First detection of deuterated methylidyne (CD) in the interstellar medium

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