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This paper presents a spectroscopic investigation of 11 HII regions in thenearby galaxy NGC 2403. The HII regions are observed with a long-slitspectrograph mounted on the 2.16 m telescope at XingLong station of NationalAstronomical Observatories of China. For each of the HII regions, spectra areextracted at different nebular radii along the slit-coverage. Oxygen abundancesare empirically estimated from the strong-line indices R23, N2O2, O3N2, and N2for each spectrophotometric unit, with both observation- and model-basedcalibrations adopted into the derivation. Radial profiles of these diverselyestimated abundances are drawn for each nebula. In the results, the oxygenabundances separately estimated with the prescriptions on the basis ofobservations and models, albeit from the same spectral index, systematicallydeviate from each other; at the same time, the spectral indices R23 and N2O2are distributed with flat profiles, whereas N2 and O3N2 exhibit apparentgradients with the nebular radius. Because our study naturally samples variousionization levels which inherently decline at larger radii within individualHII regions, the radial distributions indicate not only the robustness of R23and N2O2 against ionization variations but also the sensitivity of N2 and O3N2to the ionization parameter. The results in this paper provide observationalcorroboration of the theoretical prediction about the deviation in theempirical abundance diagnostics. Our future work is planned to investigatemetal-poor HII regions with measurable T_e, in an attempt to recalibrate thestrong-line indices and consequently disclose the cause of the discrepanciesbetween the empirical oxygen abundances.

Internal Variations in Empirical Oxygen Abundances for Giant HII Regions in the Galaxy NGC 2403