Modelling the Warm H2 Infrared Emission of the Helix Nebula Cometary
  Knots
  Molecular hydrogen emission is commonly observed in planetary nebulae. Images
taken in infrared H2 emission lines show that at least part of the molecular
emission is produced inside the ionised region. In the best-studied case, the
Helix nebula, the H2 emission is produced inside cometary knots (CKs),
comet-shaped structures believed to be clumps of dense neutral gas embedded
within the ionised gas. Most of the H2 emission of the CKs seems to be produced
in a thin layer between the ionised diffuse gas and the neutral material of the
knot, in a mini photodissociation region (PDR). However, PDR models published
so far cannot fully explain all the characteristics of the H2 emission of the
CKs. In this work, we use the photoionisation code \textsc{Aangaba} to study
the H2 emission of the CKs, particularly that produced in the interface H^+/H^0
of the knot, where a significant fraction of the H2 1-0S(1) emission seems to
be produced. Our results show that the production of molecular hydrogen in such
a region may explain several characteristics of the observed emission,
particularly the high excitation temperature of the H2 infrared lines. We find
that the temperature derived from H2 observations even of a single knot, will
depend very strongly on the observed transitions, with much higher temperatures
derived from excited levels. We also proposed that the separation between the
H_alpha and NII peak emission observed in the images of CKs may be an effect of
the distance of the knot from the star, since for knots farther from the
central star the NII line is produced closer to the border of the CK than
H_alpha.
