Study of electron density in planetary nebulae - A comparison of different density indicators

We present a comparison of electron density estimates for planetary nebulae based on different emission-line ratios. We have considered the density indicators [O II]lambda3729/lambda3726, [S II]lambda6716/lambda6731, [Cl III]lambda5517/lambda5537, [Ar IV]lambda4711/lambda4740, C III]lambda1906/lambda1909 and [Ni]lambda5202/lambda5199. The observational data were extracted from the literature. We have found systematic deviations from the density homogeneous models, in the sense that: N-e(N I) less than or similar to N-e(O II) < N-e(S II; C III; Cl III or Ar IV) and N-e(S II) ≈ N-e(C III) ≈ N-e(Cl III) ≈ N-e(Ar IV). We argue that the lower [O II] density estimates are likely due to errors in the atomic parameters used.