Time-dependent chemical evolution during cloud formation: H$_2$-regulated chemistry in diffuse molecular cloud

TL;DR

The paper shows that molecular cloud formation behind shocks creates a CO-poor diffuse phase where H$_2$ chemistry, governed by the gas-density history, sets the stage for subsequent molecular evolution. By coupling 3D MHD simulations with tracer particles to post-process a large gas-grain chemical network, the authors demonstrate that many hydrocarbons and oxygen-bearing molecules in diffuse gas are well described by quasi-steady-state chemistry once the local H$_2$ abundance is known. They also reveal that ice formation on grains alters the gas-phase C/O ratio in dense regions, shifting the balance of species such as OH and HCO$^+$ and leading to non-linear CO behavior with H$_2$. Analytic solutions for key species reproduce the numerical results in diffuse conditions and help interpret observations, including linear CH and CCH vs H$_2$ correlations and the non-linear CO–H$_2$ relation, highlighting the role of shielding effects and gas-grain interactions. Overall, the work connects dynamical cloud formation to chemical evolution and provides a framework to interpret observed molecular abundances in diffuse and translucent clouds.

Abstract

We investigate the chemical evolution of a forming molecular cloud behind an interstellar shock wave. We conduct three-dimensional magnetohydrodynamics simulations of the converging flow of atomic gas, including a simple chemical network and tracer particles that move along the local velocity field. Then we perform detailed chemical network calculations along the trajectory of each tracer particle. The diffuse part of forming molecular clouds is CO-poor; i.e., H$_2$ and CO abundances do not correlate. In diffuse regions of $n_\mathrm{H}\lesssim 10^{3}\,\mathrm{cm^{-3}}$, we find that the abundances of hydrocarbons and oxygen-bearing molecules are determined by steady-state chemistry reflecting the local H$_2$ abundance, which is determined by the gas density along the trajectory. In denser regions, the abundances are affected by water ice formation, which changes the elemental abundance of carbon and oxygen (i.e., C/O ratio) in the gas phase. Assuming quasi-steady-state chemistry given the abundances of major molecules (e.g., H$_2$) from the simple network, we derive analytic solutions for molecular abundances, which reproduce the calculation results. We also calculate the molecular column densities based on the spatial distribution of tracer particles and their molecular abundances, and compare them with observations of diffuse molecular clouds. We find that the column densities of CH, CCH, and OH are linearly correlated with those of H$_2$, which supports the empirical relation used in the observations. On the other hand, the column density of HCO$^+$ shows non-linear dependence on the H$_2$ column density, reflecting the difference in HCO$^+$ formation paths in CO-poor and CO-rich regions.

Figures (20)

• Figure 1: The initial condition of the converging flow simulation. The color shows the number density of hydrogen nuclei on the $z=0$ plane.
• Figure 2: Slices of gas density, H$_2$ abundance, and CO abundance in the $z=0$ plane at 5 Myr.
• Figure 3: Mass-weighted probability distribution function of the gas density, H$_2$, and major carbon carriers at 5 Myr.
• Figure 4: The total hydrogen column density, H$_2$ column density, and CO column density along $x$-axis at 5 Myr.
• Figure 5: The gas density, temperature, and $A_V$ of each tracer particles at 5 Myr.