The relation between elemental abundances and morphology in planetary nebulae

An investigation of the variation of elemental abundances with planetary nebula morphology is of considerable interest, since it has a bearing upon how such sources are formed, and from which progenitors they are ejected. Recent advances in morphological classification now enable us to assess such trends for a statistically significant number of sources. We find, as a result, that the distribution N [log(X/H)] of sources with respect to elemental abundance (X/H) varies between the differing morphologies. Circular sources tend to peak towards low abundance values, whilst bipolar nebulae (BPNe) peak towards somewhat higher values. This applies for most elemental species, although it is perhaps least apparent for oxygen. In contrast, elliptical sources appear to display much broader functions N [log(X/H)], which trespass upon the domains of both circular and elliptical planetary nebulae (PNe). We take these trends to imply that circular sources derive from lower-mass progenitors, bipolar sources from higher-mass stars, and that elliptical nebulae derive from all masses of progenitor, high and low. Whilst such trends are also evident in values of mean abundance [X/H], they are much less clear. Only in the cases of He/H, N/H, Ne/H and perhaps Ar/H is there evidence for significant abundance differences. Certain BPNe appear to possess low abundance ratios He/H and Ar/H, and this confirms that a few such outflows may arise from lower-mass progenitors. Similarly, we note that ratios [O/H] are quite modest in elliptical planetary nebulae, and not much different from those for circular and bipolar PNe; a result that conflicts with the expectations of at least one model of shell formation.