X-ray ionization and the Orion molecular cloud

The observed X-ray sources near the core of the Orion molecular cloud are sufficient to supply all the ionization that is needed to drive the molecular chemistry throughout that portion of the cloud in which the greatest density and diversity of molecular species is found. In small regions within that core, the ionization rate is so high that the molecular balance is significantly different from that in the bulk of the cloud core's volume. We calculate equilibrium models to determine the dependence of molecular abundances on ionization rate, and we construct an exemplary model to show how this data may be assembled to predict total column densities. The observed abundance of certain species, particularly C2H, HC3N, and CN, unaccounted for in homogeneous models, may be entirely due to the presence of small regions of high ionization rate. It is possible that high abundances of species such as these in other molecular clouds may be used as signals of stellar X-ray emission too faint or too deeply buried in the clouds to be detected by current X-ray telescopes