The First Interferometric Measurements of NH 2 D/NH 3 Ratio in Hot Corinos

Abstract

The chemical evolution of nitrogen during star and planet formation is still not fully understood. Ammonia (NH 3 ) 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 NH 3 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 NH 3 transitions with high excitation energies (≳100 K) from the vicinity of the protostars, indicating the NH 3 ice has sublimated at the inner hot region. The NH 3 column density is estimated to be ∼10 17 –10 18 cm −2 . We also detected two NH 2 D transitions, allowing us to constrain the deuterium fractionation of ammonia. The NH 2 D/NH 3 ratios are as high as ∼0.3–1 in both 4A1 and 4A2. From comparisons with the astrochemical models in the literature, the high NH 2 D/NH 3 ratios suggest that the formation of NH 3 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 N 2 and NH 3 . The implications on the physical properties of IRAS4A’s cores are discussed as well.