Integral field spectroscopy of the planetary nebula NGC 3242 and the puzzling nature of its low-ionization structures

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 Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
⁴ Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
⁵ Observatório do Valongo, Universidade Federal do Rio de Janeiro, Ladeira Pedro Antonio 43, 20080-090 Rio de Janeiro, Brazil
⁶ School of Physics and Astronomy, Cardiff University, Queen’s Buildings, The Parade, Cardiff CF24 3AA, UK
⁷ Instituto de Física e Química, Universidade Federal de Itajuba, Av. BPS 1303-Pinheirinho, 37500-903 Ítajuba, Brazil
⁸ Observatorio Astronómico de Córdoba, Universidad Nacional de Córdoba, Laprida 854, Córdoba, Argentina
⁹ Laboratório Nacional de Astrofísica, Rua dos Estados Unidos, 154, Bairro das Nações, Itajubá, MG 37504-365, Brazil
¹⁰ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands

^★ Corresponding author; lydiakonst@gmail.com

Received: 30 December 2024
Accepted: 19 March 2025

Abstract

Context. We investigate the physico-chemical properties of the planetary nebula (PN) NGC 3242 in both 1D and 2D, using integral field unit (IFU) data. This PN has a complex morphology with multiple shells and contains a pair of structures with a lower degree of ionization compared to the main nebular components. The origin of these so-called low-ionization structures (LISs) remains a mystery.

Aims. With the capabilities offered by IFU spectroscopy, we aim to gain a better understanding of the behavior of nebular properties in the LISs and examine the spatial distribution of physico-chemical parameters in NGC 3242.

Methods. Data from the Multi Unit Spectroscopic Explorer (MUSE) at the Very Large Telescope (VLT) were used in order to perform a spatially resolved physico-chemical analysis of NGC 3242. The analysis was performed both in 2D, based on emission line maps, as well as in 1D, via simulations of long-slit spectroscopy, with pseudo-slits.

Results. Through the deeper investigation of MUSE data, we detected new structures perpendicular to the pair of LISs of NGC 3242, which are mainly seen in the light of [S III] and [N II]. In addition, two arc-like structures were revealed. Moreover, an inner jet-like structure was found through its [Fe III] emission. The interaction of the jet with the rim may be related to the formation of knots and blobs. The higher value of T_e, is estimated from the [S III] diagnostic lines, followed by T_e [N II]), T_e (H I) and, finally, T_e (He I). In all cases, T_e is higher at the inner nebular structures. Regarding electron density, n_e, is lower at the LISs, while an increase is observed at the nebular rim. Diagnostic diagrams confirm that NGC 3242 is a highly ionized nebula. Moreover, for the first time, the MUSE data have unveiled the atomic line [C I] λ8727 in this PN, which is primarily emitted from the LISs. This finding suggests that these structures may consist of a molecular core surrounded by neutral and ionized gas.