SIMBAD references

We study the density structures of planetary nebulae implied by four diagnostics that sample different regions within the nebulae: [S II] λ6716/λ6731, [O II] λ3726/λ3729, [Cl III] λ5518/λ5538, and [Ar IV] λ4711/λ4740. We use a sample of 46 objects with deep spectra that allow the calculation of the electron density from these four diagnostics, and explore the impact that different atomic data have on the results. We compare the observational results with those obtained from photoionization models characterized by three different density structures. We conclude that the atomic data used in the calculations of electron density fully determine the density structures that are derived for the objects. We illustrate this by selecting three combinations of atomic data that lead to observational results that are compatible with each of the three different density structures explored with the models.