Clark et al. 2026, A&A, ... JWST Proto-PAH project: aliphatic substitution ...

The JWST Proto-PAH project: Aromatic backbones with extensive aliphatic substitution account for both the aromatic and aliphatic emission

N. Clark (Univ. Western Ontario), E. Peeters (Univ. Western Ontario), J. Cami (Univ. Western Ontario), G.C. Sloan (STScI, UNC Chapel Hill), K Volk (STScI), K.E. Kraemer (Boston College), H.L. Dinerstein (Univ. Texas Austin), M.A. Gomez-Munoz (Univ. Barcelona, IEEC), G.M. Wahlgren (STScI), N.C. Sterling (Univ. West Georgia), A. Ricca (NASA Ames, SETI), A.A. Zijlstra (Univ. Manchester) J. Li (Inst. Astrof. Canarias, Univ. La Laguna), M. Matsuura (Cardiff Univ.), D.A. Garcia-Hernandez (Inst. Astrof. Canarias, Univ. La Laguna), R. Sahai (JPL), B. Aringer (Uppsala Univ., Univ. Vienna), C. Bhatt (Univ. Western Ontario), J. Bernard-Salas (ACRI-ST), D. Das (Univ. Western Ontario), A. Manchado (Inst. Astrof. Canarias, Univ. La Laguna, CSIC)

2026, A&A, submitted

Full manuscript available locally (PDF)

PLEASE NOTE: The manuscript above is as submitted to A&A and likely to change, as is the abstract below. Even the titles and author list could change! We'll update this page as soon as we can.

We find that the aromatic and aliphatic emission bands in four carbon-rich post-AGB 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-IR aromatic features would constrain them to be large (≳200 carbon atoms), compact, and at least partially ionized; properties that predict a strong 8.6 µm feature. Yet the 8.6 µm feature is weak or absent in all four sources, ruling out any as the source of the aromatic emission, which must instead 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 CH₂ and CH₃ feature strengths vary across the sample, with the Class D2 sources showing higher CH₃ content. These carriers are far more aliphatic than the PAHs of planetary nebulae and the ISM, yet retain the aromatic skeleton that produces their aromatic features.

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