2010A&A...521A..24J

The very young brown dwarf candidate ChaHα8 was recently discovered to have a close (∼1AU) companion by means of radial velocity monitoring. We present additional radial velocity data obtained with UVES/VLT between 2007 and 2010, which significantly improve the orbit determination of the system. The combined data set spans ten years of radial velocity monitoring for ChaHα8. A Kepler fit to the data yields an orbital period of 5.2 years, an eccentricity of e=0.59, and a radial velocity semi-amplitude of 2.4km/s. A companion mass M₂sini (which is a lower limit due to the unknown orbital inclination i) of 25±7M_Jup and of 31±8M_Jup is derived when using model-dependent mass estimates for the primary of 0.07M_☉ and 0.10M_☉, respectively. Assuming random orientation of orbits in space, we find a very high probability that the companion of ChaHα8 is of substellar nature: With a greater than 50% probability (i≥60°), the companion mass is between 30 and 35M_Jup and the mass ratio M₂/M₁ smaller than 0.4; with a greater than 87% probability (i≥30°) the companion mass is between 30 and 69M_Jup and the mass ratio smaller than 0.7. The absence of any evidence of the companion in the cross-correlation function together with the size of the radial velocity amplitude also indicate a mass ratio of at most 0.7, and likely smaller. Furthermore, the new data exclude the possibility that the companion has a mass in the planetary regime (≤13M_Jup). We show that the companion contributes significantly to the total luminosity of the system: model-dependent estimates provide a minimum for the luminosity ratio L₂/L₁ of 0.2. ChaHα8 is the fourth known spectroscopic brown dwarf or very low-mass stellar binary with determined orbital parameters, and the second known very young one. With an age of only about 3Myr, it is of particular interest to very low-mass formation and evolution theories. In contrast to most other spectroscopic binaries, it has a relatively long orbital period and it might be possible to determine the astrometric orbit of the primary and, thus, the orbital inclination.