The Physical Parameters and Excitation of Jets and Knots in PNe

We are carrying out a study of the physical parameters and excitation of low-ionization structures (LIS) in planetary nebulae (PNe). Since the optical morphology and kinematics of the LIS and the main components (rims, shells and haloes) of our sample were studied previously, our main goal now is to search for: i) the density contrasts between jets/knots and the main nebular components; ii) their main excitation processes; and iii) their chemical abundances. The first results of this survey -- based on the analysis of NGC7009, NGC6543, NGC6891 and K4-47 -- are that there is no significant density contrast between LIS and their surroundings, and that most of the LIS studied (but not all) are mainly photoionized, rather than shock excited.