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 13 CH3OH, 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 H + .