First detection of triply-deuterated methanol

We report the first detection of triply-deuterated methanol, with 12 observed transitions, towards the low-mass protostar IRAS 16293-2422, as well as multifrequency observations of ¹³CH₃OH, used to derive the column density of the main isotopomer CH₃OH. The derived fractionation ratio [CD₃OH]/[CH₃OH] averaged on a 10'' beam is 1.4%. Together with previous CH₂DOH and CHD₂OH observations, the present CD₃OH observations are consistent with a formation of methanol on grain surfaces, if the atomic D/H ratio is 0.1 to 0.3 in the accreting gas. Such a high atomic ratio can be reached in the framework of gas-phase chemical models including all deuterated isotopomers of H₃⁺.