The impact of freeze-out on collapsing molecular clouds
  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.
