EAS Publications Series
Volume 46, 2011PAHs and the Universe: A Symposium to Celebrate the 25th Anniversary of the PAH Hypothesis
|Page(s)||177 - 189|
|Published online||30 March 2011|
C. Joblin and A.G.G.M. Tielens (eds)
EAS Publications Series, 46 (2011) 177-189
The formation of Polycyclic Aromatic Hydrocarbons in evolved circumstellar environments
Departement Physik, Universität Basel,
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.
© EAS, EDP Sciences 2011
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