The effects of spatially distributed ionisation sources on the temperature structure of HII region

TL;DR

This study uses 3D photoionisation models to examine how the spatial distribution of ionising stars affects the temperature structure and metallicity measurements in HII regions, revealing minimal temperature fluctuations but potential biases in metallicity indicators.

Contribution

It introduces detailed 3D models of HII regions with distributed ionising sources, highlighting their impact on temperature fluctuations and metallicity diagnostics, which was not thoroughly explored before.

Findings

Temperature fluctuations due to stellar distribution are small.

Distributed ionising sources cause lower ionisation parameters.

Distribution of sources may explain scatter in metallicity measurements.

Abstract

Spatially resolved studies of star forming regions show that the assumption of spherical geometry is not realistic in most cases, with a major complication posed by the gas being ionised by multiple non-centrally located stars or star clusters. We try to isolate the effects of multiple non-centrally located stars on the temperature and ionisation structure of HII regions, via the construction of 3D photoionisation models using the 3D Monte Carlo photoionisation code MOCASSIN. We find that the true temperature fluctuations due to the stellar distribution (as opposed to the large-scale temperature gradients due to other gas properties) are small in all cases and not a significant cause of error in metallicity studies. Strong emission lines from HII regions are often used to study the metallicity of star-forming regions. We compare integrated emission line spectra from our models and quantify any systematic errors caused by the simplifying assumption of a single, central location for all ionising sources. We find that the dependence of the metallicity indicators on the ionisation parameter causes a clear bias, due to the fact that models with a fully distributed configuration of stars always display lower ionisation parameters than their fully concentrated counterparts. The errors found imply that the geometrical distribution of ionisation sources may partly account for the large scatter in metallicities derived using model-calibrated empirical methods.