Measurement of the Spin-Orbit Alignment in the Exoplanetary System HD 189733

Abstract

We present spectroscopy of a transit of the exoplanet HD 189733b. By modeling the Rossiter-McLaughlin effect (the anomalous Doppler shift due to the partial eclipse of the rotating stellar surface), we find the angle between the sky projections of the stellar spin axis and orbit normal to be λ=-1.4d+/-1.1d. This is the third case of a ``hot Jupiter'' for which λ has been measured. In all three cases λ is small, ruling out random orientations with 99.96% confidence, and suggesting that the inward migration of hot Jupiters generally preserves spin-orbit alignment. Data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA, and was made possible by the generous financial support of the W. M. Keck Foundation.