Multiple ionisation sources in HII regions and their effect on derived nebular abundances

TL;DR

This study shows that direct temperature methods for determining nebular abundances in HII regions are highly reliable and minimally affected by multiple ionisation sources, especially at solar metallicity levels.

Contribution

The paper demonstrates the robustness of two-temperature zone direct temperature methods against the spatial distribution of ionisation sources in HII regions.

Findings

Errors less than 0.15 dex at twice solar metallicity

Errors below 0.05 dex at solar metallicity

Biases smaller than those in strong line methods

Abstract

We present a theoretical investigation of the effect of multiple ionisation sources in HII regions on the total elemental abundances derived from the analysis of collisionally excited emission lines. We focus on empirical methods based on direct temperature measurements that are commonly employed in cases when the temperature of the nebular gas can be determined from the ratio of nebular to auroral lines of (e.g.) doubly ionised oxygen. We find that direct temperature methods that employ a two-temperature zone approach (DT2T methods) are very robust against the spatial distribution of sources. Errors smaller than 0.15 dex are estimated for regions where the metallicity is twice solar and errors below 0.05 dex for solar metallicities and below. The biases introduced by the spatial distribution of the ionisation sources are thus much smaller for DT2T methods than for strong line methods, previously investigated by Ercolano, Bastian & Stasinska. Our findings are in agreement with the recent study of HII regions in NGC 300 by Bresolin et al.