Oxygen abundances and chemical evolution in low surface brightness galaxies

We report the oxygen abundances of the H II regions of a sample of low surface brightness (LSB) galaxies. We provide analytic functions describing McGaugh's calibration of the R 23 method. We use this and the equivalent width (EW) method to determine oxygen abundances, and also make direct estimates in a few cases where the temperature-sensitive [O III] λ4363 line is available. We find LSB galaxies to be metal-poor, consistent with the luminosity-metallicity (L-Z) relation of other galaxies. The large gas mass fractions of these objects provide an interesting test of chemical evolution models. We find no obvious deviation from the closed-box model of galactic chemical evolution. Based on our abundance and gas mass fraction measurements, we infer that LSB galaxies are not fundamentally different from other galaxy types but are perhaps at an early stage of evolution.