Effects of turbulent dust grain motion to interstellar chemistry

TL;DR

This study uses numerical modeling to show that turbulent dust grain motion significantly alters interstellar chemistry, affecting molecular abundances and reaction timings in various interstellar environments.

Contribution

It provides the first detailed numerical analysis of how dust grain motion influences interstellar chemical processes across different environments.

Findings

Grain motion decreases gas-phase abundances and increases surface species by up to 2-3 orders of magnitude.

Chemical reaction timings shift, with jumps occurring earlier in gas-phase and later on surfaces.

Grain motion enhances cation accretion in cold neutral medium, boosting certain molecular abundances significantly.

Abstract

Theoretical studies have revealed that dust grains are usually moving fast through the turbulent interstellar gas, which could have significant effects upon interstellar chemistry by modifying grain accretion. This effect is investigated in this work on the basis of numerical gas-grain chemical modeling. Major features of the grain motion effect in the typical environment of dark clouds (DC) can be summarised as follows: 1) decrease of gas-phase (both neutral and ionic) abundances and increase of surface abundances by up to 2-3 orders of magnitude; 2) shifts of the existing chemical jumps to earlier evolution ages for gas-phase species and to later ages for surface species by factors of about ten; 3) a few exceptional cases in which some species turn out to be insensitive to this effect and some other species can show opposite behaviors too. These effects usually begin to emerge from a typical DC model age of about 10^5 yr. The grain motion in a typical cold neutral medium (CNM) can help overcome the Coulomb repulsive barrier to enable effective accretion of cations onto positively charged grains. As a result, the grain motion greatly enhances the abundances of some gas-phase and surface species by factors up to 2-6 or more orders of magnitude in the CNM model. The grain motion effect in a typical molecular cloud (MC) is intermediate between that of the DC and CNM models, but with weaker strength. The grain motion is found to be important to consider in chemical simulations of typical interstellar medium.