The JWST Proto-PAH Project: Detection of the Methyl Radical CH 3 in the Highly Evolved C-rich Object SMP LMC 011

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Abstract
We report the first detection of the neutral methyl radical CH 3 in a highly evolved C-rich object, SMP LMC 011, using the Mid-Infrared Instrument (MIRI) Medium-Resolution Spectrometer (MRS) on JWST. CH 3 is well fitted by an excitation temperature of T ex ≃ 190 K and a column density of N tot (CH 3 ) ≃ 5.6 × 10 17 cm −2 . We also report the nondetection of ethane (C 2 H 6 ), consistent with CH 3 reacting preferentially with unsaturated radicals rather than self-recombination, which may be inefficient in SMP LMC 011. Combined with the exceptionally large benzene column density of SMP LMC 011, this supports a methyl-addition route from benzene toward alkyl-substituted aromatics. We propose that the high CH 3 abundance is driven by the erosion of hydrogenated amorphous carbon (HAC) dust grains in the dense warm torus by UV photons from the central star and/or by shocks, which release CH 3 directly into the gas phase. These results establish CH 3 as a key reactive intermediate in the formation of complex hydrocarbons in C-rich circumstellar environments. Chemical models that incorporate methyl-addition reactions are needed for a better understanding of polycyclic aromatic hydrocarbon (PAH) formation pathways in evolved stars. The potential detection of alkyl-substituted aromatics such as toluene (C 7 H 8 ) and ethylbenzene (C 8 H 10 ) in C-rich evolved stars would provide direct confirmation of CH 3 -driven aromatic growth in circumstellar environments.
Funding Information
  • MEC ∣ Spanish National Plan for Scientific and Technical Research and Innovation (PID2023-147325NB-I00)
  • European Cooperation in Science and Technology (CA21126)
  • Canadian Space Agency (24JWGO3B01)
  • Space Telescope Science Institute (NAS5-03127)

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