The JWST Proto-PAH project: Aromatic backbones with extensive aliphatic substitution account for both the aromatic and aliphatic emission
Year: 2026
Type: article
Abstract: We find that the aromatic and aliphatic emission bands in four carbon-rich post-asymptotic giant branch stars with class D emission arise from a single carrier population: extensive aliphatic decoration of an aromatic skeleton, i.e. molecules with a coherent aromatic backbone and an unusually high aliphatic content. This conclusion is based on JWST NIRSpec and MIRI/MRS spectroscopy, covering 2.88–28.7 µm. The aliphatic 3.4 µm feature is stronger than the aromatic 3.3 µm feature, reversing the ratio seen in sources with typical polycyclic aromatic hydrocarbon (PAH) emission. If the aromatic emission arose from nominal PAHs, the 3.3 µm feature – unavailable in previous observations – together with the mid-infrared aromatic features, would constrain them to be large (≳200 carbon atoms), compact, and at least partially ionized. These properties predict a strong 8.6 µm feature. However, the 8.6 µm feature is weak or absent in all four sources, ruling out PAHs as the source of the aromatic emission. Instead, the emission must arise from the same carriers as the aliphatic emission. The position of the 6.9 µm feature and the broadened 11–14 µm out-of-plane emission independently support this. The CH 2 and CH 3 feature strengths vary across the sample, with the class D2 sources showing higher CH 3 content. These carriers are far more aliphatic than the PAHs of planetary nebulae and the interstellar medium. However, they retain the aromatic skeleton that produces their aromatic features.
Source: Astronomy and Astrophysics
Institutions: Western University, Space Telescope Science Institute, Boston College, The University of Texas at Austin, Institut d'Estudis Espacials de Catalunya, Institut de Ciències del Cosmos, Universitat de Barcelona, University of West Georgia, Ames Research Center, Search for Extraterrestrial Intelligence, University of Manchester, Universidad de La Laguna, Instituto de Astrofísica de Canarias, Cardiff University, California Institute of Technology, Jet Propulsion Laboratory, Uppsala University, University of Vienna, Consejo Superior de Investigaciones Científicas
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $1,865
Funders: National Aeronautics and Space Administration, California Institute of Technology, Space Telescope Science Institute, European Commission, European Cooperation in Science and Technology, Jet Propulsion Laboratory, Canadian Space Agency, Natural Sciences and Engineering Research Council of Canada, European Regional Development Fund, Agencia Estatal de Investigación
OpenAlex ID: W7214644317