Schmidt M R, He J H, Szczerba R, Bujarrabal V, Alcolea J, Cernicharo J, Decin L, Justtanont K, Teyssier D, Menten K M, Neufeld D A, Olofsson H, Planesas P, Marston A P, Sobolev A M, de Koter A, Schöier F L
N. Copernicus Astronomical Center, Rabiańska 8, 87-100 Toruń, Poland.
Key Laboratory for the Structure and Evolution of Celestial Objects, Yunnan Observatories, Chinese Academy of Sciences, P.O. Box 110, Kunming, Yunnan Province, China.
Astron Astrophys. 2016 Aug;592. doi: 10.1051/0004-6361/201527290. Epub 2016 Aug 12.
A discrepancy exists between the abundance of ammonia (NH) derived previously for the circumstellar envelope (CSE) of IRC+10216 from far-IR submillimeter rotational lines and that inferred from radio inversion or mid-infrared (MIR) absorption transitions.
To address the discrepancy described above, new high-resolution far-infrared (FIR) observations of both ortho- and para-NH transitions toward IRC+10216 were obtained with , with the goal of determining the ammonia abundance and constraining the distribution of NH in the envelope of IRC+10216.
We used the Heterodyne Instrument for the Far Infrared (HIFI) on board to observe all rotational transitions up to the = 3 level (three ortho- and six para-NH lines). We conducted non-LTE multilevel radiative transfer modelling, including the effects of near-infrared (NIR) radiative pumping through vibrational transitions. The computed emission line profiles are compared with the new HIFI data, the radio inversion transitions, and the MIR absorption lines in the band taken from the literature.
We found that NIR pumping is of key importance for understanding the excitation of rotational levels of NH. The derived NH abundances relative to molecular hydrogen were (2.8 ± 0.5) × 10 for ortho-NH and [Formula: see text] for para-NH, consistent with an ortho/para ratio of 1. These values are in a rough agreement with abundances derived from the inversion transitions, as well as with the total abundance of NH inferred from the MIR absorption lines. To explain the observed rotational transitions, ammonia must be formed near to the central star at a radius close to the end of the wind acceleration region, but no larger than about 20 stellar radii (1 confidence level).
先前通过远红外亚毫米波转动线得出的IRC+10216恒星周包层（CSE）中氨（NH）的丰度，与通过射电反转或中红外（MIR）吸收跃迁推断出的丰度之间存在差异。
为了解决上述差异，利用[具体仪器名称未给出]对朝向IRC+10216的邻-和对-氨跃迁进行了新的高分辨率远红外（FIR）观测，目的是确定氨丰度并限制NH在IRC+10216包层中的分布。
我们使用了[具体卫星名称未给出]上的远红外外差仪（HIFI）来观测直至J = 3能级的所有转动跃迁（三条邻-氨线和六条对-氨线）。我们进行了非局部热动平衡（non-LTE）多能级辐射传输建模，包括通过振动跃迁的近红外（NIR）辐射抽运效应。将计算得到的发射线轮廓与新的HIFI数据、射电反转跃迁以及从文献中获取的ν3带中的MIR吸收线进行比较。
我们发现近红外抽运对于理解氨转动能级的激发至关重要。相对于分子氢得出的邻-氨的NH丰度为(2.8 ± 0.5) × 10⁻⁸，对-氨的为[公式：见原文]，正/反比为1，与之一致。这些值与从反转跃迁得出的丰度以及从MIR吸收线推断出的NH总丰度大致相符。为了解释观测到的转动跃迁，氨必须在靠近中心恒星处、半径接近风加速区域末端但不大于约20个恒星半径处形成（1置信水平）。
