• DOI:10.1051/0004-6361/201014853
• Corpus ID: 54989533

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

@article{Joergens2010ImprovedRV,
  title={Improved radial velocity orbit of the young binary brown dwarf candidate Cha H$\alpha$ 8},
  author={Viki Joergens and Andreas Mueller and Sabine Reffert},
  journal={Astronomy and Astrophysics},
  year={2010},
  volume={521}
}

• V. Joergens, A. Mueller, S. Reffert
• Published 11 June 2010
• Physics, Geology
• Astronomy and Astrophysics

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 sini (which is a lower limit due to the unknown orbital inclination i )o f 25± 7 MJup and of 31 ± 8 MJup is derived when using model-dependent mass estimates for the primary of 0.07 Mand 0.10 M� , 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 MJup 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 MJup 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 MJup). 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.