4 Discussion

In the color-magnitude diagram we find that Gl 86B appears in a region well below the hydrogen burning limit (Fig. 2). It is lying between the evolutionary model tracks of COND and DUSTY (see Chabrier et al. 2000). The COND model represents an atmosphere in which a rapid grain settlement below the photosphere takes place, thus ignoring the influence of condensates on the radiative transfer but taking dust into account in the equation of state. On the other hand, DUSTY models use all condensates in the equation of state as well as in the radiative transfer equations. As pointed out in Chabrier et al. (2000), these two models represent the two extreme possible cases of a brown dwarf atmospheres. As can be seen from Fig. 2 neither the DUSTY nor the COND models seem to fit Gl 86B. Assuming coevality, an age of Gl 86 of several billion years (Queloz et al. 2000), and using the NextGen track (Baraffe et al. 1998) for 1 and 10 Gyr objects we can at least infer an upper limit for the mass of Gl 86B of about ${M}_{{\rm GJ86B}} \le 60{-}70\ {M}_{{\rm Jup}}$. As this object seems to possess methane in its atmosphere we can follow the arguments by Leggett et al. (2000) and estimate a temperature to be about 1300 K which leads to a mass estimate of 40-70 ${M}_{{\rm Jup}}$ (Burrows et al. 1997).

Using the $K_{\rm S}$ band magnitude we infer an approximate spectral type of Gl 86B using the data of Kirkpatrick et al. (2000). Figure 3 is a reproduction of Fig. 8 from Kirkpatrick et al. (2000) including the Gl 86B data. As pointed out by these authors there is a gap of about 2.5 mag in the $K_{\rm S}$ band between the latest L dwarfs and the T dwarfs which is caused by the strong methane absorption features in this wavelength region. We find that Gl 86B is falling right into this gap; it seems to represent a transition object between the L and T dwarf regime. Due to the probable existance of methane it would be a so-called "early T-dwarf'', a class proposed by Leggett et al. (2000) based on their finding of three objects with similar ( $J-K \approx 1$) colours and spectroscopic confirmation of the presence of methane. We speculate that Gl 86B is a (several Gyr) old transition object between the L and T dwarf regime in whose atmosphere dust has already settled below the photosphere and does not dominate the appearance of this object. Detailed spectroscopy is necessary to prove this hypothesis.

Assuming a mass of 50 ${M}_{{\rm Jup}}$ for Gl 86B orbiting at a distance of 18.75 AU around Gl 86 introduces a Doppler-shift amplitude of the order of about 0.5 kms^-1, with a period of $\approx$100 yrs, depending on the viewing geometry. The radial velocities available might contain this additional component. However, the companion found can not account for the long term trend in the radial velocities as observed by the CORAVEL survey: CORAVEL observed a long term drift of more than about 2 kms^-1 over more than 10 years. This companion, if real, must be fairly massive, and we should have easily spotted it, unless it is hidden by the primary. New and higher precision radial velocities could clarify this point. The Gl 86 system might contain even more components.

The detection of Gl 86B was only possible by using the high angular resolution of an adaptive optics system combined with a coronographic mask to obtain a high sensitivity close to a bright star. In the near future several such systems at 8 m class telescopes will become available thus offering the opportunity to detect the orbital motion and to do spectroscopy of this very interesting object.

The Gl 86 system is one of the few systems where a brown dwarf is found as companion to a star. In addition Gl 86 is also orbited by an extrasolar planet. It is therefore the second such system as HD 168443 (Udry et al. 2000) was found by radial velocity data to also be orbited by a planet and a brown dwarf. In view of the very small number of brown dwarfs as companions to stars, the existance of already two systems hosting a brown dwarf and a planetary companion is even more puzzling and raises the question whether star-planet-BD systems are more frequent than previously thought.

Acknowledgements

It's a pleasure to acknowledge the support by the 3.6 m telescope team during the observations, especially by N. Ageorges, K. Brooks, A. Gonzalez, O. Marco, V. Meriño and E. Wenderoth. We thank A. Hatzes for his comments on an early version of this paper. Also J. Gizis, the referee, pointed out several improvements to this paper. We thank ESO's OPC and DDTC for the generous allocation of observing time. E.P. wishes to thank the CNRS-INSU Program for Planetogy for a supporting travel grant. This work has been supported in part by the National Science Foundation Science and Technology Center for Adaptive Optics, managed by the University of California at Santa Cruz under cooperative agreement No. AST-9876783.