Molecular chemistry and the missing mass problem in PNe
  Detections of molecular lines, mainly from H2$ and CO, reveal molecular
material in planetary nebulae. Observations of a variety of molecules suggest
that the molecular composition in these objects differs from that found in
interstellar clouds or in circumstellar envelopes. The success of the models,
which are mostly devoted to explain molecular densities in specific planetary
nebulae, is still partial, however. The present study aims at identifying the
influence of stellar and nebular properties on the molecular composition of
planetary nebulae by means of chemical models. A comparison of theoretical
results with those derived from the observations may provide clues to the
conditions that favor the presence of a particular molecule. A self-consistent
photoionization numerical code was adapted to simulate cold molecular regions
beyond the ionized zone. The code was used to obtain a grid of models and the
resulting column densities are compared with those inferred from observations.
Our models show that the inclusion of an incident flux of X-rays is required to
explain the molecular composition derived for planetary nebulae. We also obtain
a more accurate relation for the N(CO)/N(H2) ratio in these objects. Molecular
masses obtained by previous works in the literature were then recalculated,
showing that these masses can be underestimated by up to three orders of
magnitude. We conclude that the problem of the missing mass in planetary
nebulae can be solved by a more accurate calculation of the molecular mass.
