2016ApJ...819..133A

PSO J318.5338-22.8603 is an extremely red planetary-mass object that has been identified as a candidate member of the β Pictoris moving group based on its spatial position and tangential velocity. We present a high-resolution K-band spectrum of PSO J318.5338-22.8603. Using a forward modeling Markov Chain Monte Carlo approach, we report the first measurement of the radial velocity and v sin(i) of PSO J318.5-22, -6.0_–1.1^+0.8 km s^–1, and 17.5_–2.8^+2.3 km s^–1, respectively. We calculate the space velocity and position of PSO J318.5-22 and confirm that it is a member of the β Pictoris moving group. Adopting an age of 23 ± 3 Myr for PSO J318.5-22, we determine a mass of 8.3 ± 0.5 M_Jup and an effective temperature of 1127_–26⁺²⁴ K using evolutionary models. PSO J318.5338-22.8603 is intermediate in mass and temperature to the directly imaged planets β Pictoris b and 51 Eridani b, making it an important benchmark object in the sequence of the planetary-mass members of the β Pictoris moving group. Combining our v sin(i) measurement with recent photometric variability data, we constrain the inclination of PSO J318.5-22 to >29° and its rotational period to 5-10.2 hr. The equatorial velocity of PSO J318.5-22 indicates that its rotation is consistent with an extrapolation of the velocity-mass relationship for solar system planets.