Oxygen Abundance Determination in HiiRegions: The Strong Line Intensities–Abundance Calibration Revisited

The problem of oxygen abundance determination in H II regions based on the "strong oxygen line intensities-oxygen abundance" empirical calibration is revisited. A compilation of spectroscopic data of H II regions in spiral and irregular galaxies with a measured O III λ4363 line intensity has been carried out, resulting in a sample containing more than 700 individual measurements. Methods are devised to select out only high-precision measurements from that original sample. Te-based oxygen abundances have been recomputed in the same way for all H II regions with high-precision measurements. That sample of Te abundances is used to recalibrate the empirical relations between the oxygen abundance and the strong oxygen line intensities for both high (the upper branch of the O/H-R23 diagram) and low (the lower branch) metallicities, within the framework of the P method, where P is the excitation parameter. Concerning high-metallicity H II regions, an alternative way for deriving oxygen abundances using only measurements of the strong nebular oxygen lines is proposed. The method is based on a tight correlation between the flux in the auroral O III λ4363 line and the fluxes in the nebular O II λλ3727, 3729 and O III λλ4959, 5007 lines, called the ff relation. This relation is also used to select out high-metallicity H II regions with high-precision measurements. In contrast to previous work, the new upper branch P calibration is based only on abundances. It is found that P abundances usually agree well with the ff abundances, although faint (log R23 ≲ -0.5) low-excitation (P ≲ 0.25) H II regions may show systematic differences that can be as large as ~0.1 dex. As for the newly derived low-metallicity P calibration, it is shown to be robust. The calibrations derived from the sample containing all abundance determinations and from that containing only recent (since 1995) measurements are found to be in very good agreement. For both low- and high-metallicity H II regions, the new calibration gives P abundances that agree with abundances to within 0.1 dex.