The clockwork is moving on -- a combined analysis of TESS and Kepler measurements of Kepler-13Ab

TL;DR

This paper combines Kepler and TESS data to analyze the precession and spin-orbit misalignment of exoplanet Kepler-13Ab, revealing a nearly orthogonal stellar spin axis and a slow change in transit impact parameter.

Contribution

It presents a self-consistent model of Kepler-13Ab using combined space telescope data, accounting for stellar oblateness and gravity darkening effects.

Findings

Impact parameter is decreasing at -0.011 per year.

Stellar spin axis is nearly orthogonal with an inclination around 100°.

Transit duration and impact parameter show a linear drift over time.

Abstract

Kepler-13Ab (KOI-13) is an exoplanet orbiting a rapidly rotating A-type star. The system shows a significant spin-orbit misalignment and a changing transit duration most probably caused by the precession of the orbit. Here we present a self-consistent analysis of the system combining {\it Kepler} and TESS observations. We model the light curves asssuming a planet transits a rotating oblate star which has a strong surface temperature gradient due to rotation-induced gravity darkening. The transit chord moves slowly as an emergent feature of orbital precession excited by the oblate star with a decline rate in the impact parameter of ${\rm d}b/{\rm d}t = -0.011/{\rm yr}$, and with an actual value of $b=0.19$ for the latest TESS measurements. The changing transit duration that was measured from {\it Kepler} Q2 and Q17 quarters and the TESS measurements indicates a linear drift of the impact parameter. The solutions for the stellar spin axis suggest a nearly orthogonal aspect, with inclination around $100^\circ$.