The JWST Proto-PAH Project: Detection of the Methyl Radical CH 3 in the Highly Evolved C-rich Object SMP LMC 011
Year: 2026
Type: article
Abstract: 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.
Authors: Jialu Li, D. A. García-Hernández, A. Manchado, Debayan Das, J. Cami, Els Peeters, G. C. Sloan, B. Aringer, J. Bernard-Salas, C. Bhatt, Nicholas Clark, Harriet L. Dinerstein, M. A. Gómez-Muñoz, Kathleen E. Kraemer, M. Matsuura, R. Sahai, N. C. Sterling, Kevin Volk, G. M. Wahlgren, A. A. Zijlstra
Institutions: Universidad de La Laguna, Instituto de Astrofísica de Canarias, Consejo Superior de Investigaciones Científicas, Western University, Space Telescope Science Institute, Uppsala University, University of Vienna, The University of Texas at Austin, Universitat de Barcelona, Boston College, Cardiff University, California Institute of Technology, University of West Georgia, University of Manchester
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $2,400
Funders: National Aeronautics and Space Administration, California Institute of Technology, Space Telescope Science Institute, Jet Propulsion Laboratory, Canadian Space Agency, Natural Sciences and Engineering Research Council of Canada, European Regional Development Fund, Agencia Estatal de Investigación
OpenAlex ID: W7214977264