Constraining the environment of CH⁺ formation with CH ⁺ ₃ observations

The formation of CH⁺ in the interstellar medium (ISM) has long been an outstanding problem in chemical models. In order to probe the physical conditions of the ISM in which CH⁺ forms, we propose the use of CH⁺ ₃ observations. The pathway to forming CH⁺ ₃ begins with CH⁺, and a steady-state analysis of CH ⁺ ₃ and the reaction intermediary CH⁺ ₂ results in a relationship between the CH⁺ and CH ⁺ ₃ abundances. This relationship depends on the molecular hydrogen fraction, , and gas temperature, T, so observations of CH⁺ and CH⁺ ₃ can be used to infer the properties of the gas in which both species reside. We present observations of both molecules along the diffuse cloud sight line toward Cyg OB2 No. 12. Using our computed column densities and upper limits, we put constraints on the versus T parameter space in which CH⁺ and CH⁺ ₃ form. We find that average, static, diffuse molecular cloud conditions (i.e., , T 60 K) are excluded by our analysis. However, current theory suggests that non-equilibrium effects drive the reaction C⁺ + H₂ → CH⁺ + H, endothermic by 4640 K. If we consider a higher effective temperature due to collisions between neutrals and accelerated ions, the CH⁺ ₃ partition function predicts that the overall population will be spread out into several excited rotational levels. As a result, observations of more CH⁺ ₃ transitions with higher signal-to-noise ratios are necessary to place any constraints on models where magnetic acceleration of ions drives the formation of CH⁺.