James Webb Space Telescope Feed Post

Harvard ADS: Atmospheric abundances and bulk properties of the binary brown dwarf Gliese 229 Bab from JWST/MIRI spectroscopy

Paper abstract: We present JWST/MIRI low-resolution spectroscopy (4.75-14~\mum) of the first known substellar companion, Gliese 229 Bab, which was recently resolved into a tight binary brown dwarf. Previous atmospheric retrieval studies modeling Gliese 229 B as a single brown dwarf have reported anomalously high carbon-to-oxygen ratios (C/O) of ~ 1.1 using 1-5~\mum ground-based spectra. Here, we fit the MIRI spectrum of Gliese 229 Bab with a two-component binary model using the Sonora Elf Owl grid and additionally account for the observed K band flux ratio of the binary brown dwarf. Assuming the two brown dwarfs 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 the same abundances if we fit the MIRI spectrum with a single brown dwarf model, indicating that binarity does not strongly affect inferred abundances from mid-infrared data when the T_\rm{eff} are similar between components of the binary. We measure T_\rm{eff}=900^{+78}_{-29}~K and T_\rm{eff}=775^{+20}_{-33}~K for the two brown dwarfs. We find that the vertical diffusion coefficients of \log{K_\rm{zz}} ~4.0 are identical between the two brown dwarfs and in line with \log{K_\rm{zz}} values inferred for isolated brown dwarfs with similar T_\rm{eff}. Our results demonstrate the power of mid-infrared spectroscopy in providing robust atmospheric abundance measurements for brown dwarf companions and by extension, giant planets.