A Prograde, Low-Inclination Orbit for the Very Hot Jupiter WASP-3b

We present new spectroscopic and photometric observations of the transiting exo- planetary system WASP-3. Spectra obtained during two separate transits exhibit the Rossiter-McLaughlin (RM) effect and allow us to estimate the sky-projected angle be- tween the planetary orbital axis and the stellar rotation axis, = 3.3+2.5 −4.4 degrees. This alignment between the axes suggests that WASP-3b has a low orbital inclination rela- tive to the equatorial plane of its parent star. During our first night of spectroscopic measurements, we observed an unexpected redshift briefly exceeding the expected sum of the orbital and RM velocities by 140 m s−1. This anomaly could represent the oc- cultation of material erupting from the stellar photosphere, although it is more likely to be an artifact caused by moonlight scattered into the spectrograph.United States. National Aeronautics and Space Administration (NASA) (NNX09AD36G)United States. National Aeronautics and Space Administration (NASA) (NNX09AB33G)National Science Foundation (U.S.) (Grant No. AST-0702821