Circumstellar carbon condensation .1. Low-p,T stabilities of polyaromatic hydrocarbons

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 mast 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 rone 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 500 K, the equilibrium situation of the gas, the chemical pathways to PAHs and the PAH formation rates are discussed.