First detection of CHD${}_2$OH towards pre-stellar cores

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, CH${}_3$OH, towards two starless and two pre-stellar cores. We derive a D/H ratio of 0.8-1.9$\mathrm{\%}$ with CH${}_2$DOH 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 CHD${}_2$OH/CH${}_2$DOH are $\sim$50-80$\mathrm{\%}$, consistently high as that towards Class 0/I objects, indicating an efficient formation mechanism of CHD${}_2$OH, possibly through H atom additions to D${}_2$CO. The CH${}_2$DOH/CH${}_3$OH and CHD${}_2$OH/CH${}_3$OH 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.