The JWST Proto-PAH Project: Detection of the methyl radical CH 3 in the highly evolved C-rich object SMP LMC 011

We report the first detection of the neutral methyl radical CH 3 in a highly evolved C-rich object, SMP LMC 011, using JWST MIRI/MRS. 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 times10 17 cm-2. We also report the non-detection 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 towards 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 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.