Nickel- and iron-rich clumps in planetary nebulae: New discoveries and emission-line diagnostics

¹ Institute for Astronomy, Astrophysics, Space Applications and Remote Sensing, National Observatory of Athens, 15236 Penteli, Greece
² Department of Physics, University of Patras, Patras, 26504 Rio, Greece
³ School of Physics and Astronomy, Cardiff University, Queen’s Buildings, The Parade, Cardiff CF24 3AA, UK
⁴ Instituto de Física e Química, Universidade Federal de Itajuba, Av. BPS 1303-Pinheirinho, 37500-903, Ítajuba, Brazil
⁵ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
⁶ Departamento de Astrofísica, Universidad de La Laguna, E-38206 La Laguna, Tenerife, Spain
⁷ Observatório do Valongo, Universidade Federal do Rio de Janeiro, Ladeira Pedro Antonio 43, 20080-090 Rio de Janeiro, Brazil
⁸ Laboratório Nacional de Astrofísica, Rua dos Estados Unidos, 154, Bairro das Nações, Itajubá, MG 37504-365, Brazil
⁹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
¹⁰ Department of Physics and Astronomy, University of Western Ontario, London, ON N6A 3K7, Canada
¹¹ Institute for Earth and Space Exploration, University of Western Ontario, London, ON N6A 3K7, Canada
¹² SETI Institute, 189 Bernardo Ave, Suite 100, Mountain View, CA 94043, USA

^★ Corresponding authors: kbouvis@noa.gr; stavrosakras@noa.gr

Received: 31 March 2025
Accepted: 12 June 2025

Abstract

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.