Observational results on abundances in planetary nebulae

Recently, ultra deep spectroscopy of bright PNe allowed to detect and measure lines from elements of the fourth, fifth and even sixth row of the Mendeleev table (Pequignot Baluteau 1994, Baluteau et al. 1995, Dinerstein 2001, Dinerstein Geballe 2001) V, Cr, Co, Ni, Cu, Zn, Se, Br, Kr, Rb, Sr, Y, Te, I, Xe, Cs, Ba, Pb. When the atomic 

It is instructive to compare the abundances determined by various authors for two bright and well studied PNe. 

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. 

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. 

PNe as probes of the chemical evolution of galaxies 6.3.1. The universal Ne/H versus O/H relation 

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