CASSCF and CCI calculations of the vibrational band strengths of HCN

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Abstract
HCN has been observed to be a constituent of late type carbon‐rich stellar atmospheres and has recently been suggested to be a very important opacity source in cooler carbon stars. The transitions giving rise to the combination vibration–rotation bands in the near infrared region may be expected to be the most important HCN transitions for the structure of the carbon‐star atmospheres. In this paper CASSCF and CCI calculations of vibrational bands in the interval 3000–10 000 cm−1 are presented. Accounting for the HCN opacity extends the model atmosphere by a factor of 5, and lowers the gas pressure in the surface layers by one or two orders of magnitude. The structures, e.g., the gas pressures, in the upper layers of these models are uncertain by more than a factor of 3 as a result of the uncertainties in the potential surface of the molecule and in the method for handling the degeneracy for the vibrational bands. These uncertainties are, however, of minor importance in practical applications in present carbon‐star work.