The Relation between the Transit Depths of KIC 12557548b & the Stellar Rotation Period

TL;DR

This study confirms a statistically significant correlation between the transit depths of KIC 12557548b and the stellar rotation period, likely caused by the planet occulting starspots, with implications for understanding star-planet interactions.

Contribution

We provide robust evidence linking transit depth variations to stellar rotation and propose starspot occultation as the primary cause, refining previous hypotheses about star-planet interactions.

Findings

Transit depths are modulated with stellar rotation period.

Starspot occultation explains large transit depth variations.

Transit-timing variations are small and undetectable in Kepler data.

Abstract

Kawahara and collaborators analyzed the transits of the candidate disintegrating Mercury-mass planet KIC 12557548b and suggested that the transit depths were correlated with the phase of the stellar rotation. We analyze the transit depths of KIC 12557548b and confirm that there is indeed a robust, statistically significant signal in the transit depths at the rotation period of the spotted host star. This signal is more prominent in the first-half of the Kepler data, and is not due to leakage of the rotating spot signal into our measurement of the transit depths, or due to unocculted starspots. We investigate the suggestion that this signal could be due to an active region on the star, emitting enhanced ultraviolet or X-ray radiation leading to an increased mass loss rate of the planet; we confirm that such a scenario could cause both modulation of the transit depths of KIC 12557548b, and small enough transit-timing variations that they might not be detected in the Kepler data. Our preferred explanation for the fact that the transit depths of KIC 12557548b are modulated with the stellar rotation phase is that the candidate transiting planet is occulting starspots on this highly spotted star; such a scenario could cause transit depth variations as large as have been observed, and cause transit-timing variations small enough that they are arguably consistent with the Kepler data.