When Shape Matters: correcting the ICFs to derive the chemical abundances of bipolar and elliptical PNe

TL;DR

This paper develops new ionisation correction factors based on 3-D photoionisation models to improve chemical abundance determinations in bipolar and elliptical planetary nebulae, accounting for shape effects.

Contribution

It introduces a set of shape-dependent ICFs derived from 3-D models, improving abundance accuracy over traditional spherical assumptions.

Findings

ICFs are higher for bipolar PNe than elliptical ones.

Additional corrections can be up to 50% for sulphur in bipolars.

Corrections vary significantly with nebula shape and central star temperature.

Abstract

The extraction of chemical abundances of ionised nebulae from a limited spectral range is usually hampered by the lack of emission lines corresponding to certain ionic stages. So far, the missing emission lines have been accounted for by the ionisation correction factors (ICFs), constructed under simplistic assumptions like spherical geometry by using 1-D photoionisation modelling. In this contribution we discuss the results (Goncalves et al. 2011, in prep.) of our ongoing project to find a new set of ICFs to determine total abundances of N, O, Ne, Ar, and S, with optical spectra, in the case of non-spherical PNe. These results are based on a grid of 3-D photoionisation modelling of round, elliptical and bipolar shaped PNe, spanning the typical PN luminosities, effective temperatures and densities. We show that the additional corrections --to the widely used Kingsburgh and Barlow (1994) ICFs-- are always higher for bipolars than for ellipticals. Moreover, these additional corrections are, for bipolars, up to: 17% for oxygen, 33% for nitrogen, 40% for neon, 28% for argon and 50% for sulphur. Finally, on top of the fact that corrections change greatly with shape, they vary also greatly with the central star temperature, while the luminosity is a less important parameter.