Detection of CH$^{+}$, CH and H$_2$ molecules in the Young Planetary
  Nebula IC 4997
  We have detected CH$^{+}$ and CH molecular absorption lines from the young
compact planetary nebula IC 4997 from high resolution optical spectra. A
high-resolution infra-red (H and K bands) spectrum provides detection of H$_2$
emission lines amongst many other lines. The H$_2$ lines provide an excitation
temperature of 2100 K which may result from UV fluorescence in the envelope or
from shocks formed at the interface between an expanding outflow of ionized gas
and the neutral envelope ejected when the star was on the AGB. It is suggested
that the CH$^+$ may result from the endothermic reaction C + H$_2$
$\rightarrow$ CH$^+$ + H. Intriguingly, CH$^{+}$ and also CH show a higher
expansion velocity than H$_{\rm 2}$ emission suggesting they may be part of the
post-shocked gas.
