[P-CG20] 宇宙における物質の形成と進化

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

*大和 義英¹、古家 健次²、相川 祐理¹、Persson Magnus³、Tobin John⁴、Vianny Taquet⁵、Jorgensen Jes⁶、Kama Mihkel⁷ (1.東京大学大学院理学系研究科天文学専攻、2.国立天文台、3.Chalmers University of Technology、4.National Radio Astronomy Observatory、5.National Institute for Astrophysics、6.University of Copenhagen、7.University College London)

キーワード：アストロケミストリー

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.