The Central 1000 au of a Pre-stellar Core Revealed with ALMA. II. Almost Complete Freeze-out
Paola Caselli, Jaime E. Pineda, Olli Sipil\"a, Bo Zhao, Elena, Redaelli, Silvia Spezzano, Maria Jos\'e Maureira, Felipe Alves, Luca, Bizzocchi, Tyler L. Bourke, Ana Chac\'on-Tanarro, Rachel Friesen, Daniele, Galli, Jorma Harju, Izaskun Jim\'enez-Serra, Eric Keto, Zhi-Yun Li

TL;DR
This study provides the first observational evidence of NH$_2$D freeze-out in a pre-stellar core, revealing a nearly complete depletion zone within 1800 au, and enhances understanding of chemical processes in early star formation.
Contribution
It presents the first clear detection of NH$_2$D freeze-out in a pre-stellar core, supported by advanced chemical modeling and high-resolution ALMA observations.
Findings
NH$_2$D freeze-out occurs within 1800 au of the core center.
A complete-depletion zone is identified, matching model predictions.
NH$_2$D begins freezing out at approximately 7000 au.
Abstract
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 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 NHD freeze-out toward the L1544 pre-stellar core, suggestive of the presence of a"complete-depletion zone" within a…
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