Low-ionization pairs of knots in planetary nebulae: physical properties
  and excitation
  We obtained optical long-slit spectra of four planetary nebulae (PNe) with
low-ionization pair of knots, namely He 1-1, IC 2149, KjPn 8 and NGC 7662.
  These data allow us to derive the physical parameters and excitation of the
pairs of knots, and those of higher ionization inner components of the nebulae,
separately.
  Our results are as follows. 1) The electron temperatures of the knots are
within the range 9500 to 14500 K, similar to the temperatures of the higher
ionization rims/shells. 2) Typical knots' densities are 500 to 2000 cm^{-3}. 3)
Empirical densities of the inner rims/shells are higher than those of the pairs
of knots, by up to a factor of 10. Theoretical predictions, at variance with
the empirical results, suggest that knots should be denser than the inner
regions, by at least a factor of 10. 4) Empirical and theoretical density
contrasts can be reconciled if we assume that at least 90% of the knots' gas is
neutral (likely composed of dust and molecules). 5) By using Raga et al. (2008)
shock modeling and diagnostic diagrams appropriated for spatially resolved PNe,
we suggest that high-velocity shocked knots traveling in the photoionized outer
regions of PNe can explain the emission of the pairs of knots analysed in this
paper.
