New Observational Views on the Chemistry of Diffuse Interstellar Clouds

Since more than two decades, various space and airborne instruments for far-infrared spectroscopy deliver insights, unobtainable any other way, into the chemistry of the interstellar medium. The reactants and products of the reaction network forming astrophysical hydrides are meanwhile identified by observations, even some key species only recently. While the analysis is greatly aided by measurements and calculations of the underlying reaction rates at a range of relevant temperatures, the determination of abundances is hampered by the nonavailability of CO as surrogate for molecular hydrogen, owing to the insufficient self-shielding of the former at Av < 1 mag. This teletalk will discuss the alternative tracers for molecular hydrogen in such CO-dark environments, such as HF, OH and CH. Discrepancies from predictions of the cold chemistry of ion-neutral reactions are interpreted as indications of endothermic formation pathways. The recent literature exploiting MHD models sheds light on the role of turbulence and yields a complex, highly dynamic picture involving energy supply by the dissipation of turbulence, and by the transport of warm molecular hydrogen in a medium with a highly anisotropic shielding against the surrounding UV field. Thanks to the data collected with GREAT, the heterodyne receiver of SOFIA's instrument suite, we present findings from an analysis of the rotational ground-state transition of CH. We attribute abundance anomalies to the chemical footprint of such an environment, a conclusion corroborated by recent studies of sulfur-bearing hydrides.