THE RADIAL AND ROTATIONAL VELOCITIES OF PSO J318.5338-22.8603, A NEWLY CONFIRMED PLANETARY-MASS MEMBER OF THE β PICTORIS MOVING GROUP

PSO J318.5338-22.8603 is an extremely red planetary-mass object that has been identified as a candidate member of the beta 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 beta 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 beta Pictoris. b and 51. Eridani. b, making it an important benchmark object in the sequence of the planetary-mass members of the beta 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 degrees 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.