Determination of N/O from Far-Infrared Line Observations of Galactic H II Regions

The procedure for deriving the N/O abundance ratio in H II regions from far-infrared (FIR) observations of [N III] and [O III] lines yields systematically much higher values than that used to obtain N/O from optical observations of [N II] and [O II] lines. The authors attribute the disparity to two effects that are not accounted for in the standard derivation of N/O by either procedure. New, more precise measurements made from NASA's Kuiper Airborne Observatory are presented of the [O III] 52, 88, and [N III] 57 μm line ratios for six H II regions. The authors apply the "geometry effect" correction as derived from their models to their own and previous FIR data. They find that the N/O ratio for most Galactic H II regions is ≡0.2 - 0.3 rather than the ≡0.05 - 0.1 derived from optical studies, thus raising the average N/O ratio from less than to greater than the solar value of 0.12. This has important implications for comparisons with theories of galactic chemical evolution and heavy element nucleosynthesis.