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LibraryJ. Li (Inst. Astrof. Canarias, Univ. La Laguna), D.A. Garcia-Hernandez (Inst. Astrof. Canarias, Univ. La Laguna), A. Manchado (Inst. Astrof. Canarias, Univ. La Laguna, CSIC), D. Das (Univ. Western Ontario), J. Cami (Univ. Western Ontario), E. Peeters (Univ. Western Ontario), G.C. Sloan (STScI, UNC Chapel Hill), B. Aringer (Uppsala Univ., Univ. Vienna), J. Bernard-Salas (ACRI-ST), C. Bhatt (Univ. Western Ontario), N. Clark (Univ. Western Ontario), H.L. Dinerstein (Univ. Texas Austin), M.A. Gomez-Munoz (Univ. Barcelona, IEEC), K.E. Kraemer (Boston College), M. Matsuura (Cardiff Univ.), K. Volk (STScI), R. Sahai (JPL), N.C. Sterling (Univ. West Georgia), G.M. Wahlgren (STScI), A.A. Zijlstra (Univ. of Manchester)
2026, ApJ Letters, submitted
We report the first detection of the neutral methyl radical CH3 in a highly evolved C-rich object, SMP LMC 011, using JWST MIRI/MRS. CH3 is well fitted by an excitation temperature of Tex ~ 190 K and a column density of Ntot(CH3) ~ 5.6 x 1017 cm-2. We also report the non-detection of ethane (C2H6), consistent with CH3 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 CH3 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 CH3 directly into the gas phase. These results establish CH3 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 (C7H8) and ethylbenzene (C8H10) in C-rich evolved stars would provide direct confirmation of CH3-driven aromatic growth in circumstellar environments.
Last modified 27 August, 2026. © Gregory C. Sloan and others.