KELT-9 b’s Asymmetric TESS Transit Caused by Rapid Stellar Rotation and Spin-Orbit Misalignment

Kavli Affiliate: Robert F. Goeke

| First 5 Authors: John P. Ahlers, Marshall C. Johnson, Keivan G Stassun, Knicole D. Colon, Jason W. Barnes

| Summary:

KELT-9 b is an ultra hot Jupiter transiting a rapidly rotating, oblate
early-A-type star in a polar orbit. We model the effect of rapid stellar
rotation on KELT-9 b’s transit light curve using photometry from the Transiting
Exoplanet Survey Satellite (tess) to constrain the planet’s true spin-orbit
angle and to explore how KELT-9 b may be influenced by stellar gravity
darkening. We constrain the host star’s equatorial radius to be $1.089pm0.017$
times as large as its polar radius and its local surface brightness to vary by
$sim38$% 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 light curve asymmetry to constrain KELT-9 b’s true spin
orbit angle (${87^circ}^{+10^circ}_{-11^circ}$), agreeing with
citet{gaudi2017giant} that KELT-9 b is in a nearly polar orbit. We also apply
a gravity darkening correction to the spectral energy distribution model from
citet{gaudi2017giant} 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.

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