Three dimensional geometries and the analysis of H II regions

TL;DR

This paper demonstrates that three-dimensional shell geometries, including fractal structures, can accurately reproduce observed emission line ratios in low-density H II regions, challenging previous one-dimensional models and abundance assumptions.

Contribution

It introduces 3D photoionization models with smooth and fractal geometries that better match observations without increasing heavy element abundances.

Findings

3D shell geometries reproduce observed line ratios.

Fractal structures account for spatial variations in line intensities.

Caution is needed when inferring properties from 1D models.

Abstract

We compare emission line intensities from photoionization models of smooth and fractal shell geometries for low density H II regions, with particular focus on the low-ionization diagnostic diagram [N II]/H-alpha vs H-alpha. Building on previously published models and observations of Barnard's Loop, we show that the observed range of intensities and variations in the line intensity ratios may be reproduced with a three dimensional shell geometry. Our models adopt solar abundances throughout the model nebula, in contrast with previous one dimensional modeling which suggested the variations in line intensity ratios could only be reproduced if the heavy element abundances were increased by a factor of 1.4. For spatially resolved H II regions, the multiple sightlines that pierce and sample different ionization and temperature conditions within smooth and fractal shells produce a range of line intensities that are easily overlooked if only the total integrated intensities from the entire nebula model are computed. Our conclusion is that inference of H II region properties, such as elemental abundances, via photoionization models of one dimensional geometries must be treated with caution and further tested through three dimensional modeling.