The formation of polycyclic aromatic hydrocarbons in evolved circumstellar environments

TL;DR

This paper reviews the formation of polycyclic aromatic hydrocarbons (PAHs) in evolved star environments, detailing chemical processes, modeling results, and implications for interstellar chemistry and unidentified infrared bands.

Contribution

It provides new modeling results for the inner wind of the archetype carbon star IRC+10216 and discusses PAH formation mechanisms in various evolved stellar environments.

Findings

Benzene forms close to the star due to shock-induced chemistry

Water is produced alongside benzene in the inner wind

PAH formation may resemble sooting flames chemistry

Abstract

The formation of Polycyclic Aromatic Hydrocarbons in the circumstellar outflows of evolved stars is reviewed, with an emphasis on carbon stars on the Asymptotic Giant Branch. Evidence for PAHs present in their winds is provided by meteoritic studies and recent observations of the Unidentified Infrared bands. We detail the chemical processes leading to the closure of the first aromatic ring as well as the growth mechanisms leading to amorphous carbon grains. Existing studies on PAH formation in evolved stellar envelopes are reviewed and new results for the modelling of the inner wind of the archetype carbon star IRC+10216 are presented. Benzene, C6H6, forms close to the star, as well as water, H2O, as a result of non-equilibrium chemistry induced by the periodic passage of shocks. The growth process of aromatic rings may thus resemble that active in sooting flames due to the presence of radicals like hydroxyl, OH. Finally, we discuss possible formation processes for PAHs and aromatic compounds in the hydrogen-rich R CrB star, V854 Cen, and their implication for the carriers of the Red Emission and the Diffuse Interstellar Bands.