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

Abstract We present JWST/Mid Infrared Instrument (MIRI) low-resolution spectroscopy (4.75–14 μ m) 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 μ m 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi mathvariant="normal">[M/H]</mml:mi> <mml:mo>=</mml:mo> <mml:mn>0.0</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.03</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.04</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> as their abundances (2 σ 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>T</mml:mi> <mml:mi>eff</mml:mi> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>90</mml:mn> <mml:msubsup> <mml:mn>0</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>29</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>78</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> K and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>T</mml:mi> <mml:mi>eff</mml:mi> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>77</mml:mn> <mml:msubsup> <mml:mn>5</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>33</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>20</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> K for the two brown dwarfs. We find that the vertical diffusion coefficients of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:msub> <mml:mi>K</mml:mi> <mml:mrow> <mml:mi>z</mml:mi> <mml:mi>z</mml:mi> </mml:mrow> </mml:msub> <mml:mo>≈</mml:mo> <mml:mn>4.0</mml:mn> </mml:math> are identical between the two brown dwarfs and in line with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:msub> <mml:mi>K</mml:mi> <mml:mrow> <mml:mi>z</mml:mi> <mml:mi>z</mml:mi> </mml:mrow> </mml:msub> </mml:math> 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.