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

Abstract

We compare observations of HI 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 ($A_V\lesssim 1$) interstellar medium (ISM) to predictions from a photodissociation region (PDR) chemical model and multi-phase 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 HI absorption. These structures fall into two categories. Structures with $T_s<40\mathrm{K}$, mostly with $N(\mathrm{HC}{\mathrm{O}}^{+})\lesssim 10^{12}\mathrm{c}{\mathrm{m}}^{-2}$, are consistent with modest density, FUV radiation field, and CRIR models, typical of the diffuse molecular ISM. Structures with spin temperature $T_s>40\mathrm{K}$, mostly with $N(\mathrm{HC}{\mathrm{O}}^{+})\gtrsim 10^{12}\mathrm{c}{\mathrm{m}}^{-2}$, 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 HI. In at least one case, we rule out the PDR model parameters, suggesting that alternative mechanisms (e.g., non-equilibrium 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, multi-phase ISM agree that HCO${}^{+}$ formation occurs along sightlines with $N(\mathrm{HI})\gtrsim 10^{21}\mathrm{c}{\mathrm{m}}^{-2}$, the simulated data fail to explain HCO${}^{+}$ column densities $\gtrsim \mathrm{few}\times 10^{12}\mathrm{c}{\mathrm{m}}^{-2}$. Since a majority of our sightlines with HCO${}^{+}$ had such high column densities, this likely indicates that non-equilibrium chemistry is important for these lines of sight.

15 pages, 3 figures, 1 table. Accepted to ApJ

References in corpus (18)

• H3+ in Diffuse Interstellar Clouds: a Tracer for the Cosmic-Ray Ionization Rate
• The relation between gas and dust in the Taurus Molecular Cloud
• Models of turbulent dissipation regions in the diffuse interstellar medium
• First results from SMAUG: Characterization of Multiphase Galactic Outflows from a Suite of Local Star-Forming Galactic Disk Simulations
• The gas-to-extinction ratio and the gas distribution in the Galaxy
• The 21-SPONGE HI Absorption Survey I: Techniques and Initial Results
• The environmental dependence of the X_CO conversion factor
• APEX mapping of H3O+ in the Sgr B2 region
• Gas phase Elemental abundances in Molecular cloudS (GEMS). IV. Observational results and statistical trends
• A Far Ultraviolet Spectroscopic Explorer Survey of Interstellar Molecular Hydrogen in the Galactic Disk
• On the enhanced cosmic-ray ionization rate in the diffuse cloud towards Zeta Persei
• HCO, c-C3H and CF+ : three new molecules in diffuse, translucent and "spiral-arm'' clouds
• HP2 survey: III The California Molecular Cloud--A Sleeping Giant Revisited
• Diffuse Ionized Gas in Simulations of Multiphase, Star-Forming Galactic Disks
• Effects of turbulent diffusion on the chemistry of diffuse clouds
• Extracting the cold neutral medium from HI emission with deep learning: Implications for Galactic foregrounds at high latitude
• Recovering interstellar gas properties with HI spectral lines: A comparison between synthetic spectra and 21-SPONGE
• Revealing The CO X-factor In Dark Molecular Gas through Sensitive ALMA Absorption Observations