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Published online by Cambridge University Press: 06 October 2025
More than three decades after the pioneering imaging catalog by Balick (1987), where the small-scale and low-ionization structures (LISs) in planetary nebulae (PNe) became evident, the present study conducts a comprehensive statistical analysis of LISs’ physical-chemical and excitation properties. Gathering the largest dataset to date, we compare LISs with high-ionization components (rims/shells) across a diverse sample of PNe. Key findings include lower electron densities (NeS ii) in LISs than in adjacent rims/shells and comparable electron temperatures (Te N ii and O iii) between these two nebular componets. Various optical diagnostic diagrams, while revealing a clear excitation stratification between these groups of components, are not good enough to clearly pinpoint the main excitation mechanism behind the LISs line-emission. Photoionization and shock models highlight a notable overlap between mechanisms, emphasizing the coexistence of both excitation processes. Contrary to expectations, shocks are unlikely to be the primary excitation source for most of LISs in PNe.