The JWST Proto-PAH project: Computational modeling of the emission carriers

A. Ricca (NASA Ames, SETI), G.C. Sloan (STScI, UNC Chapel Hill), E. Peeters (Univ. Western Ontario), J. Cami (Univ. Western Ontario), N. Clark (Univ. Western Ontario), M. Matsuura (Cardiff Univ.), R. Sahai (JPL), J. Bernard-Salas (ACRI-ST), D.A. Garcia-Hernandez (Inst. Astrof. Canarias, Univ. La Laguna), J. Li (Inst. Astrof. Canarias, Univ. La Laguna), C. Bhatt (Univ. Western Ontario)

2026, ApJ, submitted

Full manuscript available locally (PDF)

PLEASE BE AWARE that we have posted the submitted manuscript as a courtesy, even though it is likely to change during the review proecess. The abstract below may also change. We will post the accepted version as soon as we get there.

The infrared spectra of many carbon-rich post-asymptotic giant branch stars are dominated by emission features from aromatic and aliphatic hydrocarbons, but the chemical structure of the carriers remains unidentified. Class D sources show unusual emission profiles with strong aliphatic emission, providing a stringent test of carrier candidates. Here we investigate whether hydrogenated carbonaceous molecular particles can be the carriers of these features. Rather than assuming candidate geometries we generate the initial structures using ab initio molecular dynamics simulations and optimize them using density functional theory. We then compare the computed emission spectra to JWST NIRSpec and MIRI/MRS observations of Class D sources. The best-fitting structures contain sizeable domains with defect-bearing aromatics and hydrogenated-aromatics connected by aliphatic bridges, in contrast to the "arophatic" cluster model of isolated two- to three-ring aromatics connected by aliphatic and olefinic bridges. As a representative example, we discuss in detail the molecular particle C₁₃₀H₉₆ (radius of 5.5 Å) and compare its computed spectrum to the JWST spectra of IRAS 05110-6616, a Class D2 source with apparent shifts of the aromatic C-H out-of-plane bands. The calculated spectra are in qualitative agreement with the observed 3 µm profile, the aliphatic C-H bending bands, the broad 8 µm feature, the 11-14 µm C-H out-of-plane region, and the bands at 16 and 21 µm. We hypothesize that UV photo-driven fragmentation of such molecular particles releases aromatics linked by aliphatics, followed by the formation of polycyclic aromatic hydrocarbons and fullerenes as these objects evolve into planetary nebulae.

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