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

TL;DR

This study precisely measures the orbit of XO-2b, confirming its near-zero eccentricity and prograde motion, and identifies an additional radial velocity variation suggesting further investigation is needed.

Contribution

The paper provides the first precise radial velocity measurements of XO-2b over two years, confirming its orbital characteristics and revealing an unexplained long-term velocity variation.

Findings

XO-2b has an eccentricity consistent with zero.

The orbit of XO-2b is prograde.

An additional radial velocity variation was detected.

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$\sigma$ ($e=0.045\pm0.024$) and the orbit of XO-2b is prograde (the sky-projected spin-orbit alignment angle $\lambda=10^{\circ}\pm72^{\circ}$). 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 $0.62\pm0.02$ $M_{\rm Jup}$. 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.