A multiwavelength study of the hierarchical triple HD 181068. A test bed for studying star-planet interaction?

HD 181068 is the only compact, triply eclipsing, hierarchical triple system containing a giant star that is known to date. With its central, highly active G-type giant orbited by a close pair of main-sequence dwarfs, the system is ideal for studying tidal interactions. We carried out a multiwavelength study to characterize the magnetic activity of the HD 181068 system. To this end, we obtained inand out-of-eclipse X-ray snapshots with XMM-Newton and an optical spectrum, which we analyzed along with the Kepler light curve. The primary giant shows strong quiescent X-ray emission at a level of 2 10 31 erg s 1 , an S-index of 0:41 0:01, and marked whitelight flares releasing up to 6 10 38 erg in the Kepler band. During the second X-ray observation, we found a three-times elevated ‐ yet decaying ‐ level of X-ray emission, which might be due to an X-ray flare. The high level of magnetic activity is compatible with the previously reported absence of solar-like oscillations in the giant, whose atmosphere, however, undergoes tidally induced oscillations imposed by the changing configuration of the dwarf-binary. We found that the driving force exciting these oscillations is comparable to the disturbances produced by a typical hot Jupiter, making the system a potential test bed for studying the e ects of tidal interactions also present in planetary systems.