Modeling the Warm Ionized Interstellar Medium and Its Impact on Elemental Abundance Studies

We present model calculations of ionization fractions for elements in the warm (T ~ 104 K), low-density photoionized interstellar medium (WIM) of the Milky Way. We model the WIM as a combination of overlapping low-excitation H II regions having n(H+)/n(H) ≳ 0.8. Our adopted standard model incorporates an intrinsic elemental abundance pattern similar to that found for warm neutral clouds in the Galaxy and includes the effects of interstellar dust grains. The radiation field is characterized by an ionizing spectrum of a star with Teff ≈ 35,000 K and an ionization parameter log(q) ≈ -4.0. The emergent emission-line strengths are in agreement with the observed ratios of [S II]/Hα, [N II]/Hα, [S II]/[N II], [O I]/Hα, [O III]/Hα, and He I/Hα in the Galactic WIM. Although the forbidden emission-line intensities depend strongly on the input model parameters, the ionization fractions of the 20 elements studied in this work are robust over a wide range of physical conditions considered in the models. These ionization fractions have direct relevance to absorption-line determinations of the elemental abundances in the warm neutral and ionized gases in the Milky Way and other late-type galaxies. We demonstrate a method for estimating the WIM contributions to the observed column densities of singly and doubly ionized atoms used to derive abundances in the warm neutral gas. We apply this approach to study the gas-phase abundances of the warm interstellar clouds toward the halo star HD 93521.