The first interferometric measurements of NH${}_2$D/NH${}_3$ ratio in hot corinos

Abstract

The nitrogen chemical evolution 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 Karl G. Jansky Very Large Array (VLA). We spatially resolved the binary (hereafter 4A1 and 4A2) and detected compact emission of NH${}_3$ transitions with high excitation energies ($\gtrsim$100 K) from the vicinity of the protostars, indicating the NH${}_3$ ice has sublimated at the inner hot region. The NH${}_3$ column density is estimated to be $\sim 10^{17}-10^{18}$ 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 $\sim 0.3-1$ in both 4A1 and 4A2. From the 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 cores are discussed as well.

20 pages, 7 figures, accepted for publication in ApJ

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Cited by in corpus (1)

• Nitrogen fractionation in ammonia and its insights on nitrogen chemistry