2025A&A...700A.155B

Context. Integral field spectroscopy (IFS) offers a distinct advantage for studying extended sources by enabling spatially resolved emission maps for several emission lines without the need for specific filters.
Aims. This study aims to conduct a detailed analysis of iron and nickel emission lines in 12 planetary nebulae (PNe) using integral field unit (IFU) data from MUSE to provide valuable insights into their formation and evolution mechanisms.
Methods. New diagnostic line ratios, combined with machine-learning algorithms, were used to distinguish excitation mechanisms such as shock and photoionization. Electron densities and elemental abundances were estimated for different atomic data using the PYNEB package. The contribution of fluorescent excitation of nickel lines was also examined.
Results. A total of 16 iron- and nickel-rich clumps are detected in seven out of 12 PNe. New clumps are discovered in NGC 3132 and IC 4406. The most prominent lines are [Fe II] 8617 Å and [Ni II] 7378 Å. Both emission lines are observed emanating directly from the low-ionization structures (LIS) of NGC 3242, NGC 7009, and NGC 6153, as well as from clumps in NGC 6369 and Tc 1. Their abundances are found to be below the solar values, indicating that a fraction of Fe and Ni remains depleted in dust grains. The depletion factors exhibit a strong correlation over a wide range of values. A machine-learning approach allows us to classify ten out of 16 clumps as shock-excited and to establish a new shock/photoionization selection criterion: log ([Ni II] 7378 Å/Hα) & log ([Fe II] 8617 Å/Hα) > -2.20.