An international study led by researchers from the Instituto de Astrofísica de Canarias (IAC) reports the first secure detection of the methyl radical (CH₃) in the environment of an evolved, carbon-rich star. The molecule was found with the James Webb Space Telescope (JWST) in SMP LMC 011, a dying star in the Large Magellanic Cloud, and points to an unexpected way of building the carbon molecules that seed the Universe with organic material. The results are published in the prestigious journal The Astrophysical Journal Letters.

The methyl radical, CH₃, is one of the simplest and most important building blocks in carbon chemistry: a single carbon atom, three hydrogens, and an unpaired electron that makes it extremely reactive, so that it grabs onto almost any molecule it meets. That same reactivity makes it very short-lived, and very hard to catch in space.

It is also almost invisible to most telescopes. Because of its symmetric shape, CH₃ produces no signal in the radio range where astronomers detect most molecules; it can only be identified through a specific vibration in the mid-infrared, at a wavelength that Earth's atmosphere blocks. Detecting it directly therefore requires a space observatory as sensitive as JWST.

A molecule that shouldn't be so abundant

Using the Mid-Infrared Instrument (MIRI) on board JWST, the team not only detected CH₃ but measured how much of it is present, and the amount is surprisingly large. Standard chemistry, in which ultraviolet light breaks apart simpler molecules such as methane, should produce only a modest quantity of methyl. Something else must be supplying the excess.

The researchers propose that the dust grains themselves are the extra source. Around these stars, carbon and hydrogen condense into a sooty, disordered material known as hydrogenated amorphous carbon (HAC). The team suggests that this dust is being erosed by UV photons from the central star and/or by shocks, releasing methyl radicals straight into the surrounding gas.

Turning the story of cosmic dust around

The finding reverses the usual picture of how large carbon molecules (i.e., the polycyclic aromatic hydrocarbons, or PAHs) relate to dust. PAHs are normally thought to form in the gas and then lock away into dust grains. This result suggests the opposite can also occur: grains can erode and feed small, reactive molecules back into the gas, where CH₃ then acts as a building block to assemble new ring-shaped (aromatic) molecules. In effect, the authors propose a two-way feedback between dust and gas-phase chemistry.

This star is also exceptionally rich in benzene (C₆H₆), the simplest aromatic ring and the basic unit of PAHs. The abundant methyl radicals can attach to benzene and its derivatives; successive methyl additions turn benzene into toluene, then ethylbenzene, and on toward ever larger alkyl-substituted aromatics. In this way CH₃ provides a route to grow molecular complexity directly from the simplest aromatic ring.

A second clue supports this picture. The team searched for ethane (C₂H₆), which would form if methyl simply reacted with itself, and did not find it. This indicates that CH₃ is instead being channelled into these growth reactions rather than dead-ending — feeding the build-up of larger aromatic molecules such as toluene and ethylbenzene, the first steps toward greater molecular complexity.

“The methyl radical is one of the most reactive and short-lived molecules we can look for in space, and it barely leaves a fingerprint that telescopes can see,” explains Jialu Li, an IAC postdoctoral researcher who led the study. “Detecting it directly, and actually measuring how much is there, was only possible thanks to the extraordinary sensitivity of JWST.”

“The amount of methyl we measure is far too high for ordinary gas-phase photochemistry to explain,” says Domingo Aníbal García-Hernández, an IAC researcher and co-author, who coordinates the European COST Action NanoSpace on carbon nanostructures in space. “The most likely explanation is that the carbon dust grains are being eroded by UV light and/or shocks, releasing this molecule back into the gas phase; so the dust does not only grow, it can also break down and feed the chemistry.”

“Evolved stars like this one are among the main factories of carbon dust and complex organic molecules in the Universe, the raw material for future stars, planets, and perhaps life,” adds Arturo Manchado, an IAC (and CSIC) researcher and co-author. “Our result identifies the methyl radical as a key piece in that process, and it tells us that current chemical models must be updated to include these reactions.”

The study is part of the international JWST Proto-PAH Project, which brings together researchers from the IAC with colleagues in Canada, the United States and Europe to trace how the smallest carbon molecules grow into the large aromatic structures that pervade the cosmos.

Article: J. Li, D. A. García-Hernández, A. Manchado, et al. “The JWST Proto-PAH Project: Detection of the methyl radical CH₃ in the highly evolved C-rich object SMP LMC 011”. The Astrophysical Journal Letters, 2026. DOI: https://doi.org/10.3847/2041-8213/aea794

Contacts at the IAC:
Jialu Li, jialu.li [at] iac.es (jialu[dot]li[at]iac[dot]es)
Domingo Aníbal García-Hernández, agarcia [at] iac.es (agarcia[at]iac[dot]es)
Arturo Manchado, amt [at] iac.es (amt[at]iac[dot]es)