Improved Measurement of the Rossiter-McLaughlin Effect in the Exoplanetary System HD 17156

We present an improved measurement of the Rossiter-McLaughlin effect for the exoplanetary system HD 17156, based on radial-velocity data gathered with the Subaru 8.2m telescope throughout the planetary transit of UT 2008 November 7. The data allow for a precise and independent determination of the projected spin-orbit angle of this system: $\lambda ={10.0}^{\circ }\pm {5.1}^{\circ }$. This result supersedes the previous claim of $\lambda ={62}^{\circ }\pm {25}^{\circ }$ by Narita et al., which was based on lower-precision data with poor statistics. Thus the stellar spin and planetary orbital axes of the HD 17156 system are likely to be well-aligned, despite the planet’s large orbital eccentricity suggesting a history of strong dynamical interactions.