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Arxiv: Water depletion and 15NH3 in the atmosphere of the coldest brown dwarf observed with JWST/MIRI Published: 10/14/2024 8:50:51 PM Updated: 10/14/2024 8:50:51 PM

Paper abstract: With a temperature of ~ 285 K WISE0855 is the coldest brown dwarfobserved so far. Using the James Webb Space Telescope (JWST) we obtainedobservations that allow us to characterize WISE0855s atmosphere focusing onvertical variation in the water steam abundance, measuring trace gas abundancesand receiving bulk parameters for this cold object. We observed the ultra cooldwarf WISE0855 using the Mid-Infrared Instrument Medium Resolution Spectrometer(MIRI/MRS) onboard JWST at a spectral resolution of up to 3750. We combined theobservation with published data from the Near Infrared Spectrograph (NIRSpec)G395M and PRISM modes yielding a spectrum ranging from 0.8 to 22 um. We applyatmospheric retrievals using petitRADTRANS to measure atmospheric abundances,the pressure-temperature structure, radius and gravity of the brown dwarf. Wealso employ publicly available clear and cloudy self-consistent grid models toestimate bulk properties of the atmosphere such as the effective temperature,radius, gravity and metallicity. Atmospheric retrievals constrain a variablewater abundance profile in the atmosphere, as predicted by equilibriumchemistry. We detect the 15NH3 isotopologue and infer a ratio of mass fractionof 14NH3/15NH3 = 332+63-43 for the clear retrieval. We measure the bolometricluminosity by integrating the presented spectrum and obtain a value oflog(L/L_{\odot}) = -7.291+/-0.008. The detected water depletion indicatesthat water condenses out in the upper atmosphere due to the very low effectivetemperature of WISE0855. The height in the atmosphere where this occurs iscovered by the MIRI/MRS data, and thus demonstrates the potential of MIRI tocharacterize cold gas giants atmospheres. Comparing the data to retrievals andself-consistent grid models, we do not detect signs for water ice clouds,although their spectral features have been predicted in previous studies.