HUBBLE-SPACE-TELESCOPE OBSERVATIONS OF PLANETARY-NEBULAE IN THE MAGELLANIC CLOUDS .5. MASS DEPENDENCE OF DREDGE-UP AND THE CHEMICAL HISTORYOF THE LARGE MAGELLANIC CLOUD

A photoionization analysis of Hubble Space Telescope UV and ground-bas ed optical spectrophotometry is given for eight more planetary nebulae (PNs) in the Large Magellanic Cloud (LMC). This allows the central st ars to be placed accurately on the H-R diagram and permits the determi nation of the He, C, N, O, Ne, S, and Ar abundances. In some cases, th e gas-phase abundances of Mg and Si may also be determined. We have co mbined these results with the analysis of two other objects published by us in the first two papers of this series. The observed abundance p atterns are qualitatively consistent with the (mass-dependent) operati on of the various chemical dredge-up processes as predicted by theory, Dredge-up of C during the thermal pulsing stage appears to be most im portant, and ''hot bottom burning'' transforms much of this C to N in the more massive stars. There is no sign of drudge-up of Ne-22. We sho w that the spread in the alpha-process element abundances can be under stood as being due to differences in core mass of the planetary nebula nucleus, which is related directly to initial mass of the precursor s tar. This is, therefore, a tracer of the age-metallicity relationship for stars in general, and we derive, for the first time, the chemical history of the LMC based on PNs. We find that the base metallicity of the LMC almost doubled similar to 2 Gyr ago. This is consistent with s tudies of field stars and of clusters that show that there was a major burst of star formation at the time.