2010A&A...516A.112N

It is difficult to determine masses and test formation models for brown dwarfs, because they are always above the main sequence, so that there is a degeneracy between mass and age. However, for brown dwarf companions to normal stars, such determinations may be possible, because one can know the distance and age of the primary star. As a result, brown dwarf companions are well-suited to testing formation theories and atmosphere models. With more adaptive optics images available, we aim at detecting orbital motion for the first time in the system TWA 5 A+B. We measured separation and position angle between TWA 5 A and B in each high-resolution image available and followed their change in time, because B should orbit around A. The astrometric measurement precision is about one milli arcsec. With ten year difference in epoch, we can clearly detect orbital motion of B around A, a decrease in separation by ∼0.0054'' per year and a decrease in position angle by ∼0.26° per year. TWA 5 B is a brown dwarf with ∼25 Jupiter masses (Neuhaeuser et al., 2000A&A...360L..39N), but having large error bars (4 to 145 Jupiter masses, Neuhaeuser et al., 2009, AIPC, 1094, 884). Given its large projected separation from the primary star, ∼86AU, and its young age (∼10Myr), it has probably formed star-like, and would then be a brown dwarf companion. Given the relatively large changes in separation and position angle between TWA 5 A and B, we can conclude that they orbit around each other on an eccentric orbit. Some evidence is found for a curvature in the orbital motion of B around A - most consistent with an elliptic (e=0.45) orbit. Residuals around the best-fit ellipse are detected and show a small-amplitude (∼18mas) periodic sinusoid with ∼5.7yr period, i.e., fully consistent with the orbit of the inner close pair TWA 5 Aa+b. Measuring these residuals caused by the photocenter wobble - even in unresolved images - can yield the total mass of the inner pair, so can test theoretical pre-main sequence models.