The JWST Proto-PAH Project: Detection of the methyl radical CH${}_3$ in the highly evolved C-rich object SMP LMC 011

Abstract

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 $T_{\mathrm{ex}}\simeq$~190\,K and a column density of $N_{\mathrm{tot}}(\mathrm{C}{\mathrm{H}}_3)\simeq 5.6\times 10^{17}$ 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.

Accepted for publication in ApJL. 12 pages, 5 figures