Preprint typeset using LATEX style emulateapj v. 11/10/09 A HIGH STELLAR OBLIQUITY IN THE WASP-7 EXOPLANETARY SYSTEM

We measure a tilt of 86 ± 6 ◦ between the sky projections of the rotation axis of the WASP-7 star, and the orbital axis of its close-in giant planet. This measurement is based on observations of the Rossiter-McLaughlin (RM) effect with the Planet Finder Spectrograph on the Magellan II telescope. The result conforms with the previously noted pattern among hot-Jupiter hosts, namely, that the hosts lacking thick convective envelopes have high obliquities. Because the planet’s trajectory crosses a wide range of stellar latitudes, observations of the RM effect can in principle reveal the stellar differential rotation profile; however, with the present data the signal of differential rotation could not be detected. The host star is found to exhibit radial-velocity noise (“stellar jitter”) with an amplitude of ≈30 m s−1 over a timescale of days. Subject headings: techniques: spectroscopic — stars: rotation — planetary systems — planets and satellites: formation — planet-star interactions — stars: individual (WASP-7) 1