On the Measurement of Elemental Abundance Ratios in Inner Galaxy HiiRegions

Although variations in elemental abundance ratios in the Milky Way Galaxycertainly exist, details remain uncertain, particularly in the inner Galaxy,where stars and H II regions in the Galactic plane are obscured optically. Inthis paper we revisit two previously studied, inner Galaxy H II regions:G333.6-0.2 and W43. We observed three new positions in G333.6-0.2 with theKuiper Airborne Observatory and reobserved the central position with theInfrared Space Observatory's Long Wavelength Spectrometer in far-infrared linesof S++, N++, N+, and O++. We also added the N+ lines at 122 and 205 micron tothe suite of lines measured in W43 by Simpson et al. (1995). The measuredelectron densities range from ~40 to over 4000 cm-3 in a single H II region,indicating that abundance analyses must consider density variations, since thecritical densities of the observed lines range from 40 to 9000 cm-3. We proposea method to handle density variations and make new estimates of the S/H and N/Habundance ratios. We find that our sulfur abundance estimates for G333.6-0.2and W43 agree with the S/H abundance ratios expected for the S/H abundancegradient previously reported by Simpson et al. The estimated N/H, S/H, and N/Sratios are the most reliable because of their small corrections for unseenionization states (< 10%). We compute models of the two H II regions toestimate corrections for the other unseen ionization states. We discuss thesepredictions and conclude that only a few of the latest models adequatelyreproduce H II region observations, including the well-known, relatively-largeobserved Ne++/O++ ratios in low- and moderate-excitation H II regions.Comment: 29 pages, 6 figures, ApJ in pres