The MACHO Project HST Follow-Up: The Large Magellanic Cloud Microlensing
  Source Stars
  We present Hubble Space Telescope (HST) WFPC2 photometry of 13 microlensed
source stars from the 5.7 year Large Magellanic Cloud (LMC) survey conducted by
the MACHO Project. The microlensing source stars are identified by deriving
accurate centroids in the ground-based MACHO images using difference image
analysis (DIA) and then transforming the DIA coordinates to the HST frame. None
of these sources is coincident with a background galaxy, which rules out the
possibility that the MACHO LMC microlensing sample is contaminated with
misidentified supernovae or AGN in galaxies behind the LMC. This supports the
conclusion that the MACHO LMC microlensing sample has only a small amount of
contamination due to non-microlensing forms of variability. We compare the
WFPC2 source star magnitudes with the lensed flux predictions derived from
microlensing fits to the light curve data. In most cases the source star
brightness is accurately predicted. Finally, we develop a statistic which
constrains the location of the Large Magellanic Cloud (LMC) microlensing source
stars with respect to the distributions of stars and dust in the LMC and
compare this to the predictions of various models of LMC microlensing. This
test excludes at > 90% confidence level models where more than 80% of the
source stars lie behind the LMC. Exotic models that attempt to explain the
excess LMC microlensing optical depth seen by MACHO with a population of
background sources are disfavored or excluded by this test. Models in which
most of the lenses reside in a halo or spheroid distribution associated with
either the Milky Way or the LMC are consistent which these data, but LMC halo
or spheroid models are favored by the combined MACHO and EROS microlensing
results.
