  In a previous study it was proposed that the Galactic dark matter being
detected by gravitational microlensing experiments such as MACHO may reside in
a population of dim halo globular clusters comprising mostly or entirely
low-mass stars just above the hydrogen-burning limit. It was shown that, for
the case of a standard isothermal halo, the scenario is consistent not only
with MACHO observations but also with cluster dynamical constraints and
number-count limits imposed by 20 Hubble Space Telescope (HST) fields. The
present work extends the original study by considering the dependency of the
results on halo model, and by increasing the sample of HST fields to 51
(including the Hubble Deep Field and Groth Strip fields). The model dependency
of the results is tested using the same reference power-law halo models
employed by the MACHO team. For the unclustered scenario HST counts imply a
model-dependent halo fraction of at most 0.5-1.1% (95% confidence), well below
the inferred MACHO fraction. For the cluster scenario all the halo models
permit a range of cluster masses and radii to satisfy HST, MACHO and dynamical
constraints. Whilst the strong HST limits on the unclustered scenario imply
that at least 95% of halo stars must reside in clusters at present, this limit
is weakened if the stars which have escaped from clusters retain a degree of
clumpiness in their distribution.
