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The primary goal of this paper is to make a direct comparison between themeasured and model-predicted abundances of He, C and N in a sample of 35well-observed Galactic planetary nebulae (PN). All observations, datareductions, and abundance determinations were performed in house to ensuremaximum homogeneity. Progenitor star masses (M < 4M_sun) were inferred usingtwo published sets of post-AGB model tracks and L and T_eff values. We concludethe following: 1) the mean values of N/O across the progenitor mass rangeexceeds the solar value, indicating significant N enrichment in the majority ofour objects; 2) the onset of hot bottom burning appears to begin around 2 solarmasses, i.e., lower than ~5 M_sun implied by theory; 3) most of our objectsshow a clear He enrichment, as expected from dredge-up episodes; 4) the averagesample C/O value is 1.23, consistent with the effects of third dredge-up; and5) model grids used to compare to observations successfully span thedistribution over metallicity space of all C/O and many He/H data points butmostly fail to do so in the case of N/O. The evident enrichment of N in PN andthe general discrepancy between the observed and model-predicted N/O abundanceratios signal the need for extra-mixing as an effect of rotation and/orthermohaline mixing in the models. The unexpectedly high N enrichment that isimplied here for low mass stars, if confirmed, will likely impact ourconclusions about the source of N in the Universe.

On the Production of He, C and N by Low and Intermediate Mass Stars: A Comparison of Observed and Model-Predicted Planetary Nebula Abundances