Emission line profiles as a probe of physical conditions in planetary nebulae

Aims. We present an analysis of physical conditions in planetary nebulae (PNe) in terms of collisionally-excited line (CEL) and optical-recombination line (ORL) profiles. We aim to investigate whether line profiles could be used to study the long-standing CEL/ORL abundance-discrepancy problem in nebular astrophysics. Methods. Using 1D photoionization models and their assumed velocity fields, we simulate the line profiles of various ionic species. We attempt to use our model to account for the observed CEL and ORL profiles. As a case study we present a detailed study of line profiles of the low-excitation planetary nebula (PN) IC 418. Results. Our results show that the profiles of classical temperature and density diagnostic lines, such as [O iii] λλ4363,5007, [S ii] λλ6716,6731, and [Ar iv] λλ4711,4740, provide a powerful tool to study nebular temperature and density variations. The method enables the CEL/ORL abundance-discrepancy problem to be studied more rigorously than before. A pure photoionization model of a chemically-homogeneous nebula seems to explain the observed disagreements in the profiles for the [O iii] λ4363 and the λ5007 lines, but cannot account for the differences between the [O iii] CELs and the O ii ORLs. We also investigate the temperature and density variations in the velocity space of a sample of PNe, which are found to be insignificant.