Bistability and Temperature Effects in Mixing Layers Between Molecular and Ionized Regions

We propose a simple method of assessing the chemical consequences of mixing at interfaces between cool neutral gas with ionized material. It is shown that the temperature within the interface has a significant effect; at low temperatures (∼ 10 K) the chemistry is significantly suppressed, but for temperatures above 100 K the chemistry initiated by proton/O-atom charge exchange gives rise to a rich and abundant chemistry. The chemical state of the cool neutral gas has an important consequence for the interface. A mainly molecular cool gas can give rise to a high ionization state in the interface; this high ionization state appears stable against parameter variations. C₂H is one of the most easily observed molecules having a high abundance in some interfaces. This revised version was published online in July 2006 with corrections to the Cover Date.