Constraining the Environment of CH+ Formation with CH3+ Observations

TL;DR

This study uses observations of CH+ and CH3+ to constrain the physical conditions of the interstellar medium where CH+ forms, revealing that typical diffuse cloud conditions are unlikely, and highlighting the need for more detailed observations to test non-equilibrium formation models.

Contribution

It introduces a novel method of using CH3+ observations to infer the physical conditions of CH+ formation regions in the ISM, providing new constraints on diffuse cloud models.

Findings

Average diffuse cloud conditions are excluded by the data.

Higher effective temperatures may explain CH+ formation via non-equilibrium effects.

More CH3+ observations are needed to test ion acceleration models.

Abstract

The formation of CH+ in the interstellar medium 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 CH3+ observations. The pathway to forming CH3+ begins with CH+, and a steady state analysis of CH3+ and the reaction intermediary CH2+ results in a relationship between the CH+ and CH3+ abundances. This relationship depends on the molecular hydrogen fraction, f_H2, and gas temperature, T, so observations of CH+ and CH3+ 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 f_H2 vs. T parameter space in which CH+ and CH3+ form. We find that average, static, diffuse molecular cloud conditions (i.e. f_H2>0.2, T~60 K) are excluded by our analysis. However, current theory suggests that non-equilibrium effects drive the reaction C+ + H_2 --> CH+ + H, endothermic by 4640 K. If we consider a higher effective temperature due to collisions between neutrals and accelerated ions, the CH3+ partition function predicts that the overall population will be spread out into several excited rotational levels. As a result, observations of more CH3+ 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+.