APOSTLE: LONGTERM TRANSIT MONITORING AND STABILITY ANALYSIS OF XO-2b

The Apache Point Survey of Transit Lightcurves of Exoplanets (APOSTLE) observed 10 transits of XO-2b over a period of 3 yr. We present measurements that confirm previous estimates of system parameters like the normalized semi-major axis (a/R ), stellar density (ρ), impact parameter (b), and orbital inclination (i orb). Our errors on system parameters like a/R and ρ have improved by ~40% compared to previous best ground-based measurements. Our study of the transit times show no evidence for transit timing variations (TTVs) and we are able to rule out co-planar companions with masses ≥0.20  M ⊕ 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 s) measured for XO-2b, and hence remain undetectable.