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Studies of deuterated isotopologues of complex organic molecules can provideimportant constraints on their origin in regions of star formation. Inparticular, the abundances of deuterated species are very sensitive to thephysical conditions in the environment where they form. Due to the lowtemperatures in regions of star formation, these isotopologues are enhanced tosignificant levels, making detections of multiply-deuterated species possible.However, for complex organic species, only the multiply-deuterated variants ofmethanol and methyl cyanide have been reported so far. The aim of this paper isto initiate the characterisation of multiply-deuterated variants of complexorganic species with the first detection of doubly-deuterated methyl formate,CHD2OCHO. We use ALMA observations from the Protostellar Interferometric LineSurvey (PILS) of the protostellar binary IRAS 16293-2422, in the spectral rangeof 329.1 GHz to 362.9 GHz. We report the first detection of doubly-deuteratedmethyl formate CHD2OCHO in the ISM. The D/H ratio of CHD2OCHO is found to be2-3 times higher than the D/H ratio of CH2DOCHO for both sources, similar tothe results for formaldehyde from the same dataset. The observations arecompared to a gas-grain chemical network coupled to a dynamical physical model,tracing the evolution of a molecular cloud until the end of the Class 0protostellar stage. The overall D/H ratio enhancements found in theobservations are of the same order of magnitude as the predictions from themodel for the early stages of Class 0 protostars. However, the higher D/H ratioof CHD2OCHO compared to the D/H ratio of CH2DOCHO is still not predicted by themodel. This suggests that a mechanism is enhancing the D/H ratio of singly- anddoubly-deuterated methyl formate that is not in the model, e.g. mechanisms forH-D substitutions.

The ALMA-PILS survey: The first detection of doubly-deuterated methyl formate (CHD2OCHO) in the ISM