Theoretical low-energy dissociative recombination for diffuse interstellar cloud models

Dissociative recombination (DR) of an electron colliding with a molecular cation, leading to dissociation of the neutral system, is a major destruction pathway in molecular plasmas. DR is important for high-temperature plasmas in fusion reactors to low-temperature, partially ionized star-forming gas clouds in the interstellar medium. Several interstellar molecular cations play important roles in observationally constraining the properties of diffuse interstellar clouds, allowing astronomers to better understand and model their collapse and ensuing star formation. Low-energy DR is of particular importance due to the low temperatures in diffuse interstellar clouds. However, low-energy DR theory has proven to be particularly challenging to study experimentally and theoretically; especially in the case of molecules with low-lying electronic resonances where the direct and indirect DR mechanisms may be simultaneously important. Only recently have storage ring experiments, carried out at the Cryogenic Storage Ring, been able to sufficiently cool the target ions such that ground-state DR rates can be inferred. Theoretically, it is not known a priori which DR mechanism dominates, if either, and important dissociation pathways may not all have be identified for the target system. Here, we present recent developments in our theoretical studies of DR of a few key interstellar ions: CH+, CF+, OH+, and H2O+. We make use of the molecular time-independent R-matrix electron-scattering suite UKRmol+, rovibrational frame transormation theory, and multichannel quantum defect theory to implicitly treat both direct and indirect mechanisms on even footing, without the need to excplicitly calculate dissociative neutral states or electronic couplings.