HALO DARK CLUSTERS OF BROWN DWARFS AND MOLECULAR CLOUDS

The discovery of massive astrophysical compact halo objects (MACHOs) i n microlensing experiments makes it compelling to understand their phy sical nature, as well as their formation mechanism. Within the present uncertainties, brown dwarfs are a viable candidate for MACHOs, and th e present paper deals with this option. According to a recently propos ed scenario, brown dwarfs are clumped with cold molecular clouds into dark clusters--in several respects similar to globular clusters--that form in the outer part of the Galactic halo. Here we analyze the dynam ics of these dark clusters and address the possibility that a sizable fraction of MACHOs are binary brown dwarfs. We also point out that Ly alpha absorption systems fit naturally within the present picture.