Spectrophotometry of southern H II regions and strong-line diagnostics.
Abstract (from CDS):
Emission-line intensities covering the wavelength range λλ3727-6731 are presented for 18 southern hemisphere Galactic H II regions. Although these observations were made three decades ago, several of the H II regions in this sample have not been extensively observed over this range of wavelengths. Combining these data with previously published line intensities for a northern hemisphere set of H II regions yields a sample of 30 nebulae distributed in galactocentric distance from 5.9 to 12.8 kpc, observed and analyzed in a similar way. Nitrogen and oxygen abundances are determined in a subset of 11 nebulae when a traditional two-zone direct treatment of forbidden-line measurements is possible. Direct abundance measurements in these 11 nebulae also allow an assessment of two strong-line diagnostics, [N II]/[O II] and [O III]/[N II], which have been used as proxies for O/H abundances in extragalactic H II regions and star-forming galaxies. The results suggest that the [N II]/[O II] and [O III]/[N II] are robust with respect to applicability to the range of conditions characteristic of Galactic nebulae, extragalactic H II regions, and star-forming galaxies. The similarity of the resultant O/H gradients between the direct and strong-line methods implies that relative abundances are reliable for each. The direct and [O III]/[N II] absolute abundances are in general agreement, while the [N II]/[O II] diagnostic gives systematically higher absolute abundances, consistent with findings of other investigators. These data allow for an independent calibration of the [O III]/[N II] diagnostic in the near-solar metallicity regime. A conversion relationship between these two strong-line diagnostics based on abundances in these Galactic H II regions is similar to the one determined for star-forming galaxies.