H$_2$ in low-ionization structures of planetary nebulae
  We report the detection of near-IR H$_2$ emission from the low-ionization
structures (knots) in two planetary nebulae. The deepest ever, high-angular
resolution H$_2$ 1-0 S(1) at 2.122$\mu$, H$_2$ 2-1 S(1) at 2.248$\mu$ and
Br$\gamma$ images of K 4-47 and NGC 7662, obtained using the Near InfraRed
Imager and Spectrometer (NIRI) at Gemini-North, are analyzed here. K 4-47
reveals a remarkable highly collimated bipolar structure not only in the
optical but also in the molecular hydrogen emission. The H$_2$ emission
emanates from the walls of the bipolar outflows and also from the pair of knots
at the tip of the outflows. The H$_2$ 1-0 S(1)/2-1 S(1) line ratio ranges from
~7 to ~10 suggesting the presence of shock interactions. Our findings can be
explained by the interaction of a jet/bullet ejected from the central star with
the surrounding asymptotic giant branch material. The strongest H$_2$ line,
v=1-0 S(1) is also detected in several low-ionization knots located at the
periphery of the elliptical planetary nebula NGC 7662, but only four of these
knots are detected in the H$_2$ v=2-1 S(1) line. These four knots exhibit an
H$_2$ line ratio between 2 and 3.5, which suggests that the emission is caused
by the UV ionizing flux of the central star. Our data confirms the presence of
H$_2$ gas in both fast- and slow-moving low-ionization knots, which has only
been confirmed before in the nearby Helix nebula and Hu 1-2. Overall, the
low-ionization structures of planetary nebulae are found to share similar
traits to photodissociation regions.
