Abstract

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 8.2-m 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 $||$=$| 10|${\rlap{^\circ}\hskip.5pt.\hskip1pt}$|0 |$ \pm $|5|${\rlap{^\circ}\hskip.5pt.\hskip1pt}$|1. This result supersedes the previous claim of |$\lambda $||$=$| 62|${}^\circ$||$ \pm $|25|${}^\circ$| by Narita et al. (2008, PASJ, 60, L1), 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.

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