XO-2b: A Prograde Planet with Negligible Eccentricity and an Additional Radial Velocity Variation

Abstract

We present precise radial velocities of XO-2 taken with the Subaru HDS, covering two transits of XO-2b with an interval of nearly two years. The data suggest that the orbital eccentricity of XO-2b is consistent with zero within 2 σ (e = 0.045±0.024), and that the orbit of XO-2b is prograde (the sky-projected spin-orbit alignment angle λ = 10°±72°). The poor constraint of λ is due to a small impact parameter (the orbital inclination of XO-2b is almost 90°). The data also provide an improved estimate of the mass of XO-2b as 0.62±0.02MJup. Further,we find a long-term radial velocity variation in this system. Additional radial velocity measurements are necessary to specify the cause of this additional variation. © 2011. Astronomical Society of Japan.