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 to
differentiate between subsequent stages of high-mass star formation regions due
to 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 counterpart. 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. It
can be related to problems in the bandpass at the frequency of the transition
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 have been obtained the model. It is one of the first times that
observations and modeling have been combined to derive chemically based
best-fit models for the evolution of high-mass star formation including
deuteration.
