Line blanketing in model atmospheres of carbon stars.

A comparison of opacity sampling (OS) and opacity distribution function (ODF) methods for the calculation of the model atmospheres of carbon stars is presented. In order to determine the structure of OS model atmospheres to an accuracy of about 30 K or better, at least 5000 frequency points with a near optimum distribution are found to be necessary. The ODF approximation is shown to cause significant errors, particularly where polyatomic opacity dominates in the surface layers, and the diatomic molecules at larger depths. The noncorrelation assumption is not found to lead to significant errors, even for very cool and carbon rich stars. Good agreement is found between multiple component ODF models and ODF models calculated using the ODF-summation procedure of Saxner and Gustafsson (1984) for single component ODFs.