Complex Organic Molecules in Protostars with ALMA Spectral Surveys (COMPASS) VIII. First interstellar detection of fully deuterated methyl cyanide

Deuteration is a powerful metric to gain insight into the formation pathways of interstellar molecules in star forming regions. Our primary goal is to search for the fully deuterated form of methyl cyanide, CD3CN, in the ISM. We used a sensitive imaging spectral survey of the protostellar system L1551 IRS5 in the Taurus molecular cloud performed as part of the Large Program carried out with the Atacama Large Millimeter/submillimeter Array (ALMA) called Complex Organic Molecules in Protostars with ALMA Spectral Surveys (COMPASS). We derived the column densities of methyl cyanide and several of its less abundant isotopologs, including deuterated ones, under the assumption of local thermodynamic equilibrium. All three deuterated isotopologs of methyl cyanide were detected toward the northern binary component of L1551 IRS5. In particular, we report the first interstellar detection of CD3CN. The measured D/H ratio increases by about half an order of magnitude from one deuterated isotopolog to the next, in a similar way as what was measured for methanol in several other low-mass protostars. This suggests that methyl cyanide predominantly forms like methanol on the surface of dust grains during the cold prestellar phase. However, the measured factor of 2 difference between the D/H ratios of methyl cyanide and methanol may either indicate that gas-phase processes during the warm-up stage contribute approximately equally to the production of methyl cyanide, or that methyl cyanide and its deuterated isotopologs are formed during the prestellar phase at a somewhat earlier time than methanol. Multiply deuterated isotopologs are crucial to understand the formation and inheritance of molecules in the ISM by coupling observations and astrochemical models. Unfortunately, no model has so far investigated the deuteration of methyl cyanide up to the fully deuterated form. [Abridged]