IFU spectroscopy of Southern Planetary Nebulae V: Low-ionization structures

Abstract: In this 5th paper of the series, we examine the spectroscopy and morphology of four southern Galactic planetary nebulae Hen 2-141, NGC5307, IC 2553, and PB6 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 PB6 and Hen 2-141 are of very high excitation classes and IC 2553 and NGC5307 are mid to high excitation objects. The elemental abundances reveal that PB6 is of Type I, Hen 2-141 and IC 2553 are of Type IIa, and NGC5307 of Type IIb/III. The observations unveil the presence of well- defined 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 NGC5307 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.