Astron. Astrophys. 332, 721-731 (1998)

5. Discussion

About 60 bipolar PNe are known in the Galaxy (CS95, Manchado et al. 1996, Corradi et al. 1997b). The present sample of 15 objects (including IC 4406, paper I, and PN G321.6+0.2.2, Corradi et al. 1997b) therefore represents a significant fraction (25 ) of this morphological class of PNe, and some general considerations can be drawn.

In Fig. 8 we plot the abundances of the 15 bipolar PNe in the usual (N/O) vs. He/H diagram ( is used here instead of N/O because errors are lower, but all the following conclusions would still hold if we use the ratio between the total N and O abundances). As noticed by Peimbert (1978), and remarked by CS95, the great majority of objects in this morphological class are type I PNe, i.e. He and/or N rich. It has to be noticed that the present sample includes four objects which are possibly the PNe in Galaxy with the highest He and/or N/O abundances known up to date: PN G321.6+0.2.2, NGC 6537, He 2-111, M 3-2, to which M 1-75 (Guerrero et al. 1995) has to be added. The points in Fig. 8 seem to be grouped in a sequence of increasing N/O for an almost constant He, up to an upper limit of log(N/O) 0.5 at which the points start to be displaced toward very high He/H (up to 0.25!) without any further increase of the N/O ratio. The sequence of increasing N/O for bipolar PNe is qualitatively reproduced by the models of Renzini & Voli (1981) for quite massive progenitors ( =3-5  ), considering an efficient H-burning at the base of the convective envelope. No theoretical models exist, however, which are able to reproduce He overabundances as large as those shown by NGC 6537, He 2-111, and M 3-2 (Marigo et al. 1996, and 1997, private communication).

Fig. 8. ) vs. He/H diagram for the 15 bipolar PNe (dots with errorbars). The three helium-rich nebulae on the top of the diagram are, from right to left, M 3-2, He 2-111, and NGC 6537. Triangles are data for the bipolar PN M 1-75 (Guerrero et al. 1995), MyCn 18, M 1-8, and M 3-3 (KB94). The region at the top-right of the dashed lines is the locus of type I PNe (Peimbert & Torres-Peimbert 1983).

The possible Ne enrichment of bipolar PNe suggested by CS95 remains controversial. The mean Ne/O abundances ratio of the present sample (0.33 0.15) is higher than that of elliptical PNe in CS95 (0.22 0.07). On the other hand, the evidence of a Ne enrichment is marginal when the Ne/H ratio is considered (1.3 0.8 ( 10^-4) for our sample vs. 1.0 0.5 for the elliptical PNe in CS95). Also when comparing with the sample of non-type I in KB94 (average Ne/H=1.3 0.5), no evidence is found of a Ne enrichment of our sample of bipolar PNe. This is certainly a point which deserves further study, since the Ne enrichment could be the signature of efficient third dredge-up in the most massive PNe progenitors (Gallino et al. 1990).

In the present sample, there are 2 bipolar objects (He 2-36, and He 2-114) which are not type I PNe. These nebulae appear to have a "moderate" bipolar shape, since their equatorial waist is not very pronounced. In fact, if one compares the sequence of increasing N/O in Fig. 8, with the images of the corresponding nebulae in Fig. 1 (see also Fig. 2 in CS95 for the full image of He 2-111), making use of Table 3, an overall correlation is found between the N/O abundance and the "degree of bipolarity", estimated as the ratio between the maximum length and the minimum width of the objects, or alternatively, between their maximum width (measured in the lobes) to their minimum width (in the equatorial waist). According to Mellema (1997), bipolar PNe are more likely to develop from large mass progenitors, because the fast post-AGB evolution of their central stars avoids that the ionization front modifies the original density distribution, as instead occurs in low-mass progenitors preventing the formation of a marked bipolar morphology even if the initial conditions (AGB mass loss geometry) were favorable. According to these models, the present data further supports the conclusion that the PNe with high N/O and He abundances are produced by massive progenitors.

5.1. Other correlations between elements

The vs. (O/H) plot for our 15 bipolar PNe is presented in Fig. 9. Up to the value =0.2, there is some marginal evidence for a possible existence of an anti-correlation between (N/O) and (O/H), which relies on the position of the 2-3 points with the lowest N/O. We conservatively conclude that our data do not support the existence of such a correlation for these values of . This is in agreement with the results of KB94. On the other hand, for we find that nebulae tend to have lower oxygen abundances (only one object does not follow this tendency, but has very large errors). Note that KB94 in their similar diagram (their Fig. 5), have only two points above . It is certainly important to obtain further confirmation of the mentioned anticorrelation, considering the discrepant results of the different authors (see the extensive discussion in KB94). Nevertheless, we think that our data support, with some caution, the existence of such an anticorrelation for objects with 0.2. This would imply that, at least for the highest mass progenitors, a significant amount of nitrogen is formed at expenses of oxygen via a quite efficient ON-cycle.

Fig. 9. vs. (O/H) diagram for the 15 bipolar nebulae.

Finally, we confirm the conclusion by KB94 that the Ar abundance does not correlate with the N/O ratio, at variance with the results from de Freitas-Pacheco et al. (1992).

© European Southern Observatory (ESO) 1998

Online publication: March 23, 1998
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