The Central 1000 au of a Prestellar Core Revealed with ALMA. II. Almost Complete Freeze-out

Prestellar 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 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 prestellar cores due to the efficient molecular freeze-out onto the surface of cold dust grains. However, N-bearing species such as NH3 and, in particular, its deuterated isotopologues appear to maintain high abundances where CO molecules are mainly in the solid phase. Thanks to ALMA, we present here the first clear observational evidence of NH2D freeze-out toward the L1544 prestellar core, suggestive of the presence of a "complete depletion zone" within a similar or equal to 1800 au radius, in agreement with astrochemical prestellar core model predictions. Our state-of-the-art chemical model coupled with a non-LTE radiative transfer code demonstrates that NH2D becomes mainly incorporated in icy mantles in the central 2000 au and starts freezing out already at similar or equal to 7000 au. Radiative transfer effects within the prestellar core cause the NH2D(1(11) - 1(01)) emission to appear centrally concentrated, with a flattened distribution within the central similar or equal to 3000 au, unlike the 1.3 mm dust continuum emission, which shows a clear peak within the central similar or equal to 1800 au. This prevented NH2D freeze-out from being detected in previous observations, where the central 1000 au cannot be spatially resolved.