JWST measures parallax of Y dwarf JADES-GS-BD-5: 64 pc, about 3× farther than known Y dwarfs
Multi-epoch NIRCam imaging over about 2 years gave a parallax of 15.7 mas for a cold brown dwarf in the JADES field, the first such measurement in a JWST extragalactic deep field.
The authors of an arXiv · preprint (not yet peer reviewed) analyse JADES-GS-BD-5, an extremely cold Y dwarf identified in the JADES/GOODS-South field. Using multi-epoch JWST/NIRCam imaging over a baseline of about 2 years, they calibrated the telescope's astrometry and obtained what they describe as the first robust parallax for a brown dwarf discovered in a JWST extragalactic deep field arxiv.org
arXiv · препринтarxiv.orgOpen source ↗.
Distance and motion
The measured proper motion is 49.8 ± 0.3 mas per year, and the parallax is 15.7 (+1.4/−1.3) mas. This corresponds to a geometric distance of 64 (+6/−5) pc, about 3 times farther than the previously known most distant Y dwarfs with parallax-measured distances arxiv.org
arXiv · препринтarxiv.orgOpen source ↗. The tangential velocity is 15.0 (+1.4/−1.3) km/s, consistent with a Galactic thin-disk object. The authors found no evidence for binary acceleration, significant photometric variability or a plausible host.
For context, Y dwarfs radiate mostly at 4–5 µm, which makes ground-based astrometry difficult. All known Y dwarfs lie within 20 pc of the Sun, and their parallaxes have so far required dedicated Spitzer and HST campaigns. The coldest end of the sequence relies almost entirely on WISE 0855 at 2.28 pc.
Classification and atmosphere
The J-band spectral morphology of JADES-GS-BD-5 is consistent with existing Y-dwarf templates. Its very low luminosity and its near-infrared spectral profile confirm that it is one of the coldest dwarfs discovered to date. The authors adopt a classification of >Y1, in line with a NIRCam colour-based proxy of cY1.5 arxiv.org
arXiv · препринтarxiv.orgOpen source ↗.
With the parallax distance, the bolometric luminosity is log(L/L☉) = −7.08 ± 0.09. Evolutionary modelling places the object in the planetary-mass regime, which makes it a benchmark for the study of cold giant-planet atmospheres.
Current atmospheric models do not reproduce its spectral energy distribution in detail. According to the authors, this points to the importance of disequilibrium chemistry and vertical mixing. Water deficiency may also contribute, but the evidence for it is currently unclear.
The paper also reports additional proper-motion measurements for eight of the other nine known JADES Y-dwarf candidates.
