First Detection of CH 3 OD in Prestellar Cores

Abstract The isotopic ratios of deuterated methanol derived around protostars are commonly used to infer the physical conditions under which they formed in the earlier prestellar stage. However, there is a discrepancy in the ratio of the singly deuterated methanol isotopologues, CH 2 DOH/CH 3 OD, between low- and high-mass protostars, which puts into question whether prestellar isotopic ratios are generally preserved during the star- and planet-forming process. Resolving this puzzle is only made harder by the complete lack of data on this ratio in the prestellar stage. This work presents observations with the IRAM 30 m telescope that securely detect CH 3 OD in the prestellar core L1448 in Perseus and tentatively in B213-C6 in Taurus. This work constrains the ratio of CH 2 DOH/CH 3 OD and the D/H ratios for both singly deuterated methanol isotopologues for the first time at the prestellar stage. Column densities calculated under the assumption of local thermal equilibrium lead to a CH 2 DOH/CH 3 OD ratio of 2.8–8.5 in L1448 and ≤5.7 in B213-C6. The values are marginally consistent with the statistically expected ratio of 3, but most assumptions put the values in an elevated range in line with values found around low-mass protostars. The D/H ratio in CH 2 DOH is between 3.6% and 6.8% in L1448 and in the range of 2.4%–5.8% in B213-C6. The D/H ratio derived for CH 3 OD is lower, namely, 1.4%–4.4% in L1448 and ≤3.8% in B213-C6.