2022ApJ...941...75Y

The chemical evolution of nitrogen during star and planet formation is still not fully understood. Ammonia (NH₃) is a key specie in the understanding of the molecular evolution in star-forming clouds and nitrogen isotope fractionation. In this paper, we present high-spatial-resolution observations of multiple emission lines of NH₃ toward the protobinary system NGC1333 IRAS4A with the Karl G. Jansky Very Large Array. We spatially resolved the binary (hereafter, 4A1 and 4A2) and detected compact emission of NH₃ transitions with high excitation energies (≳100 K) from the vicinity of the protostars, indicating the NH₃ ice has sublimated at the inner hot region. The NH₃ column density is estimated to be ∼10¹⁷-10¹⁸ cm^–2. We also detected two NH₂D transitions, allowing us to constrain the deuterium fractionation of ammonia. The NH₂D/NH₃ ratios are as high as ∼0.3-1 in both 4A1 and 4A2. From comparisons with the astrochemical models in the literature, the high NH₂D/NH₃ ratios suggest that the formation of NH₃ ices mainly started in the prestellar phase after the formation of bulk water ice finished, and that the primary nitrogen reservoir in the star-forming cloud could be atomic nitrogen (or N atoms) rather than nitrogen-bearing species such as N₂ and NH₃. The implications on the physical properties of IRAS4A's cores are discussed as well.