The 3D structure of CO depletion in high-mass prestellar regions
  Disentangling the different stages of the star-formation process, in
particular in the high-mass regime, is a challenge in astrophysics. Chemical
clocks could help alleviating this problem, but their evolution strongly
depends on many parameters, leading to degeneracy in the interpretation of the
observational data. One of these uncertainties is the degree of CO depletion.
We present here the first self-consistent magneto-hydrodynamic simulations of
high-mass star-forming regions at different scales, fully coupled with a
non-equilibrium chemical network, which includes C-N-O bearing molecules.
Depletion and desorption processes are treated time-dependently. The results
show that full CO-depletion (i.e. all gas-phase CO frozen-out on the surface of
dust grains), can be reached very quickly, in one third or even smaller
fractions of the free-fall time, whether the collapse proceeds on slow or fast
timescales. This leads to a high level of deuteration in a short time both for
typical tracers like N$_2$H$^+$, as well as for the main ion H$_3^+$, the
latter being in general larger and more extended. N$_2$ depletion is slightly
less efficient, and no direct effects on N-bearing molecules and deuterium
fractionation are observed. We show that CO depletion is not the only driver of
deuteration, and that there is a strong impact on $D_{frac}$ when changing the
grain-size. We finally apply a two-dimensional gaussian Point Spread Function
to our results to mimic observations with single-dish and interferometers. Our
findings suggest that the low-values observed in high-mass star-forming clumps
are in reality masking a full-depletion stage in the inner 0.1 pc region.
