A second substellar companion in the Gliese 86 system -: A brown dwarf in an extrasolar planetary system

We report observations using the ESO adaptive optics system ADONIS of the known extrasolar planetary system Gliese 86. This star has a known 4 M-Jup sin i planet in a 15.8 day orbit and exhibits an addi tional, large, long-period, radial velocity drift (Queloz ct al. 2000). The coronographic images reveal a faint (J = 14.7, H = 14.4, K = 13.7) object at a projected distance of r = 1. 72 +/- 0. 02 and PA = 119 +/- 1 degrees. Gliese 86 and the discovered object share the same proper motion, as confirmed by independent measurements at three different epochs indicating that this system is gravitationally bound. From the infrared colors and magnitudes we infer an approximate spectral type for Gliese 86B at the transition from L to T dwarfs, also called early T dwarf assuming the classification by Leggett et al. (2000). Although present brown dwarf evolutionary models do not cover the mass and age range probed by this objects, an upper limit of the mass of about M-GJ86B less than or equal to 70 M-Jup can be inferred from the models by Baraffe et al. (1998). Dusty model atmospheres appear not to be compatible with the IR colors.