1994A&A...290..531G

A model is presented to calculate the line profiles of thermal emission of molecules in an expanding spherically symmetric circumstellar envelope. Special attention is given to the heating and cooling mechanisms which determine the kinetic temperature of the gas. The constraints that the presence of dust put on the molecular emission model are discussed. The following processes are found to have an effect of more than 10% on the integrated intensities (relative to a standard model): cooling by ¹³CO and HCN in carbon stars, cooling by H₂O in oxygen-rich stars, and the location of the inner boundary of the molecular shell. The model including all physical features considered predicts CO(6-5) intensities which are 25% larger and CO(1-0) intensities which are 10% smaller than those of the standard model. Photoelectric heating can be the dominant source of heating in the outer layers, thereby determining the CO(1-0) intensity, depending on the beam size of the telescope and the efficiency of the shielding of UV radiation by dust.