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 _ { \alpha \cos { \delta } } , \mu _ { \delta } , \varpi ) = ( 577.21 \pm 0.24 mas yr ^ { -1 } , -3108.39 \pm 0.27 mas yr ^ { -1 } , 210.4 \pm 1.8 mas ) .
In particular , our derived absolute parallax ( \varpi ) corresponds to a distance of 4.75 \pm 0.05 pc for the faint ultracool dwarf .