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Atoms and molecules, and in particular CO, are important coolants during theevolution of interstellar star-forming gas clouds. The presence of dust grains,which allow many chemical reactions to occur on their surfaces, stronglyimpacts the chemical composition of a cloud. At low temperatures, dust grainscan lock-up species from the gas phase which freeze out and form ices. In thissense, dust can deplete important coolants. Our aim is to understand theeffects of freeze-out on the thermal balance and the evolution of agravitationally bound molecular cloud. For this purpose, we perform 3Dhydrodynamical simulations with the adaptive mesh code FLASH. We simulate agravitationally unstable cloud under two different conditions, with and withoutgrain surface chemistry. We let the cloud evolve until one free-fall time isreached and track the thermal evolution and the abundances of species duringthis time. We see that at a number density of 10$^4$ cm$^{-3}$ most of the COmolecules are frozen on dust grains in the run with grain surface chemistry,thereby depriving the most important coolant. As a consequence, we find thatthe temperature of the gas rises up to $\sim$25 K. The temperature drops onceagain due to gas-grain collisional cooling when the density reaches afew$\times$10$^4$ cm$^{-3}$. We conclude that grain surface chemistry not onlyaffects the chemical abundances in the gas phase, but also leaves a distinctimprint in the thermal evolution that impacts the fragmentation of astar-forming cloud. As a final step, we present the equation of state of acollapsing molecular cloud that has grain surface chemistry included.

The impact of freeze-out on collapsing molecular clouds