The relation between elemental abundances and morphology in planetary nebulae

Abstract

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.

The relation between Zanstra temperature and morphology in planetary nebulae

“…The present results confirm for temperature what has also been noted for expansion velocities, latitudes and abundances (e.g. Phillips 2002a, 2001b, 2003a). Whilst some of these trends would have been expected on evolutionary grounds (see the following section), and are therefore not entirely surprising, this represents the first time that such differences have been concretely defined.…”

“…Thus whilst the bulk of the nebulae arise from low‐mass progenitors, a rump of higher‐temperature sources must derive from higher‐mass stars. This, in the case of elliptical sources, is consistent with what has been found from nebular abundances (Phillips 2003a). Such a result is also not entirely surprising.…”

“…Finally, Phillips (2003a) has noted that both the circular and jet‐like nebulae possess similar elemental abundances (Phillips 2003a).…”

“…There are other indications to support this presumption. Thus, the abundances of elements in circular sources appear less than those for BPNe, whilst elliptical PNe have a very broad range of abundances (Phillips 2003a). This is consistent with the mean trends in latitude cited above, and suggests that elliptical sources are associated with a large range of progenitor masses.…”

Spitzer mid-infrared observations of seven bipolar planetary nebulae

“…Phillips & Ramos‐Larios 2005 and ), then it seems possible that C/O ratios are greater than unity in a good fraction of the present sources. More concrete evidence for such abundances appears to be a little hard to come by, although we note that C/O ratios are ∼1.6 in NGC 2818 (Phillips 2003c, and references therein), and ∼1 in NGC 6445 (van Hoof et al 2000). There appears to have been no prior detection of IR‐band features in any of these sources (Casassus et al 2001).…”