POLYCYCLIC AROMATIC-HYDROCARBONS AND MOLECULAR EQUILIBRIA IN CARBON-RICH STARS

Thermal equilibrium (TE) molecular concentrations are computed for several carbon-rich elemental compositions and various total gas pressures. Such calculations may, be used to estimate the molecular abundances in a carbon-rich stellar photosphere and its vicinity. The six elements and seventy molecules considered include observed molecules and radicals, polycyclic aromatic hydrocarbons (PAHs) up to the size of coronene, long-chain hydrocarbons, the cyanopolyyne group, and complex ring compounds. The thermodynamic data used are from the recent literature or are estimated. Condensations of graphite, silicon carbide, and quartz are also considered. The results show that long-chain hydrocarbons are abundant at moderate temperatures. PAHs also form in significant amounts at low temperatures, but only benzene survives graphite condensation. TE molecular profiles are obtained for the inner region of an envelope model representative of late-type stars.