Atmospheric Abundances and Bulk Properties of the Binary Brown Dwarf Gliese 229Bab from JWST/MIRI Spectroscopy

Jerry W. Xuan*, Marshall D. Perrin, Dimitri Mawet, Heather A. Knutson, Sagnick Mukherjee, Yapeng Zhang, Kielan K.W. Hoch, Jason J. Wang, Julie Inglis, Nicole L. Wallack, Jean Baptiste Ruffio

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract

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 [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 Teff are similar between components of the binary. In addition, we measure T eff = 90 0 − 29 + 78 K and T eff = 77 5 − 33 + 20 K for the two brown dwarfs. We find that the vertical diffusion coefficients of log K z z ≈ 4.0 are identical between the two brown dwarfs and in line with log K z z values inferred for isolated brown dwarfs of similar Teff. Our results demonstrate the power of mid-infrared spectroscopy in providing robust atmospheric abundance measurements for brown dwarf companions and, by extension, giant planets.

Original languageEnglish (US)
Article numberL32
JournalAstrophysical Journal Letters
Volume977
Issue number2
DOIs
StatePublished - Dec 20 2024

Funding

J.X. thanks Samuel Beiler for discussions about MIRI LRS, Ben Burningham for discussions on opacities and line lists as a source of bias in retrievals, Dino Hsu for discussions about the literature of brown dwarf binaries, and Rebecca Oppenheimer for reading an early draft of this Letter. J.X. is supported by the NASA Future Investigators in NASA Earth and Space Science and Technology (FINESST) award #80NSSC23K1434 and the NASA JWST grant JWST-GO-03762.003-A. Based on observations with the NASA/ESA/CSA JWST, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-03127. This work benefited from the 2023 Exoplanet Summer Program in the Other Worlds Laboratory (OWL) at the University of California, Santa Cruz, a program funded by the Heising-Simons Foundation and NASA. The data presented in this Letter were obtained from the Mikulski Archive for Space Telescopes (MAST) at the Space Telescope Science Institute.

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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