Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011ess.....2.2402b&link_type=abstract
American Astronomical Society, ESS meeting #2, #24.02
Astronomy and Astrophysics
Astronomy
Scientific paper
Barnes (2009) predicted that objects transiting fast-rotating stars would show unusual, asymmetric lightcurves if the transiter's orbit normal is not aligned with the stellar rotational pole -- i.e. in the case of a spin-orbit misalignment. Recently Gy. Szabo et al. (arXiv:1105.252) showed that KOI-13.01, one of the Kepler planet candidates announced by Borucki et al. (arXiv:1102.0541), rotates with Vsin(i)=65 km/s and shows a pronounced lightcurve asymmetry. Szabo et al. attributed this asymmetry to gravity darkening associated with rapid rotation, but did not measure the implied spin-orbit misalignment. Using the algorithm that I developed for Barnes (2009), I fit the folded transit lightcurve for KOI-13.01. I found that the lightcurve asymmetry indicates a stellar obliquity of 18.5 degrees relative to the plane of the sky (i.e., the stellar north pole is tilted toward the viewer by 18.5 degrees), and that the longitude of the ascending node for the planet is 214.5 degrees (i.e. that the planet transits the south pole of the star first while heading toward the equator at a 34.5 degree angle with respect to the stellar equator). The resulting net spin-orbit misalignment is 40.3 degrees. This technique does not, however, resolve the degeneracy between prograde and retrograde solutions -- hence the spin-orbit misalignment might then be 139.7 degrees if the planet orbits retrograde. The parent star of KOI-13.01 has a binary companion of similar mass with a separation of 1900 AU, which allows that the Kozai mechanism may be responsible for this system's spin-orbit misalignment. KOI-13.01 thus represents the first system for which the spin-orbit alignment has been measured purely photometrically. The photometric method can complement the Rossiter-McLaughlin effect to constrain spin-orbit alignment for planets orbiting fast-rotating stars.
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