Abstract We select 947 star-forming galaxies from SDSS-DR7 with [O III]λ4363 emission lines detected at a signal-to-noise ratio larger than 5σ. Their electron temperatures and direct oxygen abundances are then determined. We compare the results from different methods. t2, the electron temperature in the low ionization region, estimated from t3, that in the high ionization region, is compared using three analysis relations between t2 − t3. These show obvious differences, which result in some different ionic oxygen abundances. The results of t3, t2, O++/H+ and O+/H+ derived by using methods from IRAF and literature are also compared. The ionic abundances O++/H+ are higher than O+/H+ for most cases. The different oxygen abundances derived from Te and the strong-line ratios show a clear discrepancy, which is more obvious following increasing stellar mass and strong-line ratio R23. The sample of galaxies from SDSS with detected [O III]λ4363 have lower metallicites and higher star formation rates, so they may not be typical representatives of the whole population of galaxies. Adopting data objects from Andrews & Martini, Liang et al. And Lee et al. data, we derive new relations of stellar mass and metallicity for star-forming galaxies in a much wider stellar mass range: from 106M⊙ to 1011M⊙.

Keywords galaxies: abundances — galaxies: evolution — galaxies: ISM — galax- ies: spiral — galaxies: starburst — galaxies: stellar content

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