Further observations of the tilted planet XO-3: A new determination of spin-orbit misalignment, and limits on differential rotation

We report on observations of the Rossiter-McLaughlin (RM) effect for the XO-3 exoplanetary system. The RM effect for the system was previously measured by two different groups, but their results were statistically inconsistent. To obtain a decisive result, we observed two full transits of XO-3b with the Subaru 8.2-m telescope. By modeling these data with a new and more accurate analytic formula for the RM effect, we found the projected spin-orbit angle to be λ = 37.°3±3.°0, in good agreement with a previous finding by Winn et al. (2009 ApJ, 700, 302). In addition, an offset of ∼l22ms ^-1 was observed between the two transit datasets. This offset could be a signal of a third body in the XO-3 system, a possibility that should be checked with future observations. We also attempted to search for a possible signature of the stellar differential rotation in the RM data for the first time, and put weak upper limits on the differential rotation parameters.