L69 The Astrophysical Journal, 653: L69–L72, 2006 December 10  2006. The American Astronomical Society. All rights reserved. Printed in U.S.A. MEASUREMENT OF THE SPIN-ORBIT ALIGNMENT IN THE EXOPLANETARY SYSTEM HD 189733 1 Joshua N. Winn, 2 John Asher Johnson, 3 Geoffrey W. Marcy, 3 R. Paul Butler, 4 Steven S. Vogt, 5 Gregory W. Henry, 6 Anna Roussanova, 2 Matthew J. Holman, 7 Keigo Enya, 8 Norio Narita, 9 Yasushi Suto, 9 and Edwin L. Turner 10 Received 2006 September 15; accepted 2006 November 1; published 2006 November 30 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 . This is the third case of a “hot l p -1° .4  1° .1 Jupiter” for which l has been measured. In all three cases l is small, ruling out random orientations with 99.96% confidence, and suggesting that the inward migration of hot Jupiters generally preserves spin-orbit alignment. Subject headings: planetary systems — planetary systems: formation — stars: individual (HD 189733) — stars: rotation 1. INTRODUCTION A primary reason to study planets of other stars is to learn how typical (or unusual) are the properties of the solar system. For example, the nearly circular orbits of solar system planets were once considered normal, but we now know that eccentric orbits of Jovian planets are common (see, e.g., Halbwachs et al. 2005 or Fig. 3 of Marcy et al. 2005). Likewise, gas giants were once thought to inhabit only the far reaches of planetary systems, an assumption that was exploded by the discovery of “hot Jupiters” (Mayor & Queloz 1995; Butler et al. 1997). This inspired theo- retical work on planetary migration mechanisms that can deliver Jovian planets to such tight orbits (as recently reviewed by Thom- mes & Lissauer [2005] and Papaloizou & Terquem [2006]). Another striking pattern in the solar system is the close align- ment between the planetary orbits and the solar spin axis. The orbit normals of the eight planets are within a few degrees of one another (Cox 2000, p. 295), and the Earth’ s orbit normal i s only 7° from the solar spin axis (Beck & Giles 2005 and ref- erences therein). Presumably this alignment dates back 5 Gyr, when the Sun and planets condensed from a single spinning disk. Whether or not this degree of alignment is universal is unknown. For hot Jupiters in particular, one might wonder whether migra- tion enforces or perturbs spin-orbit alignment. For exoplanets, the angle between the stellar spin axis and planetary orbit normal (as projected on the sky) can be measured via the Rossiter-McLaughlin (RM) effect: the spectral distortion 1 Data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Tech- nology, the University of California, and NASA, and was made possible by the generous financial support of the W. M. Keck Foundation. 2 Department of Physics, and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139. 3 Department of Astronomy, University of California, MC 3411, Berkeley, CA 94720. 4 Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, NW, Washington, DC 20015-1305. 5 UCO/Lick Observatory, University of California at Santa Cruz, Santa Cruz, CA 95064. 6 Center of Excellence in Information Systems, Tennessee State University, 3500 John A. Merritt Boulevard, Box 9501, Nashville, TN 37209. 7 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cam- bridge, MA 02138. 8 Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510, Japan. 9 Department of Physics, University of Tokyo, Tokyo 113-0033, Japan. 10 Princeton University Observatory, Peyton Hall, Princeton, NJ 08544. observed during a transit due to stellar rotation. The planet hides some of the velocity components that usually contribute to line broadening, resulting in an “anomalous Doppler shift” (Ohta et al. 2005; Gime ´nez 2006; Gaudi & Winn 2007). Observations of the exoplanetary RM effect have been re- ported for HD 209458 (Bundy & Marcy 2000; Queloz et al. 2000; Winn et al. 2005; Wittenmyer et al. 2005) and HD 149026 (Wolf et al. 2007). Here we report observations of the RM effect for HD 189733. This system, discovered by Bouchy et al. (2005), consists of a K dwarf with a transiting Jovian planet ( ) in a 2.2 day orbit. Our observations are pre- M p 1.15M P J sented in § 2, our model in § 3, and our results in § 4, followed by a discussion in § 5. 2. OBSERVATIONS We observed the transit of UT 2006 August 21 with the Keck I 10 m telescope and the High Resolution Echelle Spectrometer (HIRES; Vogt et al. 1994) following the usual protocols of the California-Carnegie planet search, as summarized here. We em- ployed the red cross-disperser and placed the absorption cell I 2 into the light path to calibrate the instrumental response and the wavelength scale. The slit width was 0 .85 and the typical ex- posure time was 3 minutes, giving a resolution of 70,000 and a signal-to-noise ratio (S/N) of 300 pixel . We obtained 70 spectra -1 over 7.5 hr bracketing the predicted transit midpoint. To these were added 16 spectra that had been obtained by the California- Carnegie group at random orbital phases. We determined the relative Doppler shifts with the algorithm of Butler et al. (1996). We estimated the measurement uncer- tainties based on the scatter in the solutions for each 2 section ˚ A of the spectrum. For the spectra obtained on 2006 August 21 the typical measurement error was 0.8 m s , while for the -1 other 16 spectra the error was ≈1.3 m s . The data are given -1 in Table 1 and plotted in Figure 1, with enlarged error bars to account for the intrinsic velocity noise of the star (see § 3). We also needed accurate photometry to pin down the plan- etary and stellar radii and the orbital inclination. We observed the transit of UT 2006 July 21 with KeplerCam on the 1.2 m telescope at the Fred L. Whipple Observatory on Mt. Hopkins, Arizona. We used the Sloan Digital Sky Survey z-band filter and an exposure time of 5 s. After bias subtraction and flat- field division, we performed aperture photometry of HD 189733 and 14 comparison stars. The light curve of each com-