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Description
Solar-type protostars have been shown to harbor highly deuterated complex organics as for example witnessed by the high relative abundances of doubly and triply deuterated isotopologs. While this degree of deuteration may provide important clues to the formation of these species, spectroscopic information on multiply deuterated isotopologs is often insufficient. In particular, searches for triply deuterated methanol, CD_3_OH, are hampered to a large extent by the lack of intensity information from a spectroscopic model. The aim of the present study is to develop a spectroscopic model of CD_3_OH in low-lying torsional states sufficiently accurate to facilitate further searches for CD_3_OH in space. We have performed a new measurement campaign for CD_3_OH involving three spectroscopic laboratories which covers the 34GHz-1.1THz and the 20-900cm^-1^ ranges. The analysis was perfomed using the rho-axis-method torsion- rotation Hamiltonian model. We determined a model that describes the ground and first excited torsional states of CD_3_OH up to quantum numbers J<=55 and K<=23, and derived a line list for radio-astronomical observations. The resulting line list is accurate up to at least 1.1THz and should be sufficient for all types of radio-astronomical searches for this methanol isotopolog. This line list was used to search for CD_3_OH in data from the Protostellar Interferometric Line Survey of IRAS 16293-2422 using the Atacama Large Millimeter/submillimeter Array. CD_3_OH is securely detected in the data with a large number of clearly separated and well-reproduced lines. We detected not only lines belonging to the ground torsional state, but also several belonging to the first excited torsional state. The derived column density of CD_3_OH and abundance relative to non-deuterated isotopolog confirm the significant enhancement of this multiply deuterated variant. This is in line with other observations of multiply deuterated complex organic molecules and may serve as an important constraint on models for their formation.
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