Modeling the warm ionized interstellar medium and its impact on elemental abundance studies

We present model calculations of ionization fractions for elements in the w arm (T similar to 10(4) K), low-density photoionized interstellar medium (W IM) of the Milky Way. We model the WIM as a combination of overlapping low- excitation H II regions having n(H+)/n(H) greater than or equal to 0.8. Our adopted standard model incorporates an intrinsic elemental abundance patte rn similar to that found for warm neutral clouds in the Galaxy and includes the effects of interstellar dust grains. The radiation held is characteriz ed by an ionizing spectrum of a star with T-eff approximate to 35,000 K and an ionization parameter log (q) approximate to -4.0. The emergent emission -line strengths are in agreement with the observed ratios of [S II]/H alpha , [N II]/H alpha, [S II]/[N II], [O I]/H alpha, [O III]/H alpha, and He I/H alpha in the Galactic WIM. Although the forbidden emission-line intensitie s depend strongly on the input model parameters, the ionization fractions o f the 20 elements studied in this work are robust over a wide range of phys ical conditions considered in the models. These ionization fractions have d irect relevance to absorption-line determinations of the elemental abundanc es in the warm neutral and ionized gases in the Milky Way and other late-ty pe galaxies. We demonstrate a method for estimating the WIM contributions t o 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 st ar HD 93521.