Molecular D/H ratios in the dense gas surrounding low-mass protostars

Observations of [HDCO]/[H(2)CO] and [DCN]/[HCN] ratios have been made towards a selection of low-mass protostellar cores in three different star forming regions. Most [HDCO]/ [H(2)CO] ratios are between 0.05 and 0.07, similar to the values observed towards the dark clouds, TMC-1 and L134N. [DCN]/[HCN] ratios are similar to 0.04, higher than those seen in TMC-1, around the low-mass protostar, IRAS 16293, and in the Orion Hot Core, but similar values to the Orion Compact Ridge and L134N. These results are compared with predictions from chemical models, and to other observations made in these sources. We find best agreement between models and observations by assuming that interaction between gas phase molecules and dust gains has impacted on the chemistry during the cold pre-collapse phase of the cloud's history. There are no marked difference between molecular D/H ratios towards different regions, or between Class 0 and I protostars. However, the difference between the [DCN]/[HCN] ratios we have measured and those previously observed towards Hot Molecular Cores leads us to suggest that there are significant evolutionary differences between high and low mass star forming regions. (C) 2002 Elsevier Science Ltd. All rights reserved.