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

TL;DR

This study uses line profile analysis of planetary nebulae to explore temperature and density variations and address the CEL/ORL abundance discrepancy problem, demonstrating the effectiveness of profile diagnostics in nebular physics.

Contribution

It introduces a method using line profiles and photoionization models to study nebular conditions and the CEL/ORL discrepancy in planetary nebulae.

Findings

Line profiles of diagnostic lines reveal temperature and density variations.

Photoionization models explain some profile differences but not all.

Temperature and density variations in velocity space are generally insignificant.

Abstract

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. 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. Our results show that the profiles of classical temperature and density diagnostic lines, such as [O III] 4363,5007, [S II] 6716,6731, and [Ar IV] 4711,4740, 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] 4363 and the 5007 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.