2023A&A...675A..69J

While the abundance of elemental deuterium is relatively low (D/H ∼ a few ×10^–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 < 20 K), deuterium fractionation is driven by the H₂D⁺ ion, whereas at higher temperatures (T ≥ 20-30 K) 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 as a result of the absence of direct observational constraints of its key ions. Therefore, making use of chemical surrogates is imperative in order to explore deuterium chemistry at intermediate temperatures. Formed at an early stage of ion-molecule chemistry directly from the dissociative recombination of ${\rm{CH}}_3^ + $ (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 (ISM), carried out using the APEX 12 m telescope toward the widely studied low-mass protostellar system IRAS 16293-2422. Observed in absorption towards the envelope of the central protostar, the D/H ratio derived from the column densities of CD and CH is found to be 0.016 ± 0.003. This is an order of magnitude lower than the values found for other small molecules like C₂H and H₂CO observed in emission but whose formation, which is similar to that of CH, is also initiated via pathways involving warm deuterium chemistry. Gas-phase chemical models reproducing the CD/CH abundance ratio suggest that it reflects 'warm deuterium chemistry' (which ensues in moderately warm conditions of the ISM) and illustrates the potential use of the CD/CH ratio in constraining the gas temperatures of the envelope gas clouds it probes.