The JWST Proto-PAH Project: Detection of the Methyl Radical CH ₃ in the Highly Evolved C-rich Object SMP LMC 011
17 affiliations
- 1Instituto de Astrofísica de Canarias
- 2Universidad de La Laguna (ULL)
- 3Consejo Superior de Investigaciones Científicas (CSIC)
- 4The University of Western Ontario
- 5Space Telescope Science Institute
- 6University of North Carolina
- 7Uppsala University
- 8University of Vienna
- 9Centre d’Etudes et de Recherche de Grasse (CERGA)
- 10INCLASS Common Laboratory
- 11University of Texas at Austin
- 12Universitat de Barcelona (UB)
- 13Boston College
- 14Cardiff University
- 15California Institute of Technology
- 16University of West Georgia
- 17The University of Manchester
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.
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BibTeX
@article{li2026jwst,
title = {{The JWST Proto-PAH Project: Detection of the Methyl Radical CH ₃ in the Highly Evolved C-rich Object SMP LMC 011}},
author = {Jialu Li and D. A. García-Hernández and A. Manchado and Debayan Das and J. Cami and Els Peeters and G. C. Sloan and B. Aringer and J. Bernard-Salas and C. Bhatt and Nicholas Clark and Harriet L. Dinerstein and M. A. Gómez-Muñoz and Kathleen E. Kraemer and M. Matsuura and R. Sahai and N. C. Sterling and Kevin Volk and G. M. Wahlgren and A. A. Zijlstra},
journal = {The Astrophysical Journal Letters},
year = {2026},
month = sep,
doi = {10.3847/2041-8213/aea794},
url = {https://iopscience.iop.org/article/10.3847/2041-8213/aea794},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - The JWST Proto-PAH Project: Detection of the Methyl Radical CH ₃ in the Highly Evolved C-rich Object SMP LMC 011
AU - Jialu Li
AU - D. A. García-Hernández
AU - A. Manchado
AU - Debayan Das
AU - J. Cami
AU - Els Peeters
AU - G. C. Sloan
AU - B. Aringer
AU - J. Bernard-Salas
AU - C. Bhatt
AU - Nicholas Clark
AU - Harriet L. Dinerstein
AU - M. A. Gómez-Muñoz
AU - Kathleen E. Kraemer
AU - M. Matsuura
AU - R. Sahai
AU - N. C. Sterling
AU - Kevin Volk
AU - G. M. Wahlgren
AU - A. A. Zijlstra
PY - 2026
DA - 2026/09/30
JO - The Astrophysical Journal Letters
DO - 10.3847/2041-8213/aea794
UR - https://iopscience.iop.org/article/10.3847/2041-8213/aea794
AB - 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.
SN - 2041-8205
SN - 2041-8213
ER -