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

4. Chemical abundances

The ionic and total abundances and their errors were computed from the 1-D spectra for each region as described in paper I. We recall here that a proper scheme, which is essentially the one suggested by Kingsburgh & Barlow (1994, hereafter KB94), was used in adopting temperatures for the lines of different ions. Abundances are listed in Tables 17-29 and plotted in Figs. 2 - 5 for all the various regions into which nebulae have been divided. As with sulphur, an element whose abundance calculation is particularly uncertain, if the ionic abundance cannot be derived from the [SIII]6312 or [SIII]9069,9531 lines, then the total S/H abundances was not computed.

4.1. The core of NGC 6537

As described in paper I, errors on the ionic abundances were calculated by taking into account both the errors in the line ratios and those on the adopted temperature. The errors on total abundances are obtained by propagating the errors on the mean ionic abundances as well as on the icf. Including all that, the final errors on the total abundances are found to be typically of 10-30 for He (where no icf correction is needed, nor the temperature dependence is of importance), and larger for the other elements. As seen in the tables and figures, the largest errors always occur in the outer parts of the nebulae due to limited S/N ratio in these regions. The errors in the total abundances are indicated within parentheses in Tables 17-29. If errors are larger than 80 , the total abundance is considered to be very uncertain and we indicate it with the symbol ":". The typical errors associated with the total abundance determinations are as follows.
Oxygen: between 20 and 50 .
Nitrogen: between 20 and 80 , except for He 2-114 and few positions through NGC 2818 and Mz 1 where they are larger.
Neon: between 20 and 80 , excluding NGC 2899, He 2-114, and the outermost position of Mz1.
Argon: errors are similar to those of Ne.
For sulphur the information is often incomplete.

The computed errors clearly limit the utility of the present data in highlighting small chemical variations throughout the nebulae. Variations larger than the indicated levels, like those claimed by Guerrero et al. (1995) for the bipolar PN M 1-75, are however expected to be detected.

4.2. Abundances variations through the nebulae

Inspection of Figs. 2 - 5 and 6 shows that no variations of the He, O, and N abundances are detected through the nebulae within our present errors. The case of M 1-75 (Guerrero et al. 1995), where a significant decrease of the N/O in the core of the nebula was detected, seems therefore to remain an isolated one.

We recall here that the ionic ratio is usually taken as a measure of the total N/O ratio, because of the similar ionization potentials of the relevant species (but caution has to be taken owing to the possible importance of charge-exchange reactions with hydrogen; cf. Osterbrock, 1989). The profiles of all nebulae are presented in Fig. 6. In the figure, no changes in the ratio are observed throughout the nebulae, within the computed errors which are often of the order of 30 , certainly smaller than those which would have been deduced by combining the errors in the N and O total abundances. A possible variation in is indicated only at the centre of NGC 2440, where it results to be by a factor of 1.4 larger than in the rest of the nebula. This variation is however just at the limit of the errorbars, and we consider it as tentative. Note, however, that the same apparent central increase of the N/O ratio has been found by Guerrero (1995).

Fig. 6. profiles of the nebulae.

As for Ne, Ar, and S, the situation is different in the following sense. As noted in paper I for IC 4406, clear trends are observed in the abundance profiles of these elements, although usually within the computed errors. If we consider the face values in 5 nebulae of our sample in which the chemical information is extended to their faint outer regions (IC 4406, NGC 2440, NGC 2899, NGC 6072, and Mz 1), we find that: i) the abundance of the three elements remains practically constant throughout the nebula in NGC 6072; ii) in the other four nebulae, the abundances of neon, argon, and sulphur increase from the centre to the border by factors between 1.3 and 5, depending on the specific element and nebula. For Ne, the centre-border variations amount to a factor between 2.5 and 4; for Ar, between 1.3 and 2.5; and for S, between 1.7 and 5 (except for NGC 2899, where S is constant).

As discussed in paper I, this systematic effect may have to do with the use of an improper ionization correction scheme (KB94), when going to the outer parts of the nebula. This kind of effect is predicted by Alexander and Balick (1994) for long-slit observations of spherical PNe. On the other hand, from the various nebulae studied in the present paper we confirm the result found in paper I for IC 4406: nitrogen, which has the largest icf, is not affected by this problem. The matter deserves then some explanation that we cannot offer at present. In any case the systematic effect, which is noticeable for Ne, Ar and S, is at present an additional limitation in the search for abundance variations of these elements across the nebulae.

We conclude that no abundances changes are detected across the objects studied in the present work to within variations of, essentially, 30 , 50 , and 80 for He, O and N, respectively, and to within larger intervals for Ne, Ar, and S depending on the element and nebula. No variations are either found, within some 30 , for the ratio, which is commonly taken as a measure of the total abundance ratio. This refers to the five nebulae observed in 5-7 spatial positions, up to their faint outer regions. For the objects observed in less positions, the precision in the abundance determinations is generally higher (see Tables 17-29).

