Jialu Li

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.