Abundance Gradients in the Galaxy

Six H II regions at galactocentric distances of R=10-15 kpc have been observed in the far-IR emission lines of [O III] (52 μm, 88 μm), [N III] (57 μm), and [S III] (19 μm) using the Kuiper Airborne Observatory. These observations have been combined with Very Large Array radio continuum observations of these sources to determine the abundances of O⁺⁺, N⁺⁺, and S⁺⁺ relative to hydrogen. In addition, eight of the most recent sets of measurements of ionic line strengths in H II regions have been reanalyzed in order to attempt to reconcile differences in optical versus far-IR abundance determinations. We have in total 168 sets of observations of 117 H II regions in our analysis. The new analysis included updating the atomic constants (transition probabilities and collision cross sections), recalculation of some of the physical conditions in the H II regions (n_e and T_e), and the use of new photoionization models to determine stellar effective temperatures of the exciting stars. We also use the most recent data available for the distances for these objects, although for most we still rely on kinematic distance determinations. Our analysis finds little indication of differences between optical and infrared observations of the nitrogen abundances, but some differences are seen in the oxygen and sulfur abundances. A very significant offset continues to be seen between optical and infrared measurements of the N/O abundance ratio.