2024ApJ...977L..32X

We present JWST/Mid Infrared Instrument (MIRI) low-resolution spectroscopy (4.75-14 μm) of the first known substellar companion, Gliese 229Bab, which was recently resolved into a tight binary brown dwarf. Previous atmospheric retrieval studies modeling Gliese 229B as a single brown dwarf have reported anomalously high carbon-to-oxygen ratios (C/O) of ≈1.1 using 1-5 μm ground-based spectra. Here, we fit the MIRI spectrum of Gliese 229Bab 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 C/O = 0.65 ± 0.05 and ${\rm{[M/H]}}=0.0{0}_{-0.03}^{+0.04}$ as their abundances (2σ 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_eff are similar between components of the binary. In addition, we measure ${T}_{\mathrm{eff}}=90{0}_{-29}^{+78}$ K and ${T}_{\mathrm{eff}}=77{5}_{-33}^{+20}$ K for the two brown dwarfs. We find that the vertical diffusion coefficients of $\mathrm{log}{K}_{zz}\approx 4.0$ are identical between the two brown dwarfs and in line with $\mathrm{log}{K}_{zz}$ values inferred for isolated brown dwarfs of similar T_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.