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N.R. Hausen, R.J. Reynolds, L.M. Haffner (U. Wisconsin-Madison), S.L. Tufte (Lewis & Clark College)
Reynolds et al.\ (1998, ApJ, 494, L99) measured the [O~I]~\lambda6300/H\alpha line intensity ratio for the O star H~II region NGC 7000 and found it to be almost ten times lower than the [O~I]/H\alpha ratios of three directions which sampled the warm ionized medium. In order to investigate whether the [O~I]/H\alpha ratios of other discrete H~II regions are similar to that of NGC 7000, the Wisconsin H\alpha Mapper (WHAM) facility has now been used to measure [O~I]/H\alpha intensity ratios for four additional H~II regions surrounding O stars. These regions are S252 (observation coordinates l = 190.1\circ, b = +0.6\circ), S261 (194.1\circ, --1.9\circ), S27 (6.3\circ, +23.6\circ), and S264 (195.1\circ, --12.0\circ). The [O~I]/H\alpha ratios for these four regions range from approximately 0.001 to 0.005. This is consistent with the NGC 7000 result of [O~I]/H\alpha = 0.0033, and hence indicates a large, systematic difference between the [O~I]/H\alpha ratios in bright H~II regions in the vicinity of O stars and the ratios in the diffuse ionized component of the interstellar medium. This difference implies either a significantly lower hydrogen ionization fraction n(\mathrm{H+})/n(\mathrm{H\circ}) or higher electron temperature in the diffuse ionized gas compared to the O star H~II regions.
This work has been supported by the National Science Foundation through grant AST 96-19424.
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The author(s) of this abstract have provided an email address for comments about the abstract: hausen@astro.wisc.edu