EDP Sciences
Free Access
Issue
A&A
Volume 570, October 2014
Article Number A26
Number of page(s) 25
Section Interstellar and circumstellar matter
DOI https://doi.org/10.1051/0004-6361/201322025
Published online 09 October 2014

Online material

Appendix A: Comparison with emission-line flux standards

In this section, we compare our line measurements with data published by Dopita & Hua (1997). We have 11 PNe in common with their list of southern emission-line flux standards. They are listed in Table A.1. In Fig. A.1, we present the comparison for the lines of [O III] λ5007, Hα, [N II] λ6583, He I λ6678, [S II] λ6717+31, and [Ar III] λ7136. The differences have been normalised separately for each PNe and each line to the error of the line intensity as given by Dopita & Hua (1997).

thumbnail Fig. A.1

Comparison of our line intensities with measurements of Dopita & Hua (1997) for 11 PNe in common that are listed in Table A.1. The difference for each PNe (identified on horizontal axis) between our measurement and of Dopita & Hua (1997) is marked with a black dot using a normalised scale (vertical axis). The normalisation was done with the original errors from Dopita & Hua (1997). If the normalised absolute difference is less than 1 (dashed lines) our measurements agree within the uncertainty given by the latter authors. Open triangles present an analogous comparison for data from Acker et al. (1992) compared to Dopita & Hua (1997) for the same PNe and are given for reference.

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Because our observations are not absolutely flux calibrated, we first had to recalculate values of Dopita & Hua (1997) to the same scale (Hβ = 100) as in our Table B.1. Therefore, the deviations shown in Fig. A.1 are influenced not only by the uncertainty of the given line but also to some degree by the errors of Hβ in both our and Dopita & Hua (1997) measurements. For the 11 PNe in common with our observations and presented in Fig. A.1, the error of the Hβ flux8 in Dopita & Hua (1997) varies between 1% and 2%.

The dashed lines in Fig. A.1 represent the normalised error limits of Dopita & Hua (1997) measurements. Our results are shown with black-filled circles. The triangles in Fig. A.1 represent line measurements from the Strasbourg-ESO catalogue of Acker et al. (1992) and are shown for reference. The latter have been derived from the Catalogue’s spectroscopic survey and are of considerably lower sensitivity than our spectra.

The panel A of Fig. A.1 presents the comparison of our measurement and that of Dopita & Hua (1997) of the [O III] λ5007 line. In almost perfect agreement, it can be seen that most of our points are confined within the error bars given by Dopita & Hua (1997).

The situation is different in panel B that presents the comparison of Hα measurements. Although there is no systematic deviation between our data and Dopita & Hua (1997), the scatter is larger than in the previous plot. It cannot be blamed on the measurement of Hβ line, which is sometimes 10 times fainter then the Hα line, since it would then have comparable impact in the first panel. We checked that the internal errors of Dopita measurements for Hα/Hβ line ratio are largest for objects Nos. 1, 6, and 9 (see Table A.1) and reach 4.0, 4.5, and 3.2%, respectively. It can be noted that our measurements actually fall within the normalised errors in these cases. One possibility is therefore that the errors of Dopita & Hua (1997) are underestimated for other PNe. The discrepancy of Hα/Hβ ratios between different authors are not uncommon. In this case, we can see that they are mostly influenced by Hα measurements itself.

Table A.1

List of PNe in common with Dopita & Hua (1997).

In panel C of Fig. A.1, we show a comparison for the close [N II] λ6583 line. The same objects Nos. 1, 6, and five others show satisfactory agreement with Dopita & Hua (1997). The objects that deviate most are Nos. 2, 8, and 11. Object 2 is a high excitation nebula with [N II] lines very weak, and the error of this measurement could be larger than calculated by Dopita & Hua (1997). In the case of Nos. 8 and 11, another explanation can be invoked. Those two PNe are low ionisation objects with [N II] dominating the spectra. Dopita & Hua (1997) used slitless spectroscopic observations and measured the cumulative flux from the nebula. Since we used a narrow slit positioned on the central star, we could have missed some of the [N II] radiation from the external parts of the nebula9. This difference in observing technique will have smaller influence on ions of higher ionisation potentials, like [O III], since they are naturally concentrated towards the centre of the nebula.

In panel D of Fig. A.1, we compare the He I λ6678 line. Despite that it is usually much fainter, we find a better agreement with the data of Dopita & Hua (1997). This again advocates in favour of the errors of some Hα and [N II] measurements being underestimated by these authors. However, some influence in which we frequently had to use short snapshot spectra to measure such lines cannot be excluded.

In panels E and F of Fig. A.1, we compare our results with Dopita & Hua (1997) for the lines of another two ions. In the case of [S II] doublet λ6717+31, a similar picture to [N II] is seen strengthening our arguments presented above. For [Ar III] λ7136, we found a generally satisfactory agreement. It is worth to note that no measurable influence by second order contamination effects is noticeable in our data. It would systematically underestimate measurements of red lines, like [Ar III] λ7136, which is not observed.

In summary, the analysis presented above show that there are no systematic deviations in our line measurements compared to Dopita & Hua (1997). It also demonstrates that the discrepancies in measured line intensities between different authors can be a complex function of many factors (wavelength, blending, location of emission zone within the nebula, etc.), and the values do not always converge with increasing line strength.

Appendix B: Supplementary tables

Table B.1

Main nebular lines on the scale Hβ = 100.

Table B.2

New, reobserved, and mis-classified PNe with emission-line central stars.

Table B.3

Plasma parameters and abundances.

Appendix C: Supplementary figures

thumbnail Fig. C.1

Spectra of the new PNe with WELs stars. Dotted lines mark possible locations of stellar emission-lines identified with the ion name if the line was detected.

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thumbnail Fig. C.2

Spectra of the possible new PNe with Wolf-Rayet type central stars.

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thumbnail Fig. C.3

Spectra of the reobserved PNe with known and misclassified emission-line central stars.

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thumbnail Fig. C.4

Ratio of O+ ionic abundances derived from λ7325 and λ3727 lines as a function of electron density for observed PNe.

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thumbnail Fig. C.5

Ratio of O+ ionic abundances derived from λ7325 and λ3727 lines as a function of electron temperature Te(N ii) for observed PNe.

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© ESO, 2014

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