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

Searches for companions of brown dwarfs by direct imaging probe mainly 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 if the peak of the separation distribution of brown dwarf binaries lies right in the unprobed range. The present work for the first time extends high-precision radial velocity surveys of brown dwarfs 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: it is 18 +20 −12 % for the whole sample and 10 +18 −8 % for the subsample of ten brown dwarfs and very low-mass stars (M 0.1 M ⊙). Two spectroscopic binaries were confirmed, these are the brown dwarf candidate Cha Hα 8 (previously discovered by Joergens & Müller), 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 is not significantly exceeding 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 of the separation distribution of brown dwarfs towards smaller separations seem to occur between 1 and 3 AU; (iv) the observed continuous decrease of the binary frequency from the stellar to the substellar regime is confirmed at <3 AU providing further evidence for a continuous formation mechanism from low-mass stars to brown dwarfs. (B34)) — stars: low-mass, brown dwarfs — stars: pre-main sequence — techniques: radial velocities