Extending Gaia DR2 with HST narrow-field astrometry. II. Refining the
  method on WISE J163940.83-684738.6
  In the second paper of this series we perfected our method of linking high
precision Hubble Space Telescope astrometry to the high-accuracy Gaia DR2
absolute reference system to overcome the limitations of relative astrometry
with narrow-field cameras. Our test case here is the Y brown dwarf WISE
J163940.83-684738.6, observed at different epochs spread over a 6-yr time
baseline with the Infra-Red channel of the Wide Field Camera 3. We derived
significantly improved astrometric parameters compared to previous
determinations, finding: (mu_RAcosDc,mu_DC,parallax) =
(577.21+/-0.24mas/yr,-3108.39+/-0.27mas/yr,210.4+/-1.8mas). In particular, our
derived absolute parallax corresponds to a distance of 4.75+/-0.05pc for the
faint ultracool dwarf.
