Deuterated methanol on a solar system scale around the HH212 protostar

Context. Deuterium fractionation is a valuable tool for understanding the chemical evolution during the process that leads to the formation of a Sun-like planetary system. Aims. Methanol is thought to be mainly formed during the prestellar phase, and its deuterated form keeps a memory of the conditions at that epoch. The unique combination of high angular resolution and sensitivity provided by ALMA enables us to measure methanol deuteration in the planet formation region around a Class 0 protostar and to understand its origin. Methods. We mapped both the (CH3OH)-C-13 and CH2DOH distribution in the inner regions (similar to 100 au) of the HH212 system in Orion B. To this end, we used ALMA Cycle 1 and Cycle 4 observations in Band 7 with angular resolution down to similar to 0.015. Results. We detected 6 lines of 13CH3OH and 13 lines of CH2DOH with upper level energies of up to 438 K in temperature units. We derived a rotational temperature of (171 +/- 52) K and column densities of 7 x 10(16) cm(-2) ((CH3OH)-C-13) and 1 x 10(17) cm(-2) (CH2DOH), respectively. This yields a D/H ratio of (2.4 +/- 0.4) x 10(-2), which is lower by an order of magnitude than previously measured values using single-dish telescopes toward protostars located in Perseus. Our findings are consistent with the higher dust temperatures in Orion B with respect to the temperature derived for the Perseus cloud. The emission traces a rotating structure extending up to 45 au from the jet axis, which is elongated by 90 au along the jet axis. So far, the origin of the observed emission appears to be related with the accretion disc. Only higher spatial resolution measurements will be able to distinguish between different possible scenarios, however: disc wind, disc atmosphere, or accretion shocks.