XO-2b: a Prograde Planet with a 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$\sigma$ ($e=0.045\pm 0.024$) and the orbit of XO-2b is prograde (the sky-projected spin-orbit alignment angle $\lambda =10^{\circ }\pm 72^{\circ }$). The poor constraint of $\lambda$ is due to a small impact parameter (the orbital inclination of XO-2b is almost 90${}^{\circ }$). The data also provide an improved estimate of the mass of XO-2b as $0.62\pm 0.02$ $M_{\mathrm{Jup}}$. We also find a long-term radial velocity variation in this system. Further radial velocity measurements are necessary to specify the cause of this additional variation.

6 pages, 4 figures, 2 tables, PASJ Letters in press

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