Magnetic activity and solar-like pulsations in X-ray sources in the Kepler field of view

MOLENDA-ZAKOWICZ JOANNA, molenda@astro.uni.wroc.pl, University of Wroclaw, Poland
Frasca Antonio, antonio.frasca@oact.inaf.it, INAF - Osservatorio Astrofisico di Catania
Froehlich Hans-Erich, HEFroehlich@aip.de, AIP Potsdam, Germany
Bonanno Alfio, abo@oact.inaf.it, INAF - Osservatorio Astrofisico di Catania
Busa Innocenza, Innocenza.Busa@oact.inaf.it, INAF - Osservatorio Astrofisico di Catania
Catanzaro Giovanni, Giovanni.Catanzaro@oact.inaf.it, INAF - Osservatorio Astrofisico di Catania
Covey Kevin, kcovey@astro.cornell.edu, Lowell Observatory
Corsaro Enrico, eco@oact.inaf.it, INAF - Osservatorio Astrofisico di Catania
Metcalfe Travis, travis@ucar.edu, High Altitude Observatory
Marilli Ettore, Ettore.Marilli@oact.inaf.it, INAF - Osservatorio Astrofisico di Catania
Biazzo Katia, kbiazzo@na.astro.it, INAF - Osservatorio Astrofisico di Napoli
Leone Francesco, fleone@oact.inaf.it, INAF - Osservatorio Astrofisico di Catania

Abstract
Star spots are characteristics of solar-like activity observed in cool stars. They are tracers of magnetic flux tube emersion from the sub-photospheric convective layer and convey information about the stellar rotation and photospheric motions, such as the latitudinal drift of spots along the activity cycle and the differential rotation. These, together with the convection, are basic ingredients of the dynamo mechanism for the magnetic field intensification. The convective motions are also responsible for the excitation of solar-like pulsations. On the other hand, it seems that the magnetic field itself quenches the amplitude of those oscillations. Therefore, it is very important to study the trade-off between these two physical phenomena which are also closely related to our ability to derive the basic physical parameters of cool stars. Active stars in the Kepler field can be best selected on the basis of their coronal X-ray emission. X-ray-selected star samples contain a relevant fraction of young stars (with an age less than 1 Gyr) as well as more evolved stars in close binary systems (RS CVn type), that rotate much faster than if they were single, due to the tidal coupling of spin and orbit and, as a consequence, are amongst the brightest coronal X-ray sources. Both classes of stars are very important for the study magnetic activity and solar-like pulsations. One of the best sources of the information about X-ray stellar sources is the ROSAT All-Sky Survey and the XMM-Newton (X-ray Multi-Mirror Mission - Newton). They can be cross-matched with the Kepler Input Catalog in order to produce the list of X-ray sources in the Kepler field of view. Some of such stars have been already observed with the Kepler instrument and the exquisite quality of the Kepler photometry allowed a detailed analysis of their light curves. The study of KIC 8429280 by Frasca et al. (2011) shows how many fine details of the pattern of photospheric activity can be derived that way. An interesting point of that study is that the solar-like oscillations are not visible in the power spectrum of the Kepler short-cadence time series of KIC 8429280. That indicates that either the convective zone of this star excites oscillations of tiny amplitude or that the very strong magnetic field suppresses the pulsations. We present the first results of our study devoted to a few of the most interesting Kepler active sources which have been fully characterized, in terms of their basic stellar parameters and chromospheric activity, also thanks to high-resolution ground-based spectroscopic observations.