Asymmetric transit curves as indication of orbital obliquity: clues from the late-type dwarf companion in KOI-13

TL;DR

This study analyzes asymmetric transit curves of KOI-13 to reveal an oblique orbit of a likely hot brown dwarf around a rapidly rotating star, demonstrating a new method to detect orbital obliquity without spectroscopy.

Contribution

It introduces a novel approach to infer orbital obliquity from transit curve asymmetries in a binary star system, identifying the nature of the transiting object.

Findings

KOI-13 is a binary with rapidly rotating stars.

The transiting object is a hot brown dwarf, not a planet.

Transit asymmetry indicates an oblique orbit.

Abstract

KOI-13.01, a planet-sized companion in an optical double star was announced as one of the 1235 Kepler planet candidates in February 2011. The transit curves show significant distortion that was stable over the ~130 days time-span of the data. Here we investigate the phenomenon via detailed analyses of the two components of the double star and a re-reduction of the Kepler data with pixel-level photometry. Our results indicate that KOI-13 is a common proper motion binary, with two rapidly rotating components (v sin i ~ 65--70 km/s). We identify the host star of KOI-13.01 and conclude that the transit curve asymmetry is consistent with a companion orbiting a rapidly rotating, possibly elongated star on an oblique orbit. After correcting the Kepler light curve to the second light of the optical companion star, we derive a radius of 2.2 R_J for the transiter, implying an irradiated late-type dwarf, probably a hot brown dwarf rather than a planet. KOI-13 is the first example for detecting orbital obliquity for a substellar companion without measuring the Rossiter-McLaughlin effect from spectroscopy.