Oxygen abundance in the Sloan Digital Sky Survey

Aims. To assess the possible systematic differences among different oxygen abundance indicators and understand the origin of nitrogen and the stars responsible for nitrogen production, we present two samples of H II galaxies from the Sloan Digital Sky Survey (SDSS) spectroscopic observations, data release 3. Methods. The electron temperatures (T e ) of 225 galaxies are calculated with the photoionized H II model and T e of 3997 galaxies are calculated with an empirical method. The oxygen abundances from the T e methods of the two samples are determined reliably. The oxygen abundances from a strong line metallicity indicator, such as R 23 , P, N2, and 03N2, are also calculated. We compared oxygen abundances of H II galaxies obtained .with the T e method, R 23 method, P method, N2 method, and O3N2 method. Results. The oxygen abundances derived with the T e method are systematically lower by ∼0.2 dex than those derived with the R 23 method, consistent with previous studies based on H II region samples. No clear offset for oxygen abundance was found between T e metallicity and P, N2 and 03N2 metallicity. When we studied the relation between N/O and O/H, we found that in the metallicity regime of 12 + log(O/H) > 7.95, the large scatter of the relation can be explained by the contribution of small mass stars to the production of nitrogen. In the high metallicity regime, 12 + log(O/H) > 8.2, nitrogen is primarily a secondary element produced by stars of all masses.