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.