On the Measurement of Elemental Abundance Ratios in Inner Galaxy HiiRegions

Abstract

Although variations in elemental abundance ratios in the Milky Way Galaxy certainly exist, details remain uncertain, particularly in the inner Galaxy, where stars and H II regions in the Galactic plane are obscured optically. In this 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 the Kuiper Airborne Observatory and reobserved the central position with the Infrared Space Observatory's Long Wavelength Spectrometer in far-infrared lines of S++, N++, N+, and O++. We also added the N+ lines at 122 and 205 micron to the suite of lines measured in W43 by Simpson et al. (1995). The measured electron densities range from ~40 to over 4000 cm-3 in a single H II region, indicating that abundance analyses must consider density variations, since the critical densities of the observed lines range from 40 to 9000 cm-3. We propose a method to handle density variations and make new estimates of the S/H and N/H abundance ratios. We find that our sulfur abundance estimates for G333.6-0.2 and W43 agree with the S/H abundance ratios expected for the S/H abundance gradient previously reported by Simpson et al. The estimated N/H, S/H, and N/S ratios are the most reliable because of their small corrections for unseen ionization states (< 10%). We compute models of the two H II regions to estimate corrections for the other unseen ionization states. We discuss these predictions and conclude that only a few of the latest models adequately reproduce H II region observations, including the well-known, relatively-large observed Ne++/O++ ratios in low- and moderate-excitation H II regions.