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Arxiv: Physical Parameters and Properties of 20 Cold Brown Dwarfs in JWST Published: 9/28/2024 3:35:34 AM Updated: 9/28/2024 3:35:34 AM

Paper abstract: We present a comprehensive analysis of 20 T and Y dwarfs using spectroscopyfrom the NIRSpec CLEAR/PRISM and MIRI LRS instruments on the James Webb SpaceTelescope. To characterize the atmospheric parameters, we utilize twoatmospheric model grids: the Sonora Elf Owl and ATMO2020++. The effectivetemperatures derived from the two models are relatively consistent, andmetallicities are both close to solar values. However, significantdiscrepancies are found in other parameters, particularly in surface gravity,with the values obtained from the Sonora Elf Owl models typically being about 1dex lower than those from the ATMO2020++ models. Further comparisons using theATMO2020 models highlight that the adiabatic convective process introduced inthe ATMO2020++ models has a significant impact on the determination of surfacegravity. Using the fitted effective temperatures and absolute parallaxes fromthe literature, we derive radii for the brown dwarfs, which range fromapproximately 0.8 to 1.2 R_{\mathrm{Jup}}. The estimated masses and ages,derived using evolutionary tracks, indicate that most brown dwarfs in oursample have masses below 30 M_{\mathrm{Jup}} and are younger than 6 Gyr.Specifically, Y dwarfs have masses ranging from 2 to 20 M_{\mathrm{Jup}} andages between 0.1 and 6.7 Gyr. In addition, We discuss the determination ofatmospheric parameters using only NIRSpec or MIRI spectra. Comparisons withresults from the combined spectra show that the effective temperatures andsurface gravities derived solely from NIRSpec spectra are largely consistentwith those obtained from the combined spectra.