Photoionization modeling of Magellanic Cloud planetary nebulae. II

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Abstract

We present the results of self-consistent photoionization modeling of a further sample of 44 Magellanic Cloud Planetary Nebulae (PN), to derive a Hertzsprung-Russell (H-R) diagram for the central stars, chemical abundances and the nebular parameters. We find that, for optically thick PN, the transformation between the "observed" Hertzsprung-Russell (H-R) diagram: the log (Hβ excitation class plane, and the "true" H-R diagram: the log (L/L_sun_) log (T_eff_) plane, is particularly simple. We also find a very tight relation between the reduced mass of the nebula, that is, the ionized mass corrected to unit luminosity of the central star, and the nebular radius. From their positions on the H-R diagram, we conclude that the majority of the central stars in this sample have masses between 0.55 and 0.7 M_sun_, but that the type I PN have generally higher mass central stars, ranging up to 1.2 M_sun_. These objects are observed mostly during the fading part of their evolution, consistent with their expected more rapid evolution across the H-R diagram. We find that there is clear evidence for a spread in the abundances of the alpha-process elements in both Clouds which is probably a consequence of the spread in ages of the PN precursor stars. For the LMC there is also a correlation between core mass and metallicity for these elements. In the future, this relationship may allow us to derive age: metallicity relationships for the Clouds. For the type I PN we find that, not only do they lie on a distinct sequence on the N/O versus He/H diagram, but also, they appear to be deficient in O, with respect to the other alpha-process elements. We interpret this as evidence for not only for CN processing, dredge-up and ejection, but also for appreciable NO processing having taken place.

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