THREE-DIMENSIONAL GEOMETRIES AND THE ANALYSIS OF H II REGIONS

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 [NII]/H[alpha] versus H[alpha]. Building on previously published models and observations of Barnard...

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Bibliographic Details
Published in:The Astrophysical journal 2013-06, Vol.770 (2), p.1-5
Main Authors: Wood, Kenneth, Barnes, J E, Ercolano, Barbara, Haffner, L M, Reynolds, R J, Dale, J
Format: Article
Language:English
Subjects:
ASTROPHYSICS, COSMOLOGY AND ASTRONOMY
COMPARATIVE EVALUATIONS
Construction
DENSITY
ELEMENT ABUNDANCE
Fractal analysis
Fractals
GEOMETRY
H II regions
Mathematical models
NEBULAE
ONE-DIMENSIONAL CALCULATIONS
PHOTOIONIZATION
SIMULATION
Three dimensional
THREE-DIMENSIONAL CALCULATIONS
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Summary: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 [NII]/H[alpha] versus 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 Hu 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.
ISSN:0004-637X
1538-4357
DOI:10.1088/0004-637X/770/2/152