CO depletion and deuterium fractionation in prestellar cores

We report the detection of D2CO in a sample of starless dense cores, in which we previously measured the degree of CO depletion. The deuterium fractionation is found to be extremely high, [D2CO]/[H2CO] similar to 1%-10%, similar to that reported in low-mass protostars. This provides convincing evidence that D2CO is formed in the cold prestellar cores and later desorbed when the gas warms up in protostars. We find that the cores with the highest CO depletions have also the largest [D2CO]/[H2CO] ratios, supporting the theoretical prediction that deuteration increases with increasing CO depletion.