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 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 × 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.

Shaken or stirred: the diffuse interstellar medium with exceptionally high SiO abundance

“…5. SiO is detected in the two directions observed here where Rybarczyk et al (2022a) showed that UVdominated, equilibrium chemical models were unable to account for the high HCO + column densities. These are also the directions where shells are observed.…”

“…Six of the eight features with SiO detected have N (HCO + ) 10 12 cm −2 , which is the column density at which Rybarczyk et al (2022a) showed that equilibrium chemical models fail to explain the observed column densities of several molecular species, including HCO + . In particular, Rybarczyk et al (2022a) argued that non-equilibrium effects (i.e., shocks) are necessary to explain the HCO + column densities and H I temperatures for features in the direction of 3C111A and BL Lac, where indeed we detect SiO, and where there appears to be an association with dynamical processes (shell expansion from a previous generation of star formation). Meanwhile, only one of the structures with N (HCO + ) > 10 12 cm −2 has no detected SiO 2 .…”

“…Six of the eight features with SiO detected have N (HCO + ) 10 12 cm −2 , which is the column density at which Rybarczyk et al (2022a) showed that equilibrium chemical models fail to explain the observed column densities of several molecular species, including HCO + . In particular, Rybarczyk et al (2022a) argued that non-equilibrium effects (i.e., shocks) are necessary to explain the HCO + column densities and H I temperatures for features in the direction of 3C111A and BL Lac, where indeed we detect SiO, and where there appears to be an association with dynamical processes (shell expansion from a previous generation of star formation).…”

“…Nevertheless, numerical simulations have shown that the HCO + excitation temperature may rise above T CMB to ∼ 4-5 K for densities 300 cm −3 n 10 3 cm −3 (Gong et al 2020;Rybarczyk et al 2022a). Such behavior could lead to the HCO + column densities being underestimated by factor of ∼ 2.…”

“…Meanwhile, in more diffuse environments without ongoing star formation to provide such feedback, chemical abundances have been puzzling for many years. Standard astrochemical models have not been able to explain the high abundances of many species (HCO + , HCN, and C 2 H, among others) in the diffuse ISM, leading to the hypothesis that turbulence, and in the most extreme cases shocks, could be responsible for the high abundances (e.g., Godard et al 2009Godard et al , 2010Reach & Heiles 2021;Rybarczyk et al 2022a). Yet, direct observations of shock tracers like SiO in the diffuse (A V 1) and translucent (1 A V 4) ISM have been extremely rare Corby et al 2018).…”