BEDIN, LuigiLuigiBEDINFontanive, C.C.Fontanive2023-01-232023-01-2320200035-8711http://hdl.handle.net/20.500.12386/33013In 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<SUB>αcos </SUB><SUB>δ</SUB>, μ<SUB>δ</SUB>, ϖ) = (577.21 ± 0.24 mas yr<SUP>-1</SUP>, -3108.39 ± 0.27 mas yr<SUP>-1</SUP>, 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.STAMPAenArticle10.1093/mnras/staa5402-s2.0-85098854942https://academic.oup.com/mnras/article/494/2/2068/5775319http://arxiv.org/abs/2002.09331v12020MNRAS.494.2068BFIS/05 - ASTRONOMIA E ASTROFISICA