A Determination of the Spin‐Orbit Alignment of the Anomalously Dense Planet Orbiting HD 149026

Abstract
Wereport35radialvelocitymeasurementsof HD149026takenwiththeKeckTelescope.Ofthesemeasurements,15 were made during the transit of the companion planet HD 149026b, which occurred on 2005 June 25. These velocities provide a high-cadence observation of the Rossiter-McLaughlin effect, the shifting of photospheric line profiles that occurswhenaplanetoccultsaportionoftherotatingstellarsurface.Wecombinetheseradialvelocitieswithpreviously publishedradialvelocityandphotometricdatasetsandderiveacompositebest-fitmodelforthestar-planetsystem.This modelconfirmsandimprovespreviouslypublishedorbitalparameters,includingtheremarkablysmallplanetaryradius, the planetary mass, and the orbital inclination, found to be Rp/RJup …0:718 0:065, Mp/MJup …0:352 0:025, and I … 86:1 1:4 , respectively. Together the planetary mass and radius determinations imply a mean planetary density of1:18˛0:38 0:30 gcm 3 .Thenewdataalsoallowforthedeterminationoftheanglebetweentheapparentstellarequatorand

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