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

A candidate extrasolar planet companion to the young brown dwarf 2MASSWJ1207334-393254 (2M1207) was recently discovered by Chauvin et al. They findthat 2M1207 B's temperature and luminosity are consistent with being a young,\~5 M_Jupiter planet. The 2M1207 system is purported to be a member of the TWHya association (TWA), and situated ~70 pc away. Using a revised space motionvector for TWA, and improved proper motion for 2M1207, I use the moving clustermethod to estimate the distance to the 2M1207 system and other TWA members. Thederived distance for 2M1207 (53+-6 pc) forces the brown dwarf and planet to behalf as luminous as previously thought. The inferred masses for 2M 1207 A and Bdecrease to ~21 M_Jup and ~3-4 M_Jup, respectively, with the mass of B beingwell below the observed tip of the planetary mass function and the theoreticaldeuterium-burning limit. After removing probable Lower Centaurus-Crux (LCC)members from the TWA sample, as well as the probable non-member TWA 22, theremaining TWA members are found to have distances of 49 +- 3(s.e.m.) +-12(1sigma) pc, and an internal 1D velocity dispersion of 0.8(+0.3,-0.2) km/s.There is weak evidence that the TWA is expanding, and the data are consistentwith a lower limit on the expansion age of 10 Myr (95% confidence).Comment: ApJ, in pres