Modelling H2 Infrared Emission of the Helix Nebula Cometary Knots
  In the present work, we use a photoionisation code to study the H2 emission
of the Helix nebula (NGC 7293) cometary knots, particularly that produced in
the interface H+/H0 of the knot, where a significant fraction of the H2 1-0
S(1) emission seems to be produced. Our results show that the production of
molecular hydrogen in such region may explain several characteristics of the
observed emission, particularly the high excitation temperature of the H2
infrared lines.
