On the Spin-Orbit Misalignment of the XO-3 Exoplanetary System

Creators: Winn, Joshua N.; Johnson, John Asher; Fabrycky, Daniel; Howard, Andrew W.; Marcy, Geoffrey W.; Narita, Norio; Crossfield, Ian J.; Suto, Yasushi; Turner, Edwin L.; Esquerdo, Gil; Holman, Matthew J.

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Abstract

We present photometric and spectroscopic observations of the 2009 February 2 transit of the exoplanet XO-3b. The new data show that the planetary orbital axis and stellar rotation axis are misaligned, as reported earlier by Hébrard and coworkers. We find the angle between the sky projections of the two axes to be 37.3 ± 3.7 deg, as compared to the previously reported value of 70 ± 15 deg. The significance of this discrepancy is unclear because there are indications of systematic effects. XO-3b is the first exoplanet known to have a highly inclined orbit relative to the equatorial plane of its parent star, and as such it may fulfill the predictions of some scenarios for the migration of massive planets into close-in orbits. We revisit the statistical analysis of spin-orbit alignment in hot-Jupiter systems. Assuming the stellar obliquities to be drawn from a single Rayleigh distribution, we find the mode of the distribution to be 13^(+5)_(–2) deg. However, it remains the case that a model representing two different migration channels—in which some planets are drawn from a perfectly aligned distribution and the rest are drawn from an isotropic distribution—is favored over a single Rayleigh distribution.

Additional Information

© 2009 American Astronomical Society. Received 2009 February 17; accepted 2009 May 19; published 2009 July 1. 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 the National Aeronautics and Space Administration, and was made possible by the generous financial support of the W. M. Keck Foundation. We thank Peter McCullough for reminding us about the near-coincidence of the orbital and rotational periods of XO-3b. We are grateful to Gáspár Bakos for trading telescope time at FLWO on short notice, and to Breann Sitarski and Tracy Ly for helping with the observations at Lick Observatory. We are indebted to Lara Winn for enabling this work to be completed in a timely fashion. This work was partly supported by the NASA Origins program through awards NNX09AD36G and NNX09AB33G. J.A.J. acknowledges support from an NSF Astronomy and Astrophysics Postdoctoral Fellowship (grant AST-0702821). This work was partly supported by World Premiere International Research Center Initiative (WPI Initiative), MEXT, Japan.

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