CO depletion: a microscopic perspective

TL;DR

This study combines experiments and simulations to understand CO molecule freeze-out and release on interstellar dust grains, revealing a wide range of binding energies and their impact on CO abundance in star-forming regions.

Contribution

It provides new experimental and simulation insights into CO depletion and desorption mechanisms, emphasizing the role of weakly bound molecules and their effects on interstellar chemistry.

Findings

CO molecules accrete onto water ice below 27 K.

Desorption involves weakly bound CO molecules with low diffusion efficiency.

Wide binding energy range reduces CO depletion at high extinctions.

Abstract

In regions where stars form, variations in density and temperature can cause gas to freeze-out onto dust grains forming ice mantles, which influences the chemical composition of a cloud. The aim of this paper is to understand in detail the depletion (and desorption) of CO on (from) interstellar dust grains. Experimental simulations were performed under two different (astrophysically relevant) conditions. In parallel, Kinetic Monte Carlo simulations were used to mimic the experimental conditions. In our experiments, CO molecules accrete onto water ice at temperatures below 27 K, with a deposition rate that does not depend on the substrate temperature. During the warm-up phase, the desorption processes do exhibit subtle differences indicating the presence of weakly bound CO molecules, therefore highlighting a low diffusion efficiency. IR measurements following the ice thickness during the TPD confirm that diffusion occurs at temperatures close to the desorption. Applied to astrophysical conditions, in a pre-stellar core, the binding energies of CO molecules, ranging between 300 K and 850 K, depend on the conditions at which CO has been deposited. Because of this wide range of binding energies, the depletion of CO as a function of AV is much less important than initially thought. The weakly bound molecules, easily released into the gas phase through evaporation, change the balance between accretion and desorption, which result in a larger abundance of CO at high extinctions. In addition, weakly bound CO molecules are also be more mobile, and this could increase the reactivity within interstellar ices.