Constraining the Environment of CH+ Formation with CH3+ Observations

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+.

7 pages, 3 figures, 2 tables, accepted for publication in ApJ

References in corpus (3)

• Ultraviolet Survey of CO and H_2 in Diffuse Molecular Clouds: The Reflection of Two Photochemistry Regimes in Abundance Relationships
• Abundances and Behavior of 12CO, 13CO, and C2 in Translucent Sight Lines
• Models of turbulent dissipation regions in the diffuse interstellar medium

Cited by in corpus (13)

• Interstellar Hydrides
• Comparative study of CH+ and SH+ absorption lines observed towards distant star-forming regions
• Formation of the Methyl Cation by Photochemistry in a Protoplanetary Disk
• The Interaction of Cosmic Rays with Diffuse Clouds
• Broadband velocity modulation spectroscopy of HfF^+: towards a measurement of the electron electric dipole moment
• CH2D+, the Search for the Holy Grail
• The EDIBLES survey IV. Cosmic ray ionization rates in diffuse clouds from near-ultraviolet observations of interstellar OH${}^{+}$
• Herschel/HIFI Spectral Mapping of C${}^{+}$, CH${}^{+}$, and CH in Orion BN/KL: The Prevailing Role of Ultraviolet Irradiation in CH${}^{+}$ Formation
• Experimental determination of the dissociative recombination rate coefficient for rotationally-cold CH${}^{+}$ and its implications for the diffuse cloud chemistry
• Radiative association of C(${}^{3}P$) and H${}^{+}$: Triplet states
• Action spectroscopy of SrCl${}^{+}$ using an integrated ion trap time-of-flight mass spectrometer
• High-resolution collision energy control through ion position modulation in atom-ion hybrid systems
• Opposite effects of the rotational and translational energy on the rates of ion-molecule reactions near $0\text{K}$: the ${\text{D}}_2^{+}+{\text{NH}}_3$ and ${\text{D}}_2^{+}+{\text{ND}}_3$ reactions