2003ApJ...585L..55B

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]∼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.