Improved radial velocity orbit of the young binary brown dwarf candidate Cha Hα 8
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 M_Jup and of 31 ± 8 M_Jup is derived when using model-dependent mass estimates for the primary of 0.07 M⊙ and 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 M_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 69 M_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 (≤13 M_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. 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.
Based on observations obtained at the Very Large Telescope of the European Southern Observatory at Paranal, Chile in program 279.C-5018(A), 380.C-0619(A), 082.C-0023(A+B), 385.C-0510(A).- Publication:
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Astronomy and Astrophysics
- Pub Date:
- October 2010
- DOI:
- 10.1051/0004-6361/201014853
- arXiv:
- arXiv:1006.2383
- Bibcode:
- 2010A&A...521A..24J
- Keywords:
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- techniques: radial velocities;
- binaries: spectroscopic;
- stars: individual: Cha Hα 8;
- stars: pre-main sequence;
- stars: low-mass;
- planets and satellites: detection;
- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- Accepted for publication in A&