APOSTLE: Longterm Transit Monitoring and Stability Analysis of XO-2b

TL;DR

This study uses long-term transit observations of XO-2b to refine system parameters, confirm orbital stability, and analyze transit timing variations, finding no evidence for co-planar companions above 0.20 Earth masses.

Contribution

It provides improved measurements of XO-2b's system parameters and assesses the stability and TTV signals of potential nearby planets, including Earth-mass perturbers.

Findings

Refined system parameters with ~40% reduced errors.

No evidence of transit timing variations indicating no massive nearby companions.

Many stable configurations produce undetectable TTVs, including Earth-mass perturbers.

Abstract

The Apache Point Survey of Transit Lightcurves of Exoplanets (APOSTLE) observed 10 transits of XO-2b over a period of three years. We present measurements which confirm previous estimates of system parameters like the normalized semi-major axis (a/R_{*}), stellar density (\rho_{*}), impact parameter (b) and orbital inclination (i_{orb}). Our errors on system parameters like a/R_{*} and \rho_{*} have improved by ~40% compared to previous best ground-based measurements. Our study of the transit times show no evidence for transit timing variations and we are able to rule out co-planar companions with masses \ge 0.20 \mearth\ in low order mean motion resonance with XO-2b. We also explored the stability of the XO-2 system given various orbital configurations of a hypothetical planet near the 2:1 mean motion resonance. We find that a wide range of orbits (including Earth-mass perturbers) are both dynamically stable and produce observable TTVs. We find that up to 51% of our stable simulations show TTVs that are smaller than the typical transit timing errors (~20 sec) measured for XO-2b, and hence remain undetectable.