XO-2b: Transiting Hot Jupiter in a Metal-rich Common Proper Motion Binary

We report on a V = 11.2 early K dwarf, XO-2 (GSC 03413-00005), that hosts a R_p = 0.98±_0.01^0.03 R_J, M_p = 0.57 ± 0.06 M_J transiting extrasolar planet, XO-2b, with an orbital period of 2.615857 ± 0.000005 days. XO-2 has high metallicity, [Fe/H ] = 0.45 ± 0.02, high proper motion, μみゅー_tot = 157 mas yr⁻¹, and a common proper motion stellar companion with 31'' separation. The two stars are nearly identical twins, with very similar spectra and apparent magnitudes. Due to the high metallicity, these early K dwarf stars have a mass and radius close to solar, M_⋆ = 0.98 ± 0.02 M_☉ and R_⋆ = 0.97±_0.01^0.02 R_☉. The high proper motion of XO-2 results from an eccentric orbit (Galactic pericenter, R_per < 4 kpc) well confined to the Galactic disk (Z_max ∼ 100 pc). In addition, the phase-space position of XO-2 is near the Hercules dynamical stream, which points to an origin of XO-2 in the metal-rich, inner thin disk and subsequent dynamical scattering into the solar neighborhood. We describe an efficient Markov chain Monte Carlo algorithm for calculating the Bayesian posterior probability of the system parameters from a transit light curve.