Improved radial velocity orbit of the young binary brown dwarf candidate Cha Hα8

Abstract

The very young brown dwarf candidate ChaHa8 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 of the system. The combined data set spans ten years of radial velocity monitoring for ChaHa8. A Kepler fit to the data yields an orbital period of 5.2 yrs, an eccentricity of 0.59, and a radial velocity semi-amplitude of 2.4 km/s. A companion mass M2sini (which is a lower limit due to the unknown orbital inclination) of 25 Mjup and of 31 Mjup is derived when using model-dependent mass estimates for the primary of 0.07 and 0.10 Msun, resp. Assuming random orientation of orbits in space, we find a very high probability that the companion of Chaha8 is of substellar nature: With a greater than 87% probability, the companion mass is between 30 and 69 Mjup and the mass ratio < 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 (<13 Mjup). We show that the companion contributes significantly to the total luminosity of the system: model-dependent estimates provide a minimum luminosity ratio L2/L1 of 0.2. ChaHa8 is the 4th known spectroscopic brown dwarf or very low-mass stellar binary with determined orbital parameters, and the 2nd known very young one. With an age of only ~3 Myr 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 period and it might be possible to determine the astrometric orbit of the primary and, thus, the orbital inclination.