No abstract

Abstract. We present a comparison of electron density estimates for planetary nebulae based on different emissionline ratios. We have considered the density indicators. The observational data were extracted from the literature. We have found systematic deviations from the density homogeneous models, in the sense that:We argue that the lower [O ii] density estimates are likely due to errors in the atomic parameters used.

Abstract.We used photoionization models in order to reproduce the observed gradients of emission-line ratios for H  regions located in the normal spiral galaxy M 101 and in three barred spiral galaxies, namely NGC 1365, NGC 925, and NGC 1073. 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 0.1−0.4 dex when compared to the direct abundance determinations. NGC 1073 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 T eff / R = (400 ± 112) K kpcand 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.

No abstract