Astronomers have, for the first time, observed fully deuterated methyl cyanide (CD3CN) in the interstellar medium. The molecule was detected toward the northern component of the low‑mass protostellar system L1551 IRS5, a star‑forming region in the Taurus molecular cloud.

The discovery was made using a deep, high‑resolution imaging spectral survey carried out with the Atacama Large Millimeter/submillimeter Array (ALMA) as part of the COMPASS Large Program. Radiative‑transfer modeling under a local thermodynamic equilibrium assumption, performed with the Weeds software package, yielded the column densities of methyl cyanide and its deuterated isotopologs.

The measured deuterium‑to‑hydrogen (D/H) ratio of CD3CN relative to hydrogenated CH3CN is about three times higher than the ratio for the singly deuterated isotopolog CH2DCN. This trend is similar to the deuteration pattern observed for methanol in other low‑mass protostars and suggests a stepwise increase in deuteration with each added deuterium atom.

At present, no astrochemical models have examined methyl cyanide deuteration up to the fully deuterated form, limiting interpretation of how this species forms and evolves in star‐forming environments. Future modeling efforts are needed to test whether surface chemistry on dust grains can reproduce the observed patterns and to assess the inheritance of deuterated molecules from earlier prestellar stages.

These observations open a new window on the chemistry of complex organics in the earliest stages of star formation and highlight the importance of multiply deuterated isotopologs in tracing molecular origins.