Evolution of Single Stars. VI. Model Nuclei of Planetary Nebulae

Model evolutionary computations are presented for Population I (X_0=0.7, Z=0.03) stars of 0.6, 0.8 and 1.2 M_odot. Each model consists of a degenerate carbon-oxygen core, helium and hydrogen burning shell sources and a low mass hydrogen rich envelope. These models evolve from right to left on the H-R diagram because matter flows from their envelopes through the shell sources into their cores. Superposed on this evolution there areloops caused by the thermal pulses within helium burning shell sources. Finally the available nuclear fuel becomes exhausted and models cool down to the white dwarf stage. Neutrino energy losses are included in the computations. The initial model for every evolutionary sequence presented here was obtained from a model red subgiant stripped of most of its hydrogen rich envelope. These evolutionary sequences are believed to correspond to the evolutionary patterns of nuclei of planetary nebulae. The brightest and the youngest nuclei should be in a double shell source burning phase. Because of thermal pulses these stars are likely to heat up and cool down a number of times during the evolution. As a result a given planetary nebula may be ionized repeatedly during its lifetime.