KELT9 b's Asymmetric TESS Transit Caused by Rapid Stellar Rotation and SpinOrbit Misalignment
Abstract
KELT9 b is an ultrahot Jupiter transiting a rapidly rotating, oblate earlyAtype star in a polar orbit. We model the effect of rapid stellar rotation on KELT9 b's transit light curve using photometry from the Transiting Exoplanet Survey Satellite to constrain the planet's true spinorbit angle and to explore how KELT9 b may be influenced by stellar gravity darkening. We constrain the host star's equatorial radius to be 1.089 ± 0.017 times as large as its polar radius and its local surface brightness to vary by ∼38% between its hot poles and cooler equator. We model the stellar oblateness and surface brightness gradient and find that it causes the transit light curve to lack the usual symmetry around the time of minimum light. We take advantage of the lightcurve asymmetry to constrain KELT9 b's true spinorbit angle ( ${87^\circ }_{{11}^{^\circ }}^{{+10}^{^\circ }}$ ), agreeing with Gaudi et al. that KELT9 b is in a nearly polar orbit. We also apply a gravitydarkening correction to the spectral energy distribution model from Gaudi et al. and find that accounting for rapid rotation gives a better fit to available spectroscopy and yields a more reliable estimate for the star's polar effective temperature.
 Publication:

The Astronomical Journal
 Pub Date:
 July 2020
 DOI:
 10.3847/15383881/ab8fa3
 arXiv:
 arXiv:2004.14812
 Bibcode:
 2020AJ....160....4A
 Keywords:

 Exoplanets;
 Hot Jupiters;
 Exoplanet evolution;
 Stellar rotation;
 Gravity darkening;
 von Zeipel theorem;
 Exoplanet astronomy;
 Transit photometry;
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 Astrophysics  Earth and Planetary Astrophysics;
 Astrophysics  Solar and Stellar Astrophysics
 EPrint:
 Accepted for Publication in ApJ. arXiv admin note: text overlap with arXiv:1911.05025