4.3. Average abundances

Considering that no clear evidence is found for abundance variations throughout the nebulae, average abundances were computed by weighting the determinations in the different positions with both their errors and a "mass" factor (proportional to the square root of the total H flux from that region). For NGC 6537, following the remarks in sect.  4.1, the central region c was not given a larger weight than the surrounding ones. Average abundances are listed in Table 3.

Table 3. Average abundances.

4.4. [NII]/H line ratios vs. N/O abundances

It has been sometimes questioned whether the high ([NII]6548+6583)/H line ratios measured in bipolar PNe, and in particular in their lobes, are unequivocal signatures of nitrogen enrichment. In Fig. 7 we plot the observed [NII]/H line ratios vs. the average N/O abundances. For each nebula in our sample, the [NII]/H ratio integrated all over the slit (simulating the case of a very distant nebula which is not spatially resolved) is indicated by a filled circle, while dotted lines give the range of line ratios showed in the various positions along the slit. For some objects, measurements have been extended to extremely faint regions, which were not considered in our chemical analysis since only the [NII] and H emission could be measured over the entire observed spectral range. In Fig. 7, we also plot with empty circles data for the 32 nebulae in KB94 for which the complete information (H and [NII] fluxes, N/O abundance) is available, excluding those objects which are already included in our sample. It has to be noted that most of these 32 nebulae are not bipolar.

Fig. 7. The correlation between the [NII]/H line ratio and the N/O abundance. Dots are the ([NII]6548+6583)/H ratio for each nebula integrated all over the slit length, while the dotted lines give the range of line ratios showed by each object when fluxes are measured locally, i.e. in the spatial resolution elements along the slit. Empty circle represent the data for 32 PNe from KB94.

A trend of increasing N/O abundance with the [NII]/H line ratios is evident, but scatter is very large. In addition, for each object in our sample the internal variations of [NII]/H are also quite large ( 0.8 dex), while the N/O ratio was shown to be approximately constant throughout the nebulae. Nevertheless, the correlation in Fig. 7 can be used, for instance, to pick up nebulae with very high N/O abundances (and thus likely massive progenitors) by simply measuring their integrated [NII]/H line ratios (note that [NII]/H 3 always points to a N/O ). This can be valuable for statistical purposes, or for selecting specific classes of highly over- or under-abundant objects in spectroscopic and narrowband imaging surveys of PNe.

4.5. Comparison with previous studies

Previous abundances determinations exist for 9 of the 13 PNe discussed in this paper (Perinotto 1991, Koppen et al. 1991, KB94, Baessgen et al. 1995, and Guerrero 1995 for NGC 2440; Perinotto 1991, Guerrero 1995, and De Freitas-Pacheco et al. 1992 for NGC 2818; Lopez et al. 1991 and KB94 for NGC 2899; Perinotto 1991 and Perinotto et al. 1994 for NGC 6537; Gutierrez-Moreno et al. 1994, De Freitas-Pacheco et al. 1992, and KB94 for He 2-111; Perinotto 1991 and KB94 for M 1-13; Kaler et al. 1996 for M 1-16; Koppen et al. 1991 for M 3-2; Perinotto et al. 1994 for Mz 1). Note that the work by Perinotto (1991) is not an original work, but a critical compilation based upon original measurements by other authors. For determinations obtained before 1990, we have just referred to that extensive compilation, in which the reader can find the references to the original papers. For NGC 6072, He 2-36, He 2-84, and He 2-114 we did not find any previous chemical study. It has also to be remembered that all the studies mentioned above, except for the one by Guerrero (1995) and Perinotto et al. (1994), do not contain spatially resolved chemical information through the nebulae.

As expected considering the different quality of the observations and analysis methods, several discrepancies between the determinations of the various authors are found. Since a detailed one-by-one comparison would result quite dispersive and not really instructive, we only mention some individual peculiarities which we consider to be noteworthy. As noted in paper I for IC 4406, also in the cases of NGC 2818 and He 2-111 the O, N, and Ar abundances from de Freitas-Pacheco et al. (1992) are by a factor between two and four systematically higher than the determinations from all other authors (and, for He 2-111, their He abundance is by a factor 1.6 smaller than ours). In the two cases where the comparison is possible, a systematic difference also appears between our He abundance and those of Koppen et al. (1991), their value being lower by up to a factor 1.5. Generally, excluding some exceptions, the determinations from the different authors agree with our results within some 20 for He, and some 40-50 for O, N, and Ne. Ar, and S present larger discrepancies.

© European Southern Observatory (ESO) 1998

Online publication: March 23, 1998
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