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 $T_{\rm ex} \simeq$~190\,K and a column density of $N_{\rm tot}(\mathrm{CH_3}) \simeq 5.6\times10^{17}$ 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.