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 yr. 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 (μ_{αcos δ}, μ_δ, π) = (-902.62 ± 0.35 mas yr^-1, -88.26 ± 0.35 mas yr^-1, 168.4 ± 2.2 mas), which represent a sizable improvement over recent determinations in the literature. Applying a correction from relative (π) to absolute parallax (ϖ) we found ϖ = 169 ± 2 mas, corresponding to a distance of 5.9 ± 0.1 pc.