Circumstellar Carbon Condensation: I. Low-p,T stabilities of Polyaromatic Hydrocarbons

A. Goeres, R. Keller, E. Sedlmayr and H.-P. Gail

Polycyclic Aromatic Compounds, 8, 129-165 (1996)

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Abstract:

To investigate the carbon condensation process in the cool envelopes of carbon-rich supergiant stars (C-stars), the stabilities of linear and polyaromatic hydrocarbons (PAHs) up to 26 carbon atoms have been calculated for an H/C-ratio of 2000, according to the particular elemental mixture in those objects. The study comprises closed shell species as well as radicals. The data is based on molecular mechanics calculations for most of the heats of formation and on group additivity rules for entropies and heat capacities. Based on an analysis of the particular physical situation in the condensation zone of C-star envelopes, in the range of n_g=10^{13} to 10^{6}cm^{-3} for the gas particle density and temperatures from 1200 to 500K, the equilibrium situation of the gas, the chemical pathways to PAHs and the PAH formation rates are discussed.