2026ApJ...997...30K

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, CH₂DOH/CH₃OD, 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 CH₃OD in the prestellar core L1448 in Perseus and tentatively in B213-C6 in Taurus. This work constrains the ratio of CH₂DOH/CH₃OD 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 CH₂DOH/CH₃OD 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 CH₂DOH 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 CH₃OD is lower, namely, 1.4%-4.4% in L1448 and ≤3.8% in B213-C6.