Oxygen and nitrogen in isolated dwarf irregular galaxies

We present long-slit optical spectroscopy of 67 H II regions in 21 dwarf irregular galaxies to investigate the enrichment of oxygen, nitrogen, neon, sulfur, and argon in low-mass galaxies. Oxygen abundances are obtained via direct detection of the temperature-sensitive emission lines for 25 H II regions; for the remainder of the sample, oxygen abundances are estimated from strong-line calibrations. The direct abundance determinations are compared to the strong-line abundance calibrations of both McGaugh and Pilyugin. While the McGaugh calibration yields a statistical offset of 0.07 dex, the photoionization model grid traces the appropriate isometallicity contour shape in the R-23- O-32 diagnostic diagram. In contrast, while the Pilyugin calibration yields a negligible statistical offset, the residuals in this strong-line calibration method are correlated with ionization parameter. Thus, these observations indicate that oxygen abundances will be overestimated by the p-method for H II regions with low-ionization parameters. Global oxygen and nitrogen abundances for this sample of dwarf irregular galaxies are examined in the context of open-and closed-box chemical evolution models. While several galaxies are consistent with closed-box chemical evolution, the majority of this sample has an effective yield similar to(1)/(4) of the expected yield for a constant star formation rate and Salpeter IMF, indicating that either outflow of enriched gas or inflow of pristine gas has occurred. The effective yield strongly correlates with M-H/L-B in the sense that gas-rich galaxies are more likely to be closed systems. However, the effective yield does not appear to correlate with other global parameters such as dynamical mass, absolute magnitude, star formation rate, or surface brightness. In addition, open and closed systems are not identified easily in other global abundance measures; for example, the observed correlation between luminosity and metallicity is consistent with other recent results in the literature. A correlation is found between the observed nitrogen-to-oxygen ratio and the color of the underlying stellar population; redder dwarf irregular galaxies have higher N/O ratios than blue dwarf irregular galaxies. The relative abundance ratios are interpreted in the context of delayed release of nitrogen and varied star formation histories.