The elusive atmosphere of WASP-12 b / High-resolution transmission spectroscopy with CARMENES

To date, the hot Jupiter WASP-12 b has been the only planet with confirmedorbital decay. The late F-type host star has been hypothesized to be surroundedby a large structure of circumstellar material evaporated from the planet. Weobtained two high-resolution spectral transit time series with CARMENES andextensively searched for absorption signals by the atomic species Na, H, Ca,and He using transmission spectroscopy, thereby covering the He I triplet withhigh resolution for the first time. We apply SYSREM for atomic linetransmission spectroscopy, introduce the technique of signal protection toimprove the results for individual absorption lines, and compare the outcomesto those of established methods. No transmission signals were detected and themost stringent upper limits as of yet were derived for the individualindicators. Nonetheless, we found variation in the stellar Halpha and He Ilines, the origin of which remains uncertain but is unlikely to be activity. Toconstrain the enigmatic activity state of WASP-12, we analyzed XMM-Newton X-raydata and found the star to be moderately active at most. We deduced an upperlimit for the X-ray luminosity and the irradiating X-ray and extremeultraviolet (XUV) flux of WASP-12 b. Based on the XUV flux upper limit and thelack of the He I signal, our hydrodynamic models slightly favor a moderatelyirradiated planet with a thermospheric temperature of <= 12,000 K, and aconservative upper limit of <= 4e12 g/s on the mass-loss rate. Our study doesnot provide evidence for an extended planetary atmosphere or absorption bycircumstellar material close to the planetary orbit.