The Role of Neutral Hydrogen in Setting the Abundances of Molecular Species in the Milky Way's Diffuse Interstellar Medium. II. Comparison between Observations and Theoretical Models

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 ) less than or similar to 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+) less than or similar to 10(12) 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+) greater than or similar to 10(12) 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) greater than or similar to 10(21) cm(-2), the simulated data fail to explain HCO+ column densities greater than or similar to few x 10(12) 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.

Automated summary

By comparing observations and models, we find that structures in the diffuse interstellar medium can be explained by different chemical models based on their temperature and density. High-temperature and high-density structures are typically found in environments close to massive star formation and in directions with thermally unstable H i. However, some observations cannot be explained by photodissociation region models, suggesting the need for alternative mechanisms.