The transition from oxygen-rich to carbon stars

Recently, Willems and de Jong have suggested that optical carbon stars represent an intermediate evolutionary phase after the termination of mass loss during the oxygen-rich phase and before the carbon-enriched mass loss during the infrared carbon star phase. The authors have calculated evolutionary tracks for the transition from oxygen-rich to carbon-rich stars based on the model of Willems and de Jong. It is found that model spectra produce excellent fits to the observed energy distributions of carbon stars. By comparing observed data with the models, the mass-loss rate and the time since shell detachment can be determined for each individual object.