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Arxiv: Probing the Heights and Depths of Y Dwarf Atmospheres: A Retrieval Analysis of the JWST Spectral Energy Distribution of WISE J035934.06-540154.6 Published: 6/10/2024 8:29:31 PM Updated: 6/10/2024 8:29:31 PM

Paper abstract: We present an atmospheric retrieval analysis of the Y0 brown dwarf WISEJ035934.06-540154.6 using the low-resolution 0.96--12 \mum JWST spectrumpresented in \citet{Beiler_2023}. We obtain volume number mixing ratios of themajor gas-phase absorbers (H_2O, CH_4, CO, CO_2, PH_3, and H_2S) thatare 3--5\times more precise than previous work that used HST spectra. We alsofind an order-of-magnitude improvement in the precision of the retrievedthermal profile, a direct result of the broad wavelength coverage of the JWSTdata. We used the retrieved thermal profile and surface gravity to generate agrid of chemical forward models with varying metallicity, (C/O)_\textrm{atm},and strengths of vertical mixing as encapsulated by the eddy diffusioncoefficient K_\textrm{zz}. Comparison of the retrieved abundances with thisgrid of models suggests that the deep atmosphere of WISE 0359-54 shows signsof vigorous vertical mixing with K_\textrm{zz}=10^9 [cm^{2} s^{-1}]. Totest the sensitivity of these results to our 5-knot spline thermal profilemodel, we performed a second retrieval using the \citet{Madhusudhan_2009}thermal profile model. While the results of the two retrievals generally agreewell, we do find differences between the retrieved values of mass and volumenumber mixing ratio of H_2S with fractional differences of the median valuesof -0.64 and -0.10, respectively. In addition, the 5-knot thermal profileis consistently warmer at pressure between 1 and 70 bar. Nevertheless, ourresults underscore the power that the broad-wavelength infrared spectraobtainable with the James Webb Space Telescope have to characterize theatmospheres of cool brown dwarfs.