Low-ionization structures in planetary nebulae – III. The statistical analysis of physico-chemical parameters and excitation mechanisms

Abstract Nearly 30 years after the first detailed studies of low-ionization structures (LISs) in planetary nebulae (PNe), we perform a statistical analysis their physical, chemical and excitation properties, by collecting published data literature. The was made through contrast between LISs high-ionization - rims or shells for large sample PNe, order to highlight significant differences these structures. Our motivation find robust results based on largest gathered so far. (i) Indeed, have lower electron densities (Ne[S ii]) than rims/shells. (ii) nitrogen temperatures (Te[N are similar two groups, while bimodal distribution is observed Te [O iii] rims/shells, although high- Te[O median values. (iii) No variations total abundances He, N, O, Ne, Ar, Cl S groups. (iv) Through several diagnostic diagrams, separated from rims/shells terms excitation. From grids photoionization shock models, show that there an important overlap both mechanisms, particularly when line-ratios concerned. We found good tracer high-velocity shocks, as well indicator low-velocity shocks depends temperature-sensitive line ratios. In conclusion, mechanisms could be present, however cannot main source most PNe.