Microlensing, Brown Dwarfs and GAIA
  The GAIA satellite can precisely measure the masses of nearby brown dwarfs
and lower main sequence stars by the microlensing effect. The scientific yield
is maximised if the microlensing event is also followed with ground-based
telescopes to provide densely sampled photometry. There are two possible
strategies. First, ongoing events can be triggered by photometric or
astrometric alerts by GAIA. Second, events can be predicted using known high
proper motion stars as lenses. This is much easier, as the location and time of
an event can be forecast. Using the GAIA source density, we estimate that the
sample size of high proper motion ($>300$ mas yr$^{-1}$) brown dwarfs needed to
provide predictable events during the 5 year mission lifetime is surprisingly
small, only of the order of a hundred. This is comparable to the number of high
proper motion brown dwarfs already known from the work of the UKIDSS Large Area
Survey and the all-sky WISE satellite. Provided the relative parallax of the
lens and the angular Einstein radius can be recovered from astrometric data,
then the mass of the lens can be found. Microlensing provides the only way of
measuring the masses of individual objects irrespective oftheir luminosity. So,
microlensing with GAIA is the best way to carry out an inventory of masses in
the brown dwarf regime.
