Evidence of magnetic star-planet interactions in the hd 189733 system from orbitally phased caii k variations

Magnetic star-planet interactions (SPI) provide a detection method and insight into exoplanet magnetic fields and, in turn, exoplanet interiors and atmospheric environments. These signatures can be sporadic and difficult to confirm for single-epoch observations of a system due to inhomogeneous stellar magnetospheres and periodic variability in stellar magnetism. Thus, an ideal SPI search consists of multiple epochs containing observations on consecutive nights spanning at least one complete planetary orbit. Such data sets are rare but do exist for some of the most intensely studied hot Jupiter systems. One such system is HD 189733 for which six suitable SPI data sets exist, the result of spectroscopic monitoring to perform some of the first SPI searches and also to study the star's magnetic field. Here we perform a uniform analysis of six archival Ca II K data sets for HD 189733, spanning 2006 June through 2015 July, in order to search for magnetic SPI signatures in the chromospheric line variations. We find significant evidence for modulations of Ca II K with a 2.29±0.04 day period in the 2013 August data, which is consistent with the planet's orbital period. The peak in the orbital variations occurs at forb≈0.9, which corresponds to the SPI emission leading the planet with a phase difference of Δf≈40° from the sub-planetary point. This is consistent with the phase-lead predictions of nonlinear force-free magnetic field SPI models. The stellar magnetic field strength at the planet's orbit was greatest in 2013 August, which, due to the energy released in magnetic SPI scaling with B∗, lends strength to the SPI interpretation.