Binary brown dwarfs in the Galactic halo?

Abstract

A B STR A CT Microlensing events towards the Large Magellanic Cloud imply that a sizeable fraction of dark matter is in the form of MACHOs (Massive Astrophysical Compact Halo Objects), presumably located in the halo of the Galaxy. Within the present uncertainties, brown dwarfs are viable candidates for MACHOs. Various considerations strongly suggest that a large number of MACHOs should actually consist of binary brown dwarfs. Yet this circumstance appears to be in flat contradiction with the fact that MACHOs have been detected as unresolved objects so far. We show that such an apparent paradox does not exist within a model in which MACHOs are clumped into dark clusters along with cold molecular clouds, since dynamical friction on these clouds makes binary brown dwarfs very close. Moreover, we argue that future microlensing experiments with more accurate photometric observations can resolve binary brown dwarfs.