2022ApJ...926..190R

We compare observations of H I from the Very Large Array (VLA) and the Arecibo Observatory and observations of HCO⁺ from the Atacama Large Millimeter/submillimeter Array (ALMA) and the Northern Extended Millimeter Array (NOEMA) in the diffuse (A _ V _ <= 1) interstellar medium (ISM) to predictions from a photodissociation region (PDR) chemical model and multiphase ISM simulations. Using a coarse grid of PDR models, we estimate the density, FUV radiation field, and cosmic-ray ionization rate (CRIR) for each structure identified in HCO⁺ and H I absorption. These structures fall into two categories. Structures with T _ s _ < 40 K, mostly with N(HCO⁺) <= 10¹² cm^–2, are consistent with modest density, FUV radiation field, and CRIR models, typical of the diffuse molecular ISM. Structures with spin temperature T _ s _ > 40 K, mostly with N(HCO⁺) >= 10¹² cm^–2, are consistent with high density, FUV radiation field, and CRIR models, characteristic of environments close to massive star formation. The latter are also found in directions with a significant fraction of thermally unstable H I. In at least one case, we rule out the PDR model parameters, suggesting that alternative mechanisms (e.g., nonequilibrium processes like turbulent dissipation and/or shocks) are required to explain the observed HCO⁺ in this direction. Similarly, while our observations and simulations of the turbulent, multiphase ISM agree that HCO⁺ formation occurs along sight lines with N(H I) >= 10²¹ cm^–2, the simulated data fail to explain HCO⁺ column densities >= few x 10¹² cm^–2. Because a majority of our sight lines with HCO⁺ had such high column densities, this likely indicates that nonequilibrium chemistry is important for these lines of sight.