The Y Dwarf Companion to the White Dwarf WD 0806-66: Resolving the Discrepancy Between Atmospheric and Evolutionary Models

Abstract: James Webb Space Telescope near- and mid-infrared spectroscopy has been published by Voyer et al. (2025) and Lew et al. (2026) for the Y dwarf which is a distant companion to the white dwarf WD 0806-661 (Luhman et al. 2011). This target is important because the distance and the age of the system are well constrained by the primary star. Voyer et al. perform a retrieval analysis of the longer wavelength MIRI data, and Lew et al. perform retrieval and forward model grid analyses of the NIRSpec data. These studies produce different results, and both are discrepant with evolutionary model calculations based on the age of the system and the luminosity of the Y dwarf. Here we confirm the luminosity of the Y dwarf, and update the age of the system to 1.6 +0.6 -0.4 Gyr. We compare the combined NIRSpec and MIRI dataset to synthetic spectra generated by ATMO 2020++ atmospheric models. We find a good fit across the entire observed spectral energy distribution, except at the shortest near-infrared wavelengths, with atmospheric parameters consistent with evolutionary models. We find the Y dwarf to be slightly metal-poor, with an effective temperature of 357 +/- 3 K, a radius of 1.08 +/- 0.02 R_Jupiter, and a mass of 7 +/- 1 M_Jupiter.