The Y Dwarf Population with HST: unlocking the secrets of our coolest neighbours -- III. Near-Infrared Photometry

Y dwarfs represent the coldest class of brown dwarfs, with effective temperatures below 500K, and provide unique analogues to cold giant exoplanets. We present a large compilation of uniform near-infrared photometry from the Hubble Space Telescope for 21 Y dwarfs across multiple WFC3/IR filters, including the F105W, F125W and F160W bands. We employed refined PSF-fitting and calibration procedures to reach photometric uncertainties at the 0.02-0.05 mag level for most targets. Combined with precise parallax measurements, our data reveal well-defined Y-dwarf sequences in near-infrared colour-magnitude diagrams, observed with a markedly improved tightness. Known photometric trends emerge with minimal scatter, including the continuous redward progression in F125W-F160W with decreasing temperature, and the blueward trend in F105W-F125W with possible hints of a reversal around 350K. Comparisons to the ATMO, Sonora Elf Owl, and Lacy & Burrows atmospheric models highlight systematic discrepancies, in particular F105W-F125W and F105W-F160W colours predicted to be too red. Low-metallicity grids provide the best fits to the global Y-dwarf population, although closer inspection across wavelengths shows that these preferences likely reflect compensating effects in missing or incomplete physics rather than true population-level abundances. While some atmospheric diversity is expected among Y dwarfs, their tight observational sequences and systematic offsets from model predictions reveal that key physical and chemical processes remain inadequately captured in current grids. Our results underscore the importance of high-precision, internally consistent datasets in robustly tracing the Y-dwarf cooling sequence and providing the empirical constraints needed to advance theoretical models at the lowest temperatures.

💡 Research Summary

This paper presents a homogeneous set of high‑precision near‑infrared (NIR) photometry for 21 Y dwarfs obtained with the Hubble Space Telescope’s Wide Field Camera 3 infrared channel (WFC3/IR). The authors targeted the three broad filters F105W (Y‑band), F125W (J‑band) and F160W (H‑band), which sample the principal flux peaks of ultra‑cool brown dwarfs. By combining new observations with archival data and leveraging the precise parallaxes derived in their preceding papers (Paper I and Paper II), they construct absolute‑magnitude versus colour diagrams (CMDs) with unprecedented internal consistency and photometric uncertainties of 0.02–0.05 mag for the majority of the sample.

The data reduction pipeline follows the Anderson & King (2006) methodology, employing the hst1pass software to generate spatially variable effective PSFs (ePSFs) that are iteratively refined using bright, isolated stars in each exposure. Positions are corrected for geometric distortion, and a quality‑fit parameter (q) is used to weight measurements across multiple exposures. For the two faintest objects, WISE 0855‑0714 and WD 0806‑661B, a “second‑pass” approach is adopted, co‑adding many short exposures to recover reliable fluxes.

The resulting CMDs reveal clear, low‑scatter trends. The colour F125W–F160W becomes progressively redder as effective temperature declines, reflecting the strengthening of H₂O and CH₄ absorption in the H‑band. Conversely, F105W–F125W shows a blueward shift from ~500 K down to ~350 K, with a possible reversal at lower temperatures, likely driven by the emergence of NH₃ absorption in the Y‑band. The intrinsic scatter around these sequences is remarkably small (≈0.1 mag), indicating that the Y‑dwarf population is more homogeneous than previously inferred from heterogeneous ground‑based datasets.

When compared with three state‑of‑the‑art atmospheric model grids—ATMO, Sonora Elf Owl, and Lacy & Burrows—the authors find systematic discrepancies. All models predict F105W‑related colours that are too red compared with the observations, suggesting that current treatments of low‑temperature chemistry, cloud microphysics, or non‑equilibrium processes are incomplete. Low‑metallicity (