Periodic transit timing variations and refined system parameters of the exoplanet XO-6b

TL;DR

This study refines the parameters of exoplanet XO-6b, detects significant transit timing variations, and suggests possible causes, including an unseen low-mass companion, through combined photometric and radial-velocity observations.

Contribution

The paper provides refined orbital and planetary parameters for XO-6b and identifies periodic transit timing variations, proposing a new explanation involving resonant perturbations from an unseen planet.

Findings

XO-6b is about 10% larger, with a significant change in orbital inclination.

Detected transit timing variations with a semi-amplitude of 14 minutes and a period of 450 days.

Radial velocity data do not support a third stellar-mass object causing the variations.

Abstract

Only a few exoplanets are known to orbit around fast rotating stars. One of them is XO-6b, which orbits an F5V-type star. Shortly after the discovery, we started multicolor photometric and radial-velocity follow-up observations of XO-6b, using the telescopes of Astronomical Institute of the Slovak Academy of Sciences. Our main scientific goals were to better characterize the planetary system and to search for transit timing variations. We refined several planetary and orbital parameters. Based on our measurements, the planet XO-6b seems to be about 10% larger, which is, however, only about $2\sigma$ difference, but its orbit inclination angle, with respect to the plane of the sky, seems to be significantly smaller, than it was determined originally by the discoverers. In this case we found about $9.5\sigma$ difference. Moreover, we observed periodic transit timing variations of XO-6b with a semi-amplitude of about 14 min and with a period of about 450 days. There are two plausible explanations of such transit timing variations: (1) a third object in the system XO-6 causing light-time effect, or (2) resonant perturbations between the transiting planet XO-6b and another unknown low-mass planet in this system. From the O-C diagram we derived that the assumed third object in the system should have a stellar mass, therefore significant variations are expected in the radial-velocity measurements of XO-6. Since this is not the case, and since all attempts to fit radial velocities and O-C data simultaneously failed to provide a consistent solution, more realistic is the second explanation.