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

Abstract

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.

Photoionization models of Planetary Nebulae

“…3D models are used for different purposes: 1) reproducing the morphology given by imaging the object, 2) taking into account the size and the position of the spectrometer aperture, 3) constraining the velocity field by high spectral resolution observations. Examples of 3D models can be found in Zhang (2008), Morisset & Stasinska (2008), Wright et al (2011), Monteiro et al (2013), Danehkar et al (2013), and Akras et al (2016).…”

Excitation of emission lines by fluorescence and recombination in IC 418

“…Our high‐dispersion SED gives the chance to note the effect of the velocity field of the gas on the intensities of some lines mostly excited by fluorescence. Several authors have used different velocity fields to model forbidden‐line profiles in PN (Gesicki, Acker & Szczerba ; Neiner et al ; Zhang ). We have assumed two accelerating velocity fields, V ∝ R and V ∝ R 4 , with a maximum velocity of 40 km s −1 at the outer edge of the nebula, which are similar to fields considered in the literature.…”

The dark secrets of gaseous nebulae -- highlights from deep spectroscopy

“…These results suggest that ionized regions of vastly different temperatures (and radial and thermal velocities) co-exist in those nebulae (Barlow et al, 2006). Zhang (2008) shows that the differences in the observed line profiles of the O ii ORLs and the [O iii] CELs are hard to explain in a chemically homogeneous nebula.…”