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Published October 1, 2005 | Published
Journal Article Open

Measurement of Spin-Orbit Alignment in an Extrasolar Planetary System


We determine the stellar, planetary, and orbital properties of the transiting planetary system HD 209458 through a joint analysis of high-precision radial velocities, photometry, and timing of the secondary eclipse. Of primary interest is the strong detection of the Rossiter-McLaughlin effect, the alteration of photospheric line profiles that occurs because the planet occults part of the rotating surface of the star. We develop a new technique for modeling this effect and use it to determine the inclination of the planetary orbit relative to the apparent stellar equator (λ = -4º.4 ± 1º.4), and the line-of-sight rotation speed of the star (v sin /_★ = 4.70 ± 0.16 km s^(-1)). The uncertainty in these quantities has been reduced by an order of magnitude relative to the pioneering measurements by Queloz and collaborators. The small but nonzero misalignment is probably a relic of the planet formation epoch, because the expected timescale for tidal coplanarization is larger than the age of the star. Our determination of v sin /★ is a rare case in which rotational line broadening has been isolated from other broadening mechanisms.

Additional Information

© 2005 American Astronomical Society. Received 2005 April 25; accepted 2005 June 3. We acknowledge helpful discussions with T. Brown, E. Chiang, S. Gaudi, D. Lin, and G. Torres. We are grateful to J. Wisdom for advice on calculating the rate of tidal heating.Work by J.N.W. was supported by NASA through Hubble Fellowship grant HST-HF-01180.02-A, awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS 5-26555. The visit of E. L. T. to the University of Tokyo was supported by an invitation fellowship program for research in Japan from the Japan Society for Promotion of Science (JSPS). This work was partly supported by a Grant-in-Aid for Scientific Research from JSPS (grants 14102004 and 16340053) and by NASA grant NAG5-13148.

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