Propagation of ionizing radiation in HII regions: the effects of optically thick density fluctuations
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Giammanco, C.; et al. Propagation of ionizing radiation in HII regions: the effects of optically thick density fluctuations. Astronomy and Astrophysics, 424(3): 877-885 (2004). [http://hdl.handle.net/10481/29037]
PatrocinadorThis work was supported by the Spanish DGES (Dirección General de Enseñanza Superior) via Grants PB91-0525, PB94-1107 and PB97-0219 and by the Ministry of Science and Technology via grant AYA2001-0435. A.Z. acknowledges support from the Consejería de Educación y Ciencia de la Junta de Andalucía, Spain. A.Z. and C.G. acknowledge support from the Isaac Newton Group (ING) during the preparation of this article. The JKT is operated on the island of La Palma by the ING in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias.
The accepted explanation of the observed dichotomy of two orders of magnitude between in situ measurements of electron density in HII regions, derived from emission line ratios, and average measurements based on integrated emission measure, is the inhomogeneity of the ionized medium. This is expressed as a "filling factor", the volume ratio of dense to tenuous gas, measured with values of order 10^-3. Implicit in the filling factor model as normally used, is the assumption that the clumps of dense gas are optically thin to ionizing radiation. Here we explore implications of assuming the contrary: that the clumps are optically thick. A first consequence is the presence within HII regions of a major fraction of neutral hydrogen. We estimate the mean H^o/H^+ ratio for a population of HII regions in the spiral galaxy NGC 1530 to be the order of 10, and support this inference using dynamical arguments. The optically thick clumpy models allow a significant fraction of the photons generated by the ionizing stars to escape from their HII region. We show, by comparing model predictions with observations, that these models give an account at least as good as, and probably better than that of conventional models, of the radial surface brightness distribution and of selected spectral line diagnostics for physical conditions within HII regions. These models explain how an HII region can appear, from its line ratios, to be ionization bounded, yet permit a major fraction of its ionizing photons to escape.