We report the first detection of doubly-deuterated methanol ( CHD _ { 2 } OH ) , as well as firm detections of the two singly-deuterated isotopomers of methanol ( CH _ { 2 } DOH and CH _ { 3 } OD ) , towards the solar-type protostar IRAS16293 - 2422 .
From the present multifrequency observations , we derive the following abundance ratios : [ CHD _ { 2 } OH ] / [ CH _ { 3 } OH ] = 0.2 \pm 0.1 , [ CH _ { 2 } DOH ] / [ CH _ { 3 } OH ] = 0.9 \pm 0.3 , [ CH _ { 3 } OD ] / [ CH _ { 3 } OH ] = 0.04 \pm 0.02 .
The total abundance of the deuterated forms of methanol is greater than that of its normal hydrogenated counterpart in the circumstellar material of IRAS16293 - 2422 , a circumstance not previously encountered .
Formaldehyde , which is thought to be the chemical precursor of methanol , possesses a much lower fraction of deuterated isotopomers ( \sim 20 % ) with respect to the main isotopic form in IRAS16293 - 2422 .
The observed fractionation of methanol and formaldehyde provides a severe challenge to both gas-phase and grain-surface models of deuteration .
Two examples of the latter model are roughly in agreement with our observations of CHD _ { 2 } OH and CH _ { 2 } DOH if the accreting gas has a large ( 0.2-0.3 ) atomic D/H ratio .
However , no gas-phase model predicts such a high atomic D/H ratio , and hence some key ingredient seems to be missing .