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Arxiv: Atmospheric abundances and bulk properties of the binary brown dwarf Gliese 229 Bab from JWST/MIRI spectroscopy Published: 11/15/2024 10:45:51 PM Updated: 11/15/2024 10:45:51 PM

Paper abstract: We present JWST/MIRI low-resolution spectroscopy (4.75-14~\mum) of thefirst known substellar companion, Gliese 229 Bab, which was recently resolvedinto a tight binary brown dwarf. Previous atmospheric retrieval studiesmodeling Gliese 229 B as a single brown dwarf have reported anomalously highcarbon-to-oxygen ratios (C/O) of ~ 1.1 using 1-5~\mum ground-basedspectra. Here, we fit the MIRI spectrum of Gliese 229 Bab with a two-componentbinary model using the Sonora Elf Owl grid and additionally account for theobserved K band flux ratio of the binary brown dwarf. Assuming the two browndwarfs share the same abundances, we obtain \rm C/O=0.65\pm0.05 and \rm[M/H]=0.00^{+0.04}_{-0.03} as their abundances (2\sigma statistical errors),which are fully consistent with the host star abundances. We also recover thesame abundances if we fit the MIRI spectrum with a single brown dwarf model,indicating that binarity does not strongly affect inferred abundances frommid-infrared data when the T_\rm{eff} are similar between components of thebinary. We measure T_\rm{eff}=900^{+78}_{-29}~K andT_\rm{eff}=775^{+20}_{-33}~K for the two brown dwarfs. We find that thevertical diffusion coefficients of \log{K_\rm{zz}} ~4.0 are identicalbetween the two brown dwarfs and in line with \log{K_\rm{zz}} values inferredfor isolated brown dwarfs with similar T_\rm{eff}. Our results demonstratethe power of mid-infrared spectroscopy in providing robust atmosphericabundance measurements for brown dwarf companions and by extension, giantplanets.