Schmid, V.; Van Winkel, H.; Tkachenko, A.

1 LESIA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universite´s, UPMC Univ. Paris 06, Univ. Paris Diderot, Sorbonne PariskCite´, France
2Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
3Department of Astrophysics/IMAPP, Radboud University Nijmegen, 6500 GL Nijmegen, The Netherlands
4 Institut d’Astrophysique et de G´eophysique, Universit´e de Li`ege, Quartier Agora, Alle du 6 aoˆut 19C, B-4000 Li`ege, Belgium

We present high-precision photometric light curves of five O-type stars observed with the refurbished {it Kepler/} satellite during its Campaign 0. For one of the stars, we also assembled high-resolution ground-based spectroscopy with the {sc hermes} spectrograph attached to the 1.2-m Mercator telescope. The stars EPIC202060097 (O9.5V) and EPIC202060098 (O7V) exhibit monoperiodic variability due to rotational modulation with an amplitude of 5.6 mmag and 9.3 mmag and a rotation period of 2.63 d and 5.03 d, respectively. EPIC202060091 (O9V) and EPIC202060093 (O9V:pe) reveal variability at low frequency but the cause is unclear. EPIC202060092 (O9V:p) is discovered to be a spectroscopic binary with at least one multiperiodic βCep-type pulsator whose detected mode frequencies occur in the range [0.11,6.99] d−1 and have amplitudes between 0.8 and 2.0 mmag. Its pulsation spectrum is shown to be fully compatible with the ones predicted by core-hydrogen burning O-star models. Despite the short duration of some 33,d and the limited data quality with a precision near 100 μmag of these first K2 data, the diversity of possible causes for O-star variability already revealed from campaigns of similar duration by the MOST and CoRoT satellites is confirmed with {it Kepler}. We provide an overview of O-star space photometry and give arguments why future K2 monitoring during Campaigns 11 and 13 at short cadence, accompanied by time-resolved high-precision high-resolution spectroscopy opens up the possibility of in-depth O-star seismology.

Reference: Accepted for publication in MNRAS
Status: Manuscript has been accepted