Interstellar chemical differentiation across grain sizes

TL;DR

This study reveals that chemical abundances on interstellar dust grains vary significantly with grain size due to ion accretion and temperature effects, impacting surface chemistry more than gas-phase chemistry.

Contribution

It demonstrates that grain size-dependent effects are crucial for accurate modeling of interstellar surface chemistry, challenging the assumption of uniform icy species distribution.

Findings

Surface species densities vary by up to 2-4 orders of magnitude across grain sizes.

Small grains have higher ice areal densities in dark clouds due to ion accretion.

Differences in surface densities between small and large grains are more pronounced in the CNM model.

Abstract

In this work we investigate the effects of ion accretion and size-dependent dust temperatures on the abundances of both gas-phase and grain-surface species. While past work has assumed a constant areal density for icy species, we show that this assumption is invalid and the chemical differentiation over grain sizes are significant. We use a gas-grain chemical code to numerically demonstrate this in two typical interstellar conditions: dark cloud (DC) and cold neutral medium (CNM). It is shown that, although the grain size distribution variation (but with the total grain surface area unchanged) has little effect on the gas-phase abundances, it can alter the abundances of some surface species by factors up to $\sim2-4$ orders of magnitude. The areal densities of ice species are larger on smaller grains in the DC model as the consequence of ion accretion. However, the surface areal density evolution tracks are more complex in the CNM model due to the combined effects of ion accretion and dust temperature variation. The surface areal density differences between the smallest ($\sim 0.01\mu$m) and the biggest ($\sim 0.2\mu$m) grains can reach $\sim$1 and $\sim$5 orders of magnitude in the DC and CNM models, respectively.