The identification of HCN and HNC in carbon stars:: model atmospheres, synthetic spectra and fits to observations in the 2.7-4.0 μm region

Model carbon star atmospheres and synthetic spectra have been calculated using the recent HCN/HNC vibration rotation line list of Harris et al. The calculations are repeated using only HCN lines and show that HNC has a significant effect upon the temperature, density and optical depth of a stellar atmosphere. We fit synthetic spectra in the 2.7-4.0 mum region to observed ISO spectra of the carbon stars WZ Cas and TX Psc obtained by Aoki et al. These fits allow us to identify absorption by HNC in the spectrum of WZ Cas at 2.8-2.9 mum, and to determine new independent estimates of effective temperature and log (N-C/N-O). The findings reported here indicate that absorption by both HCN and HNC is needed to fully explain the observed stellar spectra and represent the first identification of HNC in a star. Q-branch absorption by the HCN Deltaupsilon(2) = 1, Deltaupsilon(3) = 1 and Deltaupsilon(1) = 1, Deltaupsilon(2) = 1 bands at 3.55 and 3.86 mum, respectively, is identified in the spectrum of WZ Cas.