Low-ionization structures in planetary nebulae - I: physical, kinematic and excitation properties
Though the small-scale, low-ionization knots, filaments and jets (LISs) of planetary nebulae (PNe) are known for ~30yr, some of their observational properties are not well established. In consequence our ability to include them in the wider context of the formation and evolution of PNe is directly affected. Why most structures have lower densities than the PN shells hosting them? Is their intense emission in low-ionization lines the key to their main excitation mechanism? Therefore, if considered altogether, can LISs line ratios, chemical abundances and kinematics enlighten the interplay between the different excitation and formation processes? Here we present a spectroscopic analysis of five PNe that possess LISs confirming that all nebular components have comparable electron temperatures, whereas the electron density is systematically lower in LISs than in the surrounding nebula. Chemical abundances of LISs versus other PN components do not show significant differences as well. By using diagnostic diagrams from shock models, we demonstrate that LISs' main excitation is due to shocks, whereas the other components are mainly photo-ionized. We also propose new diagnostic diagrams involving a few emission lines ([NII], [OIII], [SII]) and $\rm{log}$(f$_{shocks}$/f$_{\star}$), where f$_{shocks}$ and f$_{\star}$ are the ionization photon fluxes due to the shocks and the central star ionizing continuum, respectively. A robust relation differentiating the structures is found, with the shock-excited clearly having $\rm{log}$(f$_{shocks}$/f$_{\star}$)$>$-1; while the photo-ionized show $\rm{log}$(f$_{shocks}$/f$_{\star}$)$<$-2. A transition zone, with -2$<\rm{log}$(f$_{shocks}$/f$_{\star}$)$<$-1 where both mechanisms are equally important, is also defined.