LARGE PLANETARY-NEBULAE AND THEIR SIGNIFICANCE TO THE LATE STAGES OF STELLAR EVOLUTION

We present spectrophotometry of 75 large planetary nebulae, defined here as having Shklovsky radii greater than 0.15 pc. The data are used to calculate nebular parameters and compositions, stellar Zanstra temperatures and luminosities, and core masses. We identify nine new Peimbert type I nebulae (N/O > 0.8, He/ H > 0.15), as well as several others with milder enrichments. He/H correlates with N/O much as found before. The scatter of the combined available abundance data suggests that more than one dredge-up scenario is operating in the predecessor stars. The distribution in core mass, which is heavily dependent upon the uncertain distances, is rather wide. About 40% of the stars that are on cooling tracks are above 0.7 M⊙, and over 15% are above 0.8 M⊙. All the available data on large planetaries demonstrate a clear positive correlation between nitrogen enrichment and core mass (Mc). Mean N/O is constant as Mc increases from 0.55 to 0.8 M⊙, then jumps quickly to values higher than calculated through third dredge-up, providing evidence for some combination of more efficient dredge-up than expected and the existence of hot bottom (envelope) burning. N/O is also anticorrelated with O/H, a relation that becomes apparent only for N/O ≳ 1, or Mc ≳ 0.9. This correlation is difficult to see among the more heavily observed compact nebulae, since (because of evolutionary rates) they tend to have preferentially lower core masses, and consequently lower enrichment factors. The radii of the nebulae whose stars lie along specific cooling tracks increase monotonically with decreasing central star temperature as expected, giving some credence to the relative Shklovsky distances. For a given central star temperature, the nebular radii also increase with increasing core mass, showing empirically (under the assumption of a common expansion velocity) that in this part of the log L-log T plane the higher mass cores evolve the more slowly in agreement with theoretical prediction. However, theoretical evolutionary rates for the stars of large nebulae appear to be much too slow, predicting nebulae much larger than observed as the stars cool toward the white dwarfs.