Physics
Scientific paper
Mar 1982
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1982phdt........25h&link_type=abstract
Thesis (PH.D.)--THE UNIVERSITY OF ROCHESTER, 1982.Source: Dissertation Abstracts International, Volume: 42-08, Section: B, page:
Physics
Scientific paper
The infrared fine structure lines, because of their relatively weak dependence on density and temperature, provide not only a means of determining abundances in HII regions but also a way of examining the excitation conditions of these gaseous nebulae. In particular, the ArII (6.99 (mu)m)/ArIII (8.99 (mu)m) and SIII (18.71 (mu)m)/SIV (10.51 (mu)m) ratios give an abundance independent way of comparing observation and theory. 8-13 (mu)m observations of several compact HII regions have been obtained in order to help understand the physical conditions which exist in these nebulae, while previous observations of ArII, ArIII, SIII and SIV for 11 HII regions are used to compute observed ionic ratios of ArII/ArIII and SIII/SIV which can be compared with model calculations. Model calculations of the ionization structure of HII regions are performed assuming a constant electron temperature and the on-the-spot approximation for the treatment of the diffuse radiation field. Charge transfer reactions are included and the model atmospheres of Kurucz (1979) are used for the flux distribution of the exciting stars. These model calculations indicate that assuming a classical, uniform density, dust-free HII region results in predicted ArII/ArIII and SIII/SIV ratios which disagree with observations by one to two orders of magnitude. It does not appear that optically thin clumping or neutral condensations with shadowing can account for the observed line ratios, however the addition of selectively absorbing dust with an appreciable optical depth in the ultraviolet (of order unity) provides a simple mechanism which yields results in rough agreement with the observations.
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