Deuterated methanol on Solar System scale around the HH212 protostar
  Context: Methanol is thought to be mainly formed during the prestellar phase
and its deuterated form keeps memory of the conditions at that epoch. Thanks to
the unique combination of high angular resolution and sensitivity provided by
ALMA, we wish to measure methanol deuteration in the planet formation region
around a Class 0 protostar and to understand its origin. Aims: We mapped both
the $^{13}$CH$_3$OH and CH$_2$DOH distribution in the inner regions ($\sim$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 $\sim$0.15$"$.
Results: We detected 6 lines of $^{13}$CH$_3$OH and 13 lines of CH$_2$DOH with
upper level energies up to 438 K in temperature units. We derived a rotational
temperature of (171 $\pm$ 52) K and column densities of 7$\times$10$^{16}$
cm$^{-2}$ ($^{13}$CH$_3$OH) and 1$\times$10$^{17}$ cm$^{-2}$ (CH$_2$DOH),
respectively. Consequently, the D/H ratio is (2.4 $\pm$ 0.4)$\times$10$^{-2}$,
a value lower by an order of magnitude with respect to what was previously
measured using single dish telescopes toward protostars located in Perseus. Our
findings are consistent with the higher dust temperatures in Orion B with
respect to that derived for the Perseus cloud. The emission is tracing a
rotating structure extending up to 45 au from the jet axis and elongated by 90
au along the jet axis. So far, the origin of the observed emission appears to
be related with the accretion disk. Only higher spatial resolution measurements
however, will be able to disentangle between different possible scenarios: disk
wind, disk atmosphere, or accretion shocks.
