Extending Gaia DR2 with HST narrow-field astrometry: the WISE J154151.65-225024.9 test case

One field containing WISE J154151.65-225024.9 was observed by Hubble Space Telescope at three different epochs taken in ~5 yrs. We measured positions of sources in all images and successfully linked these positions to the Gaia DR2 absolute system to derive the astrometric parameters for this faint close-by Y1 brown dwarf. The developed procedure avoids traditional limitations of relative imaging-astrometry with narrow-field cameras, extending Gaia DR2 to fainter magnitudes. We found (mu_RA,mu_d,parallax) = (-902.62+/-0.35mas/yr, -88.26+/-0.35mas/yr,168.4+/-2.2mas), which represent a sizable improvement over recent determinations in the literature. Applying a correction from relative to absolute parallax we found an absolute parallax of 169+/-2 mas, corresponding to a distance of 5.9+/-0.1 pc.