2023A&A...677A..49I

Solar-type prestellar cores and protostars frequently display large amounts of deuterated organic molecules and, in particular, high relative abundances of doubly and triply deuterated isotopologs. Recent findings on CHD₂OH and CD₃OH toward IRAS 16293-2422 suggest that even fully deuterated methanol, CD₃OD, may be detectable as well. However, searches for CD₃OD are hampered in particular by the lack of intensity information from a spectroscopic model. The objective of the present investigation is to develop a spectroscopic model of CD₃OD in low-lying torsional states that is sufficiently accurate to facilitate searches for this isotopolog in space. We carried out a new measurement campaign for CD₃OD involving two spectroscopic laboratories that covers the 34 GHz-1.1 THz range. A torsion-rotation Hamiltonian model based on the rho-axis method was employed for our analysis. Our resulting model describes the ground and first excited torsional states of CD₃OD well up to quantum numbers J ⩽ 51 and K_a ⩽ 23. We derived a line list for radio-astronomical observations from this model that is accurate up to at least 1.1 THz and should be sufficient for all types of radio-astronomical searches for this methanol isotopolog. This line list was used to search for CD₃OD in data from the Protostellar Interferometric Line Survey of IRAS 16293-2422 obtained with the Atacama Large Millimeter/submillimeter Array. While we found several emission features that can be attributed largely to CD₃OD, their number is still not sufficiently high enough to establish a clear detection. Nevertheless, the estimate of 2 × 10¹⁵ cm^–2 derived for the CD₃OD column density may be viewed as an upper limit that can be compared to column densities of CD₃OH, CH₃OD, and CH₃OH. The comparison indicates that the CD₃OD column density toward IRAS 16293-2422 is in line with the enhanced D/H ratios observed for multiply deuterated complex organic molecules.