The impact of freeze-out on collapsing molecular clouds

Abstract

Atoms and molecules, and in particular CO, are important coolants during the evolution of interstellar star-forming gas clouds. The presence of dust grains, which allow many chemical reactions to occur on their surfaces, strongly impacts the chemical composition of a cloud. At low temperatures, dust grains can lock-up species from the gas phase which freeze out and form ices. In this sense, dust can deplete important coolants. Our aim is to understand the effects of freeze-out on the thermal balance and the evolution of a gravitationally bound molecular cloud. For this purpose, we perform 3D hydrodynamical simulations with the adaptive mesh code FLASH. We simulate a gravitationally unstable cloud under two different conditions, with and without grain surface chemistry. We let the cloud evolve until one free-fall time is reached and track the thermal evolution and the abundances of species during this time. We see that at a number density of 10${}^4$ cm${}^{-3}$ most of the CO molecules are frozen on dust grains in the run with grain surface chemistry, thereby depriving the most important coolant. As a consequence, we find that the temperature of the gas rises up to $\sim$25 K. The temperature drops once again due to gas-grain collisional cooling when the density reaches a few$\times$10${}^4$ cm${}^{-3}$. We conclude that grain surface chemistry not only affects the chemical abundances in the gas phase, but also leaves a distinct imprint in the thermal evolution that impacts the fragmentation of a star-forming cloud. As a final step, we present the equation of state of a collapsing molecular cloud that has grain surface chemistry included.

Increased the number of significant digits in EQ 2. It mattered. Accepted for publication in MNRAS letters

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• Astrochemical Correlations in Molecular Clouds
• TRAO Survey of the nearby filamentary molecular clouds, the universal nursery of stars (TRAO FUNS). II. Filaments and Dense cores in IC 5146
• Fast deuterium fractionation in magnetized and turbulent filaments
• The turbulent life of dust grains in the supernova-driven, multi-phase interstellar medium
• Understanding the Formation and Evolution of Interstellar Ices: A Bayesian Approach
• Exploiting Network Topology for Accelerated Bayesian Inference of Grain Surface Reaction Networks
• The role of ices in star-forming clouds
• The cosmic journey of dust grains -- from nucleation to planetary system
• The Cygnus Allscale Survey of Chemistry and Dynamical Environments: CASCADE III. The large scale distribution of DCO+, DNC and DCN in the DR21 filament