Chemical evolution in the early phases of massive star formation: II. Deuteration

The chemical evolution in high-mass star-forming regions is still poorly constrained. Studying the evolution of deuterated molecules allows distinguishing between subsequent stages of high-mass star formation regions based on the strong temperature dependence of deuterium isotopic fractionation. We observed a sample of 59 sources including 19 infrared dark clouds, 20 high-mass protostellar objects, 11 hot molecular cores and 9 ultra-compact Hii regions in the (3-2) transitions of the four deuterated molecules, DCN, DNC, DCO⁺, and N₂D⁺ as well as their non-deuterated counterparts. The overall detection fraction of DCN, DNC, and DCO⁺ is high and exceeds 50% for most of the stages. N₂D⁺ was only detected in a few infrared dark clouds and high-mass protostellar objects. This may be related to problems in the bandpass at the transition frequency and to low abundances in the more evolved, warmer stages. We find median D/H ratios of 0.02 for DCN, 0.005 for DNC, 0.0025 for DCO⁺, and 0.02 for N₂D⁺. While the D/H ratios of DNC, DCO⁺, and N₂D⁺ decrease with time, DCN/HCN peaks at the hot molecular core stage. We only found weak correlations of the D/H ratios for N₂D⁺ with the luminosity of the central source and the FWHM of the line, and no correlation with the H₂ column density. In combination with a previously observed set of 14 other molecules (Paper I), we fitted the calculated column densities with an elaborate 1D physico-chemical model with time-dependent D-chemistry including ortho- and para-H₂ states. Good overall fits to the observed data were obtained with the model. This is one of the first times that observations and modeling were combined to derive chemically based best-fit models for the evolution of high-mass star formation including deuteration.