Measurement of Spin‐Orbit Alignment in an Extrasolar Planetary System

We determine the stellar, planetary, and orbital properties of the transitingplanetary system HD 209458, through a joint analysis of high-precision radialvelocities, photometry, and timing of the secondary eclipse. Of primaryinterest is the strong detection of the Rossiter-McLaughlin effect, thealteration of photospheric line profiles that occurs because the planet occultspart of the rotating surface of the star. We develop a new technique formodeling this effect, and use it to determine the inclination of the planetaryorbit relative to the apparent stellar equator [lambda = (-4.4 +/- 1.4)degrees], and the line-of-sight rotation speed of the star [v*sin(I) = (4.70+/- 0.16) km/s]. The uncertainty in these quantities has been reduced by anorder of magnitude relative to the pioneering measurements by Queloz andcollaborators. The small but nonzero misalignment is probably a relic of theplanet formation epoch, because the expected timescale for tidalcoplanarization is larger than the age of the star. Our determination ofv*sin(I) is a rare case in which rotational line broadening has been isolatedfrom other broadening mechanisms.Comment: 21 pages, 6 figures. To match the version accepted by ApJ. Errors have been corrected in Table 1. (In previous versions, some confidence limits were mistakenly 80% rather than 90% limits.