Relative C, N, O abundances in red giants, planetary nebulae, novae and symbiotic stars.

Abstract

C/N and O/N abundance ratios deduced from UV data are presented for 24 symbiotic stars. There is little scatter in these ratios, indicating that the symbiotic phenomenon only occurs in a well defined evolutionary stage. Comparison with related objects such as red giants, novae, and planetary nebulae reveals that symbiotic objects best fit the CNO abundance ratios of normal red giants. This finding strongly supports the idea that symbiotics are binaries, in which a hot ionizing source illuminates the stellar material of a red giant star. There is no indication that the nebular material in symbiotic systems has experienced novalike nuclear processing. The importance of the C/N versus O/N diagram for the study of evolutionary connections between different types of objects is stressed. For planetary nebulae, there is no obvious correlation with the surface abundance ratios of their progenitor types. This suggests that planetary nebulae are not the remnants of the normal progenitor red-giant winds, but have been formed by rapid heavy mass loss at the end of the AGB evolution.