2000A&A...361..601D

We have investigated the excitation of the molecule HCN in the circumstellar envelopes of carbon stars, particularly the HCN(J=1-0) maser emission using a model which takes into account the collisional and radiative excitation, as well as the effect of the overlap of hyperfine lines. We calculate consistently both the gas temperature and the mass loss rate based on the energy balance equation and the fitting of CO observations. In this way we can obtain more realistic data of the envelope. In optically thin envelopes such as that around Y CVn, our results suggest that the HCN(J=1-0) transition is inverted by the absorption of 3 µm stellar photons which transfers the population to the (001) vibrational state and by subsequent cascades down to the fundamental state. Mutual amplification of hyperfine components results in HCN line profiles containing several strong maser spikes. In thicker envelopes, such as IRC+10216, we find that HCN and H¹³CN are excited mainly by the absorption of infrared photons emitted by the warm dust at 14µm. The HCN(J=1-0) and H¹³CN(J=1-0) line profiles calculated by our model contain several features as observed. We also discuss the implication of our results on the mass loss history of IRC+10216.