IFU Spectroscopy of Southern Planetary Nebulae V: Low-Ionisation Structures

Abstract

In this 5th paper of the series, we examine the spectroscopy and morphology of four southern Galactic planetary nebulae Hen 2-141, NGC 5307, IC 2553, and PB 6 using new integral field spectroscopy data. The morphologies and ionization structures of the sample are given as a set of emission-line maps. In addition, the physical conditions, chemical compositions, and kinematical characteristics of these objects are derived. The results show that PB 6 and Hen 2-141 are of very high excitation classes and IC 2553 and NGC 5307 are mid to high excitation objects. The elemental abundances reveal that PB 6 is of Type I, Hen 2-141 and IC 2553 are of Type IIa, and NGC 5307 of Type IIb/III. The observations unveil the presence of welldefined low-ionization structures or "knots" in all objects. The diagnostic diagrams reveal that the excitation mechanism of these knots is probably by photo-ionization of dense material by the nebular central stars. The physical analysis of six of these knots show no significant differences with their surrounding nebular gas, except their lower electron densities. In spite of the enhancement of the low-ionization emission lines of these knots, their chemical abundances are nearly comparable to their surrounding nebulae, with the exception of perhaps slightly higher nitrogen abundances in the NGC 5307 knots. The integrated spectrum of IC 2553 reveals that nearly all key lines that have led researchers to characterize its central star as a weak-emission line star type are in fact of nebular origin.

H2 emission in the low-ionization structures of the planetary nebulae NGC 7009 and NGC 6543

“…One of the most intriguing characteristic of LISs is the electronic density (determined from the common diagnostic line ratios), which is systematically lower than or at most equal to the electronic density of the surrounding nebular gas (e.g. Balick et al 1993;Hajian et al 1997;Gonçalves et al 2003Gonçalves et al , 2009Monteiro et al 2013;Akras & Gonçalves 2016;Ali & Dopita 2017). This finding contradicts the formation models of knots in which they are considered more dense than the surrounding nebular gas (e.g.…”

“…On the other hand, there are studies which attribute the enhancement of low-ionization lines in some LISs only to the UV stellar radiation of the central star (e.g. Hajian et al 1997;Gonçalves 2004;Ali & Dopita 2017). Despite all this effort so far, LISs are still poorly understood.…”

Molecular Hydrogen Microstructures in Planetary Nebulae

“…Gonçalves and co-workers claimed that LISs should also be dense structures composed of molecular gas, similar to CKs [13]. This could explain their systematic lower electron densities compared to the surrounding nebula values (e.g., [6,7,13]).…”

“…Due to their substantial different kinematic properties, all the microstructures were included under one class called low-ionisation structures (LISs, [5]). The intriguing enhancement of the low-ionisation emission-lines in these microstructures relative to the surrounding nebular gas has been attributed to either UV radiation from the CS (e.g., [6]) or a combination with shocks (e.g., [7]). Still, little is known about these features.…”

Analysis of Multiple Shell Planetary Nebulae Based on HST/WFPC2 Extended 2D Diagnostic Diagrams

“…An extension of this diagnostic diagram to two dimensions was first explored by [6]. Moreover, different diagnostic diagrams for spatially resolved PNe have been used in the last years to distinguish shock ionization from photoionization regions (see e.g., [19,21,[25][26][27]). The high spatial resolution of HST images make them a perfect tool to investigate on line emission ratios at the entire surface of nebulae at each region individually.…”