Proof of shock-excited H$_2$ in low-ionization structures of PNe
  We report the detection of near-IR H$_2$ lines emission from low-ionization
structures (LISs) in planetary nebulae. The deepest, high-angular resolution
H$_2$ 1-0 S(1) at 2.122 $\mu$m, and H$_2$ 2-1 S(1) at 2.248 $\mu$m images of K
4-47 and NGC 7662, obtained using NIRI@Gemini-North, are presented here. K 4-47
reveals a remarkable high-collimated bipolar structure. The H$_2$ emission
emanates from the walls of the bipolar outflows and a pair of knots at the tips
of these outflows. The H$_2$ 1-0 S(1)/2-1 S(1) line ratio is $\sim$7-10 which
indicates shock interaction due to both the lateral expansion of the gas in the
outflows and the high-velocity knots. The strongest line, H$_2$ v=1-0 S(1), is
also detected in several LISs located at the periphery of the outer shell of
the elliptical PN NGC 7662, whereas only four knots are detected in the H$_2$
v=2-1 S(1) line. These knots have H$_2$ v=1-0 S(1)/v=2-1 S(1) values between 2
and 3. These data confirm the presence of molecular gas in both highly (K 4-47)
and slowly moving LISs (NGC 7662). The H$_2$ emission in K 4-47 is powered by
shocks, whereas in NGC 7662 is due to photo-ionization by the central star.
Moreover, a likely correlation is found between the H$_2$ v=1-0 S(1)/H$_2$
v=2-1 S(1) and [N II]/H$\alpha$ line ratios.
