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Evidence of Possible Spin-orbit Misalignment Along the Line of Sight in Transiting Exoplanet Systems
Of the 26 transiting exoplanet systems with measurements of theRossiter-McLaughlin (RM) effect, eight have now been found to besignificantly spin-orbit misaligned in the plane of the sky (i.e., RMmisalignment angle |?| >~ 30° and inconsistent with? = 0°). Unfortunately, the RM effect does not constrain thecomplement misalignment angle between the orbit of the planet and thespin of its host star along the line of sight (LOS). I use a simplemodel of stellar rotation benchmarked with observational data tostatistically identify 10 exoplanet systems from a sample of 75 forwhich there is likely a significant degree of spin-orbit misalignmentalong the LOS: HAT-P-7, HAT-P-14, HAT-P-16, HD 17156, Kepler-5,Kepler-7, TrES-4, WASP-1, WASP-12, and WASP-14. All 10 systems have hoststellar masses M * in the range 1.2 M sun <~ M* <~ 1.5 M sun, and the probability of thisoccurrence by chance is less than one in ten thousand. In addition, theplanets in the candidate-misaligned systems are preferentially massiveand eccentric. The coupled distribution of misalignment from the RMeffect and from this analysis suggests that transiting exoplanets aremore likely to be spin-orbit aligned than expected given predictions fora transiting planet population produced entirely by planet-planetscattering or Kozai cycles and tidal friction. For that reason, thereare likely two populations of close-in exoplanet systems: a populationof aligned systems and a population of apparently misaligned systems inwhich the processes that lead to misalignment or to the survival ofmisaligned systems operate more efficiently in systems with massivestars and planets.
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