A Moving Cluster Distance to the Exoplanet 2M1207b in the TW Hydrae Association

Abstract

A candidate extrasolar planet companion to the young brown dwarf 2MASSW J1207334-393254 (hereafter 2M1207) was recently discovered by Chauvin et al. They find that the temperature and luminosity of 2M1207b are consistent with a young, ~5MJ planet. The 2M1207 system is purported to be a member of the TW Hya association (TWA) and situated ~70 pc away. Using a revised space motion vector for TWA and improved proper motion for 2M1207, I use the moving cluster method to estimate the distance to the 2M1207 system and other TWA members. The derived distance for 2M1207 (53 ± 6 pc) forces the brown dwarf and planet to be half as luminous as previously thought. The inferred masses for 2M1207A and 2M1207b decrease to ~21 and ~3-4MJ, respectively, with the mass of 2M1207b well below the observed tip of the planetary mass function and the theoretical deuterium-burning limit. After removing probable Lower Centaurus Crux (LCC) members from the TWA sample, as well as the probable nonmember TWA 22, the remaining TWA membership is found to have distances of 49 ± 3 (s.e.m.) ± 12 (1 σ) pc and an internal one-dimensional velocity dispersion of 0.8 km s-1. There is weak evidence that the TWA is expanding, and the data are consistent with a lower limit on the expansion age of 10 Myr (95% confidence).