The JWST proto-PAH project: Detection of the methyl radical CH 3 in the highly evolved C-rich object SMP LMC 011

Li, Jialu, García-Hernández, D. A., Manchado, A., Das, Debayan, Cami, J., Peeters, Els, Sloan, G. C., Aringer, B., Bernard-Salas, J., Bhatt, C., Clark, Nicholas, Dinerstein, Harriet L., Gómez-Muñoz, M. A., Kraemer, Kathleen E., Matsuura, M. ORCID: https://orcid.org/0000-0002-5529-5593, Sahai, R., Sterling, N. C., Volk, Kevin, Wahlgren, G. M. and Zijlstra, A. A. 2026. The JWST proto-PAH project: Detection of the methyl radical CH 3 in the highly evolved C-rich object SMP LMC 011. The Astrophysical Journal Letters 1009 (2) , L47. 10.3847/2041-8213/aea794

PDF - Published Version Available under License Creative Commons Attribution. Download (1MB)

Abstract

We report the first detection of the neutral methyl radical CH3 in a highly evolved C-rich object, SMP LMC 011, using the Mid-Infrared Instrument (MIRI) Medium-Resolution Spectrometer (MRS) on JWST. CH3 is well fitted by an excitation temperature of Tex ≃ 190 K and a column density of Ntot(CH3) ≃ 5.6 × 1017 cm−2. We also report the nondetection 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 toward 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 polycyclic aromatic hydrocarbon (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.

Item Type:	Article
Date Type:	Publication
Status:	Published
Schools:	Schools > Physical, Chemical & Environmental Sciences Schools > Physics and Astronomy
Publisher:	American Astronomical Society
ISSN:	2041-8205
Date of First Compliant Deposit:	5 October 2026
Date of Acceptance:	14 September 2026
Last Modified:	05 Oct 2026 13:13
URI:	https://orca.cardiff.ac.uk/id/eprint/189970

Actions (repository staff only)

Edit Item