Abstract 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 (N e S ii ) in LISs than in adjacent rims/shells and comparable electron temperatures (T e 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.
Mari et al. (Fri,) studied this question.
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