A highly inclined orbit for the 110-day period M-dwarf companion KOI-368.01

TL;DR

This study detects a highly inclined orbit of a late M-dwarf companion to KOI-368, revealing significant spin-orbit misalignment and providing insights into binary system formation theories.

Contribution

It presents the first measurement of true obliquity for a long-period M-dwarf companion, using Kepler transit data and spectroscopic follow-up.

Findings

Measured true obliquity of 69 degrees.

Detected secondary eclipse constraining companion temperature.

Determined orbital eccentricity to be 0.1429.

Abstract

We report the detection of asymmetry in the transit light curves of the 110-day period companion to KOI-368, a rapidly rotating A-dwarf. The significant distortion in the transit light curve is attributed to spin-orbit misalignment between the transiting companion and the gravity darkened host star. Our analysis was based on 11 Long Cadence and 2 Short Cadence transits of KOI-368.01 from the Kepler mission, as well as stellar parameters determined from our follow-up spectroscopic observation. We measured the true obliquity between the orbit normal and the stellar rotation axis to be 69 +9/-10 deg. We also find a secondary eclipse event with depth 29 +/- 3 ppm at phase 0.59, from which the temperature of the companion is constrained to 3060 +/- 50 K, indicating that KOI-368.01 is a late M-dwarf. The eccentricity is also calculated from the eclipse to be 0.1429 +/- 0.0007. The long period, high obliquity, and low eccentricity of KOI-368.01 allow us to limit a number of proposed theories for the misalignment of binary systems.