2005A&A...437..837D

We used photoionization models in order to reproduce the observed gradients of emission-line ratios for HII regions located in the normal spiral galaxy M101 and in three barred spiral galaxies, namely NGC1365, NGC925, and NGC1073. The behavior of the following nebular parameters across the disk of these galaxies was determined: temperature of the ionizing star (T_eff), ionization parameter (U), and the abundance ratios O/H, N/O, and S/O. Our O/H predictions were found to be consistent with some empirical abundance determinations, but are overestimated by a factor of 0.1-0.4dex when compared to the direct abundance determinations. NGC1073 seems to be overabundant in nitrogen compared to other spiral galaxies. No gradient of S/O was found in the studied galaxies, and we derived a positive T_eff gradient of ΔT_eff/ΔR=(400±112)K/kpc and U ranging from -3.0 to -2.3. The N/O vs. O/H diagram in general is consistent with chemical evolution models that assume that the nitrogen synthesis has both a primary and a secondary component compared to oxygen. However, a very strong N/O dependence on the O/H at high abundance was found.