Models and observations of deuterated molecules in protostellar cores

Measuring the deuterium fractionation in different molecules can allow one to determine the physical conditions in the gas and to differentiate between gas-phase and grain surface chemical processing. Observations of molecular D/H ratios in different species towards the dense gas surrounding low-mass protostars are presented and are compared with model simulations. These consider gas-phase chemistry, accretion and desorption, and reactions on grain surfaces during the initial stages of core collapse.