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16-20 Jupiter mass RV companion orbiting the brown dwarf candidate ChaHa8.pdf

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16-20 Jupiter mass RV companion orbiting the brown dwarf candidate ChaHa8

a r X i v : 0 7 0 7 .3 7 4 4 v 1 [ a s t r o - p h ] 2 5 J u l 2 0 0 7 16–20MJup RV companion orbiting the brown dwarf candidate ChaHα 81 V. Joergens and A. Mu?ller Max-Planck Institute for Astronomy, Ko?nigstuhl 17, D-69117 Heidelberg, Germany viki@mpia.de, amueller@mpia.de accepted for publication in ApJ Letter ABSTRACT We report the discovery of a 16–20MJup radial velocity companion around the very young (～3Myr) brown dwarf candidate ChaHα 8 (M5.75–M6.5). Based on high-resolution echelle spec- tra of ChaHα 8 taken between 2000 and 2007 with UVES at the VLT, a companion was detected through RV variability with a semi-amplitude of 1.6 km s?1. A Kepler fit to the data yields an or- bital period of the companion of 1590 days and an eccentricity of e=0.49. A companion minimum mass M2 sin i between 16 and 20MJup is derived when using model-dependent mass estimates for the primary. The mass ratio q ≡ M2/M1 might be as small as 0.2 and, with a probability of 87%, it is less than 0.4. ChaHα 8 harbors most certainly the lowest mass companion detected so far in a close (～ 1AU) orbit around a brown dwarf or very low-mass star. From the uncertainty in the orbit solution, it cannot completely be ruled out that the companion has a mass in the planetary regime. Its discovery is in any case an important step towards RV planet detections around BDs. Further, ChaHα 8 is the fourth known spectroscopic brown dwarf or very low-mass binary system with an RV orbit solution and the second known very young one. Subject headings: binaries: spectroscopic — planetary systems — stars: individual ([NC98] Cha HA 8) — stars: low-mass, brown dwarfs — stars: pre-main sequence — techniques: radial velocities 1. Introduction Search for planetary or brown dwarf (BD) com- panions to BDs are of primary interest for un- derstanding planet and BD formation. There ex- ists no widely accepted model for the formation of BDs (e.g. recent review by Luhman et al. 2007). The frequency of BDs in multiple