2002A&A...381.1026R

We present observations of [HDCO]/[H₂CO] and [DCN]/[HCN] ratios towards a selection of low-mass protostellar cores in three different star forming regions. The best fit to the observed [HDCO]/[H₂CO] ratios is ∼0.05-0.07, similar to the values observed towards the dark clouds, TMC-1 and L134N. [DCN]/[HCN] ratios are ∼0.04, higher than those seen in TMC-1, around the low-mass protostar IRAS 16293 and the Orion Hot Core, but similar values to the Orion Compact Ridge and L134N. We compare our results with predictions from detailed, 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 grains has impacted on the chemistry during the cold pre-collapse phase of the cloud's history. The abundance of deuterated species indicates that the dense gas out of which a low mass protostar forms, evolves and collapses on a timescale of ≤50000 years. We find no marked difference between molecular D/H ratios towards different regions, or between Class 0 and Class I protostars. However, the striking 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.