Atomic data and the density structures of planetary nebulae
  We study the density structures of planetary nebulae implied by four
diagnostics that sample different regions within the nebulae: [S II]
$\lambda6716/\lambda6731$, [O II] $\lambda3726/\lambda3729$, [Cl III]
$\lambda5518/\lambda5538$, and [Ar IV] $\lambda4711/\lambda4740$. 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.
