The Central 1000 au of a Pre-stellar Core Revealed with ALMA. II. Almost
  Complete Freeze-out
  Pre-stellar cores represent the initial conditions in the process of star and
planet formation. Their low temperatures ($<$10 K) allow the formation of thick
icy dust mantles, which will be partially preserved in the future
protoplanetary disks, ultimately affecting the chemical composition of
planetary systems. Previous observations have shown that carbon- and
oxygen-bearing species, in particular CO, are heavily depleted in pre-stellar
cores due to the efficient molecular freeze-out onto the surface of cold dust
grains. However, N-bearing species such as NH$_3$ and, in particular, its
deuterated isotopologues, appear to maintain high abundances where CO molecules
are mainly in solid phase. Thanks to ALMA, we present here the first clear
observational evidence of NH$_2$D freeze-out toward the L1544 pre-stellar core,
suggestive of the presence of a"complete-depletion zone" within a $\simeq$1800
au radius, in agreement with astrochemical pre-stellar core model predictions.
Our state-of-the-art chemical model coupled with a non-LTE radiative transfer
code demonstrates that NH$_2$D becomes mainly incorporated in icy mantles in
the central 2000 au and starts freezing-out already at $\simeq$7000 au.
Radiative transfer effects within the pre-stellar core cause the
NH$_2$D(1$_{11}$-1$_{01}$) emission to appear centrally concentrated, with a
flattened distribution within the central $\simeq$3000 au, unlike the 1.3 mm
dust continuum emission which shows a clear peak within the central
$\simeq$1800 au. This prevented NH$_2$D freeze-out to be detected in previous
observations, where the central 1000 au cannot be spatially resolved.
