Though the small-scale , low-ionization knots , filaments and jets ( LISs ) of planetary nebulae ( PNe ) are known for \sim 30Â yr , 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 ( [ NÂ ii ] , [ OÂ iii ] , [ SÂ ii ] ) and 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 log ( f _ { shocks } /f _ { \star } ) > -1 ; while the photo-ionized show log ( f _ { shocks } /f _ { \star } ) < -2 .
A transition zone , with -2 < log ( f _ { shocks } /f _ { \star } ) < -1 where both mechanisms are equally important , is also defined .