1) Speaker: M.S. Murga (INASAN)

Title: PAH formation in AGB circumstellar envelopes: the role of surface chemistry and the need for alternative pathways

Abstract:Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous in the interstellar medium (ISM) and play a key role in its physical and chemical processes. Their primary formation site is believed to be the circumstellar envelopes of stars on the asymptotic giant branch (AGB), where the conditions (temperature, pressure) favor the formation and growth of aromatic structures. While the gas-phase mechanisms for the formation of the first aromatic ring and subsequent molecular growth have been studied extensively, the contribution of heterogeneous reactions on the surfaces of materials such as silicon carbide (SiC) and amorphous carbon remains poorly understood. Nevertheless, the presence of solid particles and catalytic processes on their surfaces can significantly influence the abundance and size distribution of the resulting PAHs.

This study presents a new model describing the formation and growth of PAHs in AGB circumstellar envelopes. This model integrates a high-temperature gas-phase reaction network with surface reactions and self-consistently couples PAH chemistry with dust evolution. Silicon carbide (SiC) particles are considered nucleation seeds, where various hydrocarbons, including PAHs, accrete, leading to the growth of dust particles, which is further enhanced by their coagulation. Special attention in the model is given to catalytic pathways of benzene formation. The proposed mechanism involves the sequential addition of three acetylene (C₂H₂) molecules to the surface of dust particles (SiC, amorphous carbon, PAHs), leading to the growth of a radical chain, its cyclization into benzene (C₆H₆), and subsequent desorption of the molecule.To study the acetylene cyclotrimerization reaction, the following were conducted: 1) theoretical calculations using molecular dynamics and quantum chemistry methods; 2) experiments measuring PAH yield with and without catalysts.

Using the developed model, the PAH abundance in the circumstellar envelope of the AGB star IRC+10216 was calculated. A comparison of model scenarios with and without surface reactions demonstrates that heterogeneous catalysis enhances PAH formation efficiency and alters their size distribution. Our results also indicate that gas-phase reactions alone cannot facilitate the growth of PAHs to the sizes observed in the ISM. This leads to the conclusion that it is necessary to search for and study alternative pathways, particularly the formation and growth of aromatic structures on dust surfaces.

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