Improved radial velocity orbit of the young binary brown dwarf candidate Cha Hα 8^*

¹ Max-Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany e-mail: viki@mpia.de
² Zentrum für Astronomie Heidelberg, Institut für Theoretische Astrophysik, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
³ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
⁴ Zentrum für Astronomie Heidelberg, Landessternwarte, Königstuhl 12, 69117 Heidelberg, Germany

Received: 23 April 2010
Accepted: 9 June 2010

Abstract

The very young brown dwarf candidate Cha Hα 8 was recently discovered to have a close (~1 AU) 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 Cha Hα 8. 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 2.4 km s^-1. A companion mass M₂ sin i (which is a lower limit due to the unknown orbital inclination i) of 25 ± 7  and of 31 ± 8  is derived when using model-dependent mass estimates for the primary of 0.07  and 0.10 , respectively. Assuming random orientation of orbits in space, we find a very high probability that the companion of Cha Hα 8 is of substellar nature: With a greater than 50% probability (i ≥ 60), the companion mass is between 30 and 35  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 69  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 (≤13 ). 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. Cha Hα 8 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 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 orbital period and it might be possible to determine the astrometric orbit of the primary and, thus, the orbital inclination.