Photospheric Abundances in Central Stars of Planetary Nebulae, and Evolutionary Implications

Abstract

The available information about spectral characteristics and photospheric abundances of 115 CSPN is presented. About 33% of CSPN in the sample are H-deficient. Many (but not all) H-deficient CSPN show extremely high C abundances. There is also some evidence of a variety of C abundances among H-rich CSPN. The available evidence suggests that most CSPN leaving the AGB with a H-rich photosphere will become DA white dwarfs. The “late He shell flash + fast wind” (born-again) mechanism may offer a satisfactory way of explaining some of the H-deficient CSPN. However, at least in some cases, the complete ejection of the H-rich stellar envelope, and exposure of He-burned matter at the remnant stellar surface, must happen already at the time of departure from the AGB.