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 (1991) and Pilyugin (2000). While the McGaugh (1991) calibration yields a statistical offset of 0.07 dex, the photoionization model grid traces the appropriate iso-metallicity contour shape in the R23-O32 diagnostic diagram. In contrast, while the Pilyugin (2000) 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 have an effective yield ∼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 The National Astronomy and Ionosphere Center is operated by Cornell University under a cooperative agreement with the National Science Foundation.