EXOPLANETARY SPIN-ORBIT ALIGNMENT: RESULTS FROM THE ENSEMBLE OF ROSSITER-MCLAUGHLIN OBSERVATIONS
Author(s)Fabrycky, Daniel C.; Winn, Joshua Nathan
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One possible diagnostic of planet formation, orbital migration, and tidal evolution is the angle ψ between a planet's orbital axis and the spin axis of its parent star. In general, ψ cannot be measured, but for transiting planets one can measure the angle λ between the sky projections of the two axes via the Rossiter-McLaughlin effect. Here, we show how to combine measurements of λ in different systems to derive statistical constraints on ψ. We apply the method to 11 published measurements of λ, using two different single-parameter distributions to describe the ensemble. First, assuming a Rayleigh distribution (or more precisely, a Fisher distribution on a sphere), we find that the peak value is less than 22° with 95% confidence. Second, assuming that a fraction f of the orbits have random orientations relative to the stars, and the remaining fraction (1 – f) are perfectly aligned, we find f < 0.36 with 95% confidence. This latter model fits the data better than the Rayleigh distribution, mainly because the XO-3 system was found to be strongly misaligned while the other 10 systems are consistent with perfect alignment. If the XO-3 result proves robust, then our results may be interpreted as evidence for two distinct modes of planet migration.
DepartmentMassachusetts Institute of Technology. Department of Physics
American Astronomical Society
Fabrycky, Daniel C., and Joshua N. Winn. “EXOPLANETARY SPIN-ORBIT ALIGNMENT: RESULTS FROM THE ENSEMBLE OF ROSSITER-MCLAUGHLIN OBSERVATIONS.” The Astrophysical Journal 696.2 (2009): 1230-1240.
Author's final manuscript