[P-CG20] Origin and evolution of materials in space

[PCG20-05] Constraining the primary nitrogen reservoir and formation history of ammonia ices in star-forming regions through VLA observations of ammonia deuteration

*Yoshihide Yamato¹, Kenji Furuya², Yuri Aikawa¹, Persson Vilhelm Magnus³, Tobin John⁴, Taquet Vianny⁵, Jorgensen Kristian Jes⁶, Kama Mihkel⁷ (1.Department of Astronomy, Graduate School of Science, The University of Tokyo, 2.National Astronomical Observatory of Japan, 3.Chalmers University of Technology, 4.National Radio Astronomy Observatory, 5.National Institute for Astrophysics, 6.University of Copenhagen, 7.University College London)

Keywords:Astrochemistry

Investigating molecular isotopic ratios such as D/H ratios around low-mass protostars is essential to understand the chemical origin of our Solar system. In the hot (> 100 K) region around protostars, major volatiles such as H₂O, CH₃OH, and NH₃ have sublimated from the dust grain surface to the gas phase, allowing for constraining its abundance or isotopic ratio with radio molecular line observations. We observed multiple NH₃ and NH₂D transitions toward the protobinary system NGC1333 IRAS4A (4A1 and 4A2) with Karl G. Jansky Very Large Array (VLA) at a high angular resolution (∼1 arcsec or ∼300 au). We detected NH₃ high excitation lines toward both of the binary, indicating the hot NH₃ gas in the vicinity of the protostars. Two NH₂D high excitation lines are also tentatively detected. Employing the local thermodynamical equilibrium (LTE) analysis, we found remarkably high NH₂D/NH₃ ratios of ∼1.0 and ∼0.5 with excitation temperatures of ∼100 K and ∼160K for 4A1 and 4A2, respectively. Such high NH₂D/NH₃ ratios may indicate the relatively late formation of NH₃ ices in the parent molecular cloud, and that the primary nitrogen reservoir in the molecular cloud can be the atomic nitrogen rather than icy nitrogen-bearing species such as N₂ and NH₃. Compared with recent ALMA observations of deuterated water, it is suggested that the formation stages of water ices and ammonia ices are significantly different in the interstellar medium.