An accurate distance to 2M1207Ab

Context: In April 2004, the first image was obtained of a planetary mass companion (now known as 2M1207 b) in orbit around a self-luminous object different from our own Sun (the young brown dwarf 2MASSW J1207334-393254, hereafter 2M1207 A). That 2M1207 b probably formed via fragmentation and gravitational collapse offered proof that such a mechanism can form bodies in the planetary mass regime. However, the predicted mass, luminosity, and radius of 2M1207 b depend on its age, distance, and other observables, such as effective temperature.
Aims: To refine our knowledge of the physical properties of 2M1207 b and its nature, we accurately determined the distance to the 2M1207 A and b system by measuring of its trigonometric parallax at the milliarcsec level.
Methods: With the ESO NTT/SUSI2 telescope, we began a campaign of photometric and astrometric observations in 2006 to measure the trigonometric parallax of 2M1207 A.
Results: An accurate distance (52.4± 1.1 pc) to 2M1207A was measured. From distance and proper motions we derived spatial velocities that are fully compatible with TWA membership.
Conclusions: With this new distance estimate, we discuss three scenarios regarding the nature of 2M1207 b: (1) a cool (1150±150 K) companion of mass 4±1 M_{Jup}, (2) a warmer (1600±100 K) and heavier (8±2 M_{Jup}) companion occulted by an edge-on circumsecondary disk, or (3) a hot protoplanet collision afterglow.

Based on observations collected at the European Southern

Observatory, Chile (76.C-0543, 077.C-0112, 078.C-0158, 079.C-0229) and at Valinhos meridian circle.