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 (μ_{αcos δ}, μ_δ, ϖ) = (577.21 ± 0.24 mas yr^-1, -3108.39 ± 0.27 mas yr^-1, 210.4 ± 1.8 mas). In particular, our derived absolute parallax (ϖ ) corresponds to a distance of 4.75 ± 0.05 pc for the faint ultracool dwarf.