[论文解读] Nitrogen hydrides in interstellar gas II. Analysis of Herschel/HIFI observations towards W49N and G10.6-0.4 (W31C)

C. M. Persson, M. De Luca+15 more|UCL Discovery (University College London)|Aug 20, 2012

Astrophysics and Star Formation Studies参考文献 74被引用 44

一句话总结

本研究分析了赫歇尔/HIFI对大质量恒星形成区W49N和G10.6−0.4中氮氢化物（NH、o-NH₂、o-NH₃、para-NH₃）的亚毫米波观测，揭示了这些物种在分子分数较高的稀薄/半透明气体中同时存在。主要发现包括氨的正交-对位比显著偏低（约0.5–0.7）、不同径向速度组分间丰度比保持恒定，以及NH⁺丰度的上限表明其丰度低于NH，挑战了当前的星际化学模型。

ABSTRACT

We have used the Herschel-HIFI instrument to observe interstellar nitrogen hydrides along the sight-lines towards W49N and G10.6-0.4 in order to elucidate the production pathways leading to nitrogen-bearing species in diffuse gas. All detections show absorption by foreground material over a wide range of velocities, as well as absorption associated directly with the hot-core source itself. As in the previously published observations towards G10.6-0.4, the NH, NH2 and NH3 spectra towards W49N show strikingly similar and non-saturated absorption features. We decompose the absorption of the foreground material towards W49N into different velocity components in order to investigate whether the relative abundances vary among the velocity components, and, in addition, we re-analyse the absorption lines towards G10.6-0.4 in the same manner. Abundances, with respect to molecular hydrogen, in each velocity component are estimated using CH. The analysis points to a co-existence of the nitrogen hydrides in diffuse or translucent interstellar gas with a high molecular fraction. Towards both sources, we find that NH is always at least as abundant