Abstract
The excitation parameter U is the radius of the HII region (in pc), multiplied by the 2∕3 power of the electron density. This quantity characterizes an HII region; it is equivalent to [3R ∗ N Lc∕α t]1∕3 (R ∗: radius of the exciting star, N Lc: Lyman continuum photons per unit surface, α t: this is the effective recombination probability). Thus, U relates the size and density of an HII region to parameters that depend only on the properties of the exciting star.
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Wilson, T.L., Hüttemeister, S. (2018). Solutions for Chapter 14: Recombination Lines. In: Tools of Radio Astronomy - Problems and Solutions. Astronomy and Astrophysics Library. Springer, Cham. https://doi.org/10.1007/978-3-319-90820-5_30
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DOI: https://doi.org/10.1007/978-3-319-90820-5_30
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