Extending Gaia  DR2 with HST  narrow-field astrometry – II. Refining the method on WISE J163940.83−684738.6★

Abstract

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 α cos δ , µ δ , ̟) = (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 (̟) corresponds to a distance of 4.75±0.05 pc for the faint ultracool dwarf.

The Y dwarf population with HST: unlocking the secrets of our coolest neighbours – I. Overview and first astrometric results

“…In particular, we achieved a 0.56-mas uncertainty on the parallax, a 3-fold improvement over our previous estimate without this newest epoch (Bedin & Fontanive 2020; see Table 2). This is also considerably better than the precisions of a few to tens of mas typically reached in distance measurements for nearby Y dwarfs (Beichman et al 2014;Martin et al 2018;Kirkpatrick et al 2019).…”

“…By anchoring these three epochs of HST data acquired over ∼6 years to the Gaia DR2 reference frame, we were able to constrain its astrometric parameters to unprecedented Gaia-like precisions (Bedin & Fontanive 2020). The achieved precisions of ∼2 mas on the parallax and at the submas level on the proper motion already represented considerable improvements over previous estimates.…”

“…The same tasks were already performed for the previous observational datasets in Bedin & Fontanive (2020), reregistering each frame onto the observational plane at epoch 2013.12, and the new data were similarly linked to the same reference system. This provides us with a common reference frame for all images from the various available epochs, defined by Gaia DR2 anchor sources that can be re-positioned to the relevant epochs from their respective Gaia DR2 astrometric solutions.…”

“…As discussed in Bedin & Fontanive (2020), the parallax estimate derived in that work for W1639−6847 relied entirely on the epoch with the lowest astrometric precision (degraded by a nearby star and anomalously high background level; epoch plotted in green in Figure 2), thus requiring an additional epoch of observations to be further validated and refined. We demonstrate here that our new epoch, optimally timed to measure the maximum parallax elongation (orange epoch in Figure 2), is sufficient to improve the accuracy of the parallax by a factor of more than 3 over our previous estimate.…”

“…We used this target in Bedin & Fontanive (2020) to refine our HST -based astrometric method (see Section 4.3) first presented in Bedin & Fontanive (2018). W1639−6847 was a prime target for our approach, with three epochs of HST observations taken between 2013 and 2019, and several independent -but rather inconsistent -measurements of its parallax and proper motion available in the literature (Tinney et al 2012(Tinney et al , 2014Pinfield et al 2014;Martin et al 2018;Kirkpatrick et al 2019).…”