Molecular chemistry and the missing mass problem in PNe

TL;DR

This study uses advanced chemical modeling to show that X-ray irradiation influences molecular composition in planetary nebulae, helping to resolve the missing mass problem by providing more accurate molecular mass estimates.

Contribution

It introduces a self-consistent photoionization model including X-ray effects to better estimate molecular masses in planetary nebulae, addressing the missing mass issue.

Findings

X-ray flux is essential to explain observed molecular compositions.

Revised molecular mass estimates are up to 1000 times higher than previous calculations.

The missing mass problem can be mitigated with improved molecular mass calculations.

Abstract

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.