Binary frequency of very young brown dwarfs at separations smaller than 3 AU

Searches for companions of brown dwarfs by direct imaging mainly probe orbital separations greater than 3-10 AU. On the other hand, previous radial velocity surveys of brown dwarfs are mainly sensitive to separations smaller than 0.6 AU. It has been speculated that the peak of the separation distribution of brown dwarf binaries lies right in the unprobed range. This work extends high-precision radial velocity surveys of brown dwarfs for the first time out to 3 AU. Based on more than six years UVES/VLT spectroscopy the binary frequency of brown dwarfs and (very) low-mass stars (M4.25-M8) in Chamaeleon I was determined: 18(-12)(+20)% for the whole sample and 10(-8)(+18)% for the subsample of ten brown dwarfs and very low-mass stars (M less than or similar to 0.1 M-circle dot). Two spectroscopic binaries were confirmed, the brown dwarf candidate ChaH alpha 8 (previously discovered by Joergens & Muller) and the low-mass star CHXR 74. Since their orbital separations appear to be 1 AU or greater, the binary frequency at < 1 AU might be less than 10%. Now for the first time companion searches of (young) brown dwarfs cover the whole orbital separation range, and the following observational constraints for models of brown dwarf formation can be derived: (i) the frequency of brown dwarf and very low-mass stellar binaries at < 3 AU does not significantly exceed that at > 3 AU; i. e. direct imaging surveys do not miss a significant fraction of brown dwarf binaries; (ii) the overall binary frequency of brown dwarfs and very low-mass stars is 10-30%; (iii) the decline in the separation distribution of brown dwarfs towards smaller separations seems to occur between 1 and 3 AU; (iv) the observed continuous decrease in the binary frequency from the stellar to the substellar regime is confirmed at < 3 AU providing further evidence of a continuous formation mechanism from low-mass stars to brown dwarfs.