James Webb Space Telescope Feed Post

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

Paper abstract: We present an atmospheric retrieval analysis of the Y0 brown dwarf WISE J035934.06-540154.6 using the low-resolution 0.96--12 \mum JWST spectrum presented in \citet{Beiler_2023}. We obtain volume number mixing ratios of the major gas-phase absorbers (H_2O, CH_4, CO, CO_2, PH_3, and H_2S) that are 3--5\times more precise than previous work that used 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)_\textrm{atm}, and strengths of vertical mixing as encapsulated by the eddy diffusion coefficient K_\textrm{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_\textrm{zz}=10^9 [cm^{2} s^{-1}]. To test the sensitivity of these results to our 5-knot spline thermal profile model, we performed a second retrieval using the \citet{Madhusudhan_2009} 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 H_2S with fractional differences of the median values of -0.64 and -0.10, respectively. In addition, the 5-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 James Webb Space Telescope have to characterize the atmospheres of cool brown dwarfs.