2023A&A...669L...6L

The inheritance of material across the star and planet formation process is traced by deuterium fractionation. We report here the first detection of doubly deuterated methanol towards pre-stellar cores. We study the deuterium fractionation of methanol, CH₃OH, towards two starless and two pre-stellar cores. We derive a D/H ratio of 0.8-1.9% with CH₂DOH in pre-stellar cores H-MM1 and L694-2, consistent with measurements in more evolved Class 0/I objects and comet 67P/Churyumov-Gerasimenko, suggesting a direct chemical link arising in the pre-stellar stage. Furthermore, the column density ratios of CHD₂OH/CH₂DOH are ∼50-80%, as consistently high as towards Class 0/I objects, indicating an efficient formation mechanism of CHD₂OH, possibly through H atom additions to D₂CO. The CH₂DOH/CH₃OH and CHD₂OH/CH₃OH column density ratios in the two pre-stellar cores are larger than that in the two starless cores B68 and L1521E, representing an evolutionary trend of methanol deuteration in early-stage cores.