Spectra of carbon-rich asymptotic giant branch stars between 0.5 and 2.5 μm:: Theory meets observation

We present a hydrostatic analysis of five carbon rich stars, BH Cru, T Cae, S Cen, RU Pup and Y Hya in the wavelength range between 0.5 and 2.5 mum. All except BH Cru, which is a Mira star, show only modest variability. We identify the absorption features of the molecules CO, CN and C-2. The overall energy distribution, which is very sensitive to the effective temperature in the investigated wavelength range, as well as the bands of these molecules put strict limits on the possible values of effective temperature and C/O. We show that our model atmospheres and corresponding synthetic spectra are able to reproduce the observed spectra quite accurately from about 0.7 to 2.5 mum. The discrepancies are mainly due to uncertainties in the molecular input data. We discuss briefly why the variations of the molecular features are small and why dynamic phenomena do not play a very important role in this wavelength range. We identify colour indices based on commonly available filters and potentially suitable for the empirical determination of fundamental parameters of carbon stars.