Complex Organic Molecules in Protostars with ALMA Spectral Surveys (COMPASS) VII. First interstellar detection of fully deuterated methanol

Fully deuterated methanol, CD3OD, had no robust interstellar identification prior to this work. The COMPASS program aims to continue the search for CD3OD in star-forming regions, determine the deuteration pattern of methanol, and set constraints on the deuteration process.

The team used a sensitive ALMA imaging spectral survey of the protostellar system IRAS4A in the NGC1333 molecular cloud, carried out as part of the large program Complex Organic Molecules in Protostars with ALMA Spectral Surveys. Column densities of methanol and several less abundant isotopologs, including deuterated ones, were derived under the assumption of local thermodynamic equilibrium.

Toward the binary component IRAS4A2, the survey detected several methanol isotopologs and yielded the first interstellar detection of CD3OD. The methanol deuteration pattern in IRAS4A2 is similar to that measured in other low-mass protostars, with an apparent D/H ratio that increases from singly to multiply deuterated isotopologs. Chemical models of prestellar and protostellar cores focused on deuteration do not reproduce this pattern quantitatively, and the discrepancy is exacerbated by the CD3OD detection.

LTE modeling also produced low column density ratios of methanol to its 13C and 18O isotopologs, while the 13C to 18O and 18O to 17O ratios are consistent with local interstellar medium expectations. The authors suspect the methanol column density is underestimated, possibly because the emission is clumpy rather than monolithic. The first detection of a fully deuterated complex organic molecule highlights the power of deuterium fractionation as a diagnostic tool for investigating the formation and inheritance of organic molecules during star formation.