HST astrometry in the 30 Doradus region: measuring proper motions of
  individual stars in the Large Magellanic Cloud
  We present measurements of positions and relative proper motions in the 30
Doradus region of the Large Magellanic Cloud (LMC). We detail the construction
of a single-epoch astrometric reference frame, based on specially-designed
observations obtained with the two main imaging instruments ACS/WFC and
WFC3/UVIS onboard the Hubble Space Telescope (HST). Internal comparisons
indicate a sub milli-arc-second (mas) precision in the positions and the
presence of semi-periodic systematics with a mean amplitude of ~0.8 mas. We
combined these observations with numerous archival images taken with WFPC2 and
spanning 17 years. The precision of the resulting proper motions for
well-measured stars around the massive cluster R 136 can be as good as ~20
microarcsec/yr, although the true accuracy of proper motions is generally lower
due to the residual systematic errors. The observed proper-motion dispersion
for our highest-quality measurements is ~0.1 mas/yr. Our catalog of positions
and proper motions contains 86,590 stars down to V~25 and over a total area of
~70 square arcmin. We examined the proper motions of 105 relatively bright
stars and identified a total of 6 candidate runaway stars. We are able to
tentatively confirm the runaway status of star VFTS 285, consistent with the
findings from line-of-sight velocities, and to show that this star has likely
been ejected from R 136. This study demonstrates that with HST it is now
possible to reliably measure proper motions of individual stars in the nearest
dwarf galaxies such as the LMC.
