5.1. NGC 7027 and IC 418: two test cases
It is instructive to compare the abundances determined by various authors for two bright and well studied PNe.
NGC 7027 is the PN with the highest optical surface brightness despite of 3.5 mag absorption by dust and is a benchmark for PN spectroscopists. It is a very high excitation nebula, with lines of [Ne VI] now measured (Bernard Salas 2001). The central star temperature is estimated to be 140000 -180000 K, the gas density is around 5 × 104 cm-3. The nebula is surrounded by a dusty neutral shell. Table 5 lists the abundances derived for this object. Substantial differences are seen among the results obtained by various authors. It is interesting to recall that the concommittent models of Shields (1978) and of Péquignot et al. (1978) produced [O II] and [N II] intensities about one order of magnitude smaller than observed. Multidensity geometries and modifications of the stellar continuum failed to resolve this difficulty. Péquignot et al. (1978) postulated the existence of efficient charge transfer processes, and obtained an excellent fit to the observations by adjusting the charge transfer rates. These charge transfer rates were later confirmed by atomic physics computations. In spite of the different approaches adopted by Shields ((1978) and Péquignot et al. (1978), the resulting abundances are rather similar. On the other hand, they are significantly different from the abundances obtained later for this object. This is not only due to the use of different atomic data or to the number and quality of observational constraints (e.g. ISO spectroscopy provided high quality measurements on a large number of IR lines): when models are not entirely satisfactory, the abundances finally adopted are a matter of the author's personal choice.
|FIR data from ISO + optical + UV, empirical|
|b Kwitter & Henry (1996), optical + UV data, model|
|c Keyes et al. (1990), optical + UV data, model|
|d Middlemass (1990), optical + UV + FIR data, model|
|e Perinotto et al. (1980), optical + UV data, empirical|
|f Péquignot et al. (1978), optical + UV + FIR data, model|
|g Shields (1978), optical + UV + FIR data, model|
|h Aller 1954, optical data, empirical|
IC 418 is also a bright and relatively dense (n ~ 5 × 104 cm-3) PN, but with a central star of low effective temperature (T ~ 38000 K), so that fewer ions are observed. The nebula is surrounded by an extended neutral shell. Here again, there are substantial differences among the published abundances. In this case, the differences in O/H cannot be attributed to ionization correction, since O is observed in all its ionization stages. It is actually the observational data which strongly differ from one author to another! Besides, results from empirical methods depend, as we know, on the assumptions made for the temperature structure. As for models, they do not give satisfactory fits for this object and therefore do not provide reliable abundances.
|a Henry et al. (2000), optical + UV data, model|
|b Hyung et al. (1994), opt echelle + UV + a few IR data, model|
|c de Freitas Pacheco et al. (1992), optical data, empirical|
|d Gutierrez Moreno (1988), optical data, empirical|
|e Aller & Czyzak (1983), optical data, hybrid method|
|f Harrington et al. (1980), optical + UV data, empirical|
|g Barker (1978), optical data, empirical|
|h Torres-Peimbert & Peimbert (1977), optical data, empirical with t2 = 0.035|
These two examples may serve as a warning that abundances are not necessarily as well determined as might be thought from error bars quoted in the literature.