Oxygen and Nitrogen in Isolated Dwarf Irregular Galaxies

We present long slit optical spectroscopy of 67 HII 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 HII 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). 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 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. 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