  While the abundance of elemental deuterium is relatively low (D/H ~ a few
1E-5), orders of magnitude higher D/H abundance ratios have been found for many
interstellar molecules, enhanced by deuterium fractionation. In cold molecular
clouds (T < 20K) deuterium fractionation is driven by the H2D+ ion, whereas at
higher temperatures (T > 20-30K) gas-phase deuteration is controlled by
reactions with CH2D+ and C2HD+. While the role of H2D+ in driving cold
interstellar deuterium chemistry is well understood, thanks to observational
constraints from direct measurements of H2D+, deuteration stemming from CH2D+
is far less understood, caused by the absence of direct observational
constraints of its key ions. Therefore, making use of chemical surrogates is
imperative for exploring deuterium chemistry at intermediate temperatures.
Formed at an early stage of ion-molecule chemistry, directly from the
dissociative recombination of CH3+ (CH2D+), CH (CD) is an ideal tracer for
investigating deuterium substitution initiated by reactions with CH2D+. This
paper reports the first detection of CD in the interstellar medium, carried out
using the APEX 12m telescope toward the widely studied low-mass protostellar
system IRAS 16293-2422. Gas-phase chemical models reproducing the observed
CD/CH abundance ratio of 0.016 suggests that it reflects `warm deuterium
chemistry' (which ensues in moderately warm conditions of the interstellar
medium) and illustrates the potential use of the CD/CH ratio in constraining
the gas temperatures of the envelope gas clouds it probes.
