H$_2$ emission in the low-ionisation structures of the Planetary Nebulae
  NGC 7009 and NGC 6543
  Despite the many studies in the last decades, the low-ionisation structures
(LISs) of planetary nebulae (PNe) still hold several mysteries. Recent imaging
surveys have demonstrated that LISs are composed of molecular gas. Here we
report H$_2$ emission in the LISs of NGC 7009 and NGC 6543 by means of very
deep narrow-band H$_2$ images taken with NIRI@Gemini. The surface brightness of
the H2 1-0 S(1) line is estimated to be (0.46-2.9)$\times$10$^{-4}$ erg
s$^{-1}$ cm$^{-2}$ sr$^{-1}$ in NGC 7009 and (0.29-0.48)$\times$10$^{-4}$ erg
s$^{-1}$ cm$^{-2}$ sr$^{-1}$ in NGC 6543, with signal-to-noise ratios of 10-42
and 3-4, respectively. These findings provide further confirmation of hidden
H$_2$ gas in LISs. The emission is discussed in terms of the recent proposed
diagnostic diagram R(H$_2$)=H$_2$ 1-0 S(1)/H$_2$ 2-1 S(1) versus
R(Br$\gamma$)=H$_2$ 1-0 S(1)/Br$\gamma$, which was suggested to trace the
mechanism responsible for the H$_2$ excitation. Comparing our observations to
shock and ultraviolet (UV) molecular excitation models, as well as a number of
observations compiled from the literature showed that we cannot conclude for
either UV or shocks as the mechanism behind the molecular emission.
