Far-infrared measurements of N/O in H II regions - Evidence for enhanced CN process nucleosynthesis in the inner Galaxy

Abstract

Measurements of the [N III] 57.3 μm, [O III] 51.8 μm, and 88.4 μm fine-structure emission lines, obtained with the KAO, are presented for 13 Galactic H II regions spanning a large range in galactocentric distance, and also for two H II regions in the 30 Doradus complex of the LMC. Using newly revised atomic constants, N<SUP>++</SUP>/O<SUP>++</SUP> ionic ratios are derived for each of these nebulae. The authors argue that this ratio is proportional to the elemental ratio N/O, and its differences around the Galaxy reflect differences in nucleosynthetic history. It is found that N/O varies significantly with galactocentric distance. H II regions inside 7 kpc have N/O that is, on the average, a factor of 3 to 4 higher than those at larger radii. Comparison of the inferred N/O with O/H gives evidence for secondary enrichment of nitrogen in the heavily processed regions of the inner Galaxy. This is expected for nitrogen production by quiescent CN processing in stellar envelopes.