A survey of [ DCO] /[ HCO] and [ ND⁺] /[ NH⁺] ratios towards protostellar cores^*

School of Mathematics and Physics, Queens University Belfast, Belfast, BT7 1NN, UK e-mail: helenroberts77@gmail.com

Received: 20 October 2006
Accepted: 18 June 2007

Abstract

Aims.We use observations and models of molecular D/H ratios to probe the physical conditions and chemical history of the gas and to differentiate between gas-phase and grain-surface chemical processing in star forming regions.

Methods.As a follow up to previous observations of HDCO/H₂CO and DCN/HCN ratios in a selection of low-mass protostellar cores, we have measured D₂CO/H₂CO and N₂D⁺/N₂H⁺ ratios in these same sources. For comparison, we have also measured N₂D⁺/N₂H⁺ ratios towards several starless cores and have searched for N₂D⁺ and deuterated formaldehyde towards hot molecular cores (HMCs) associated with high mass star formation. We compare our results with predictions from detailed chemical models, and to other observations made in these sources.

Results. Towards the starless cores and low-mass protostellar sources we have found very high N₂D⁺ fractionation, which suggests that the bulk of the gas in these regions is cold and heavily depleted. The non-detections of N₂D⁺ in the HMCs indicate higher temperatures. We did detect HDCO towards two of the HMCs, with abundances 1-3% of H₂CO. These are the first detections of deuterated formaldehyde in high mass sources since Turner (1990) measured HDCO/H₂CO and D₂CO/H₂CO towards the Orion Compact Ridge.