First detection of CD toward IRAS 16293-2422

Formed at an early stage of ion-molecule chemistry directly from the dissociative recombination of CH_3_^+^ (CH_2_D^+^), CH (CD) is an ideal tracer for investigating deuterium substitution initiated by reactions with CH_2_D^+^. This paper reports the first detection of CD in the interstellar medium (ISM), carried out using the APEX 12m telescope towards the widely studied low-mass protostellar system IRAS 16293-2422. Observed in absorption toward 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 %that derived using other small molecules like C_2_H and H_2_CO 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.