Effects of photon escape on diagnostic diagrams for HII regions

Abstract

In this article we first outline the mounting evidence that a significant fraction of the ionizing photons emitted by OB stars within HII regions escape from their immediate surroundings and explain how an HII region structure containing high density contrast in homogeneities facilitates this escape. Next we describe sets of models containing inhomogeneities which are used to predict tracks in the commonly used diagnostic diagrams (based on ratios of emission lines) whose only independent variable is the photon escape fraction, xi. We show that the tracks produced by the models in two of the most cited of these diagrams conform well to the distribution of observed data points, with the models containing optically thick inhomogeneities ("CLUMPY" models) yielding somewhat better agreement than those with optically thin inhomogeneities ("FF" models). We show how variations in the ionization parameter U, derived from emission line ratios, could be due to photon escape. Using a rather wide range of assumptions about the filling factor of dense clumps we find, for a selected set of regions observed in M51 photon escape fraction ranging between 30% and 50%. We show, using oxygen as the test element, that models with different assumptions about the gas inhomogeneity will give variations in the abundance values derived from diagnostic diagrams, but do not claim here to have a fully developed set of diagnostic tools to improve abundance determinations made in this way. We finally propose a combination of line ratios with the absolute Halpha luminosity of a given HII region, which allows us to determine the photon escape fraction, and hence resolve the degeneracy between U and xi.