Probing the Heights and Depths of Y Dwarf Atmospheres: A Retrieval Analysis of the JWST Spectral Energy Distribution of WISE J035934.06–540154.6

We present an atmospheric retrieval analysis of the Y0 brown dwarf WISE J035934.06‑540154.6 using the low-resolution 0.96–12 μm James Webb Space Telescope (JWST) spectrum presented in Beiler et al. We obtain volume number mixing ratios of the major gas-phase absorbers (H₂O, CH₄, CO, CO₂, PH₃, and H₂S) that are three to five times more precise than previous work that used Hubble Space Telescope (HST) spectra. We also find an order-of-magnitude improvement in the precision of the retrieved thermal profile, a direct result of the broad wavelength coverage of the JWST data. We used the retrieved thermal profile and surface gravity to generate a grid of chemical forward models with varying metallicity, (C/O)_atm, and strengths of vertical mixing as encapsulated by the eddy diffusion coefficient K _zz. Comparison of the retrieved abundances with this grid of models suggests that the deep atmosphere of WISE 0359‑54 shows signs of vigorous vertical mixing with K _zz = 10⁹ [cm² s^‑1]. To test the sensitivity of these results to our five-knot spline thermal profile model, we performed a second retrieval using the Madhusudhan & Seager thermal profile model. While the results of the two retrievals generally agree well, we do find differences between the retrieved values of mass and volume number mixing ratio of H₂S with fractional differences of the median values of ‑0.64 and ‑0.10, respectively. In addition, the five-knot thermal profile is consistently warmer at pressure between 1 and 70 bar. Nevertheless, our results underscore the power that the broad-wavelength infrared spectra obtainable with the JWST have to characterize the atmospheres of cool brown dwarfs.