The Position of PN Nuclei in the H-R Diagram: Present Status

Abstract

Recent improvements in the determination of the distances to various planetary nebulae are discussed. This increases the accuracy of the luminosities to a selected group of nebulae. The central star temperature determination has also been substantially improved for a large group of very faint hot objects. This involves especially new measurements of the central star magnitudes. The results are combined in giving an improved H-R diagram. A comparison between model calculations and observed positions on the H-R diagram gives a value for the age of the central star since it left the AGB. Another value for this age can be obtained from the size of the nebula and the observed expansion velocity. A discrepancy between these two ages seems to exist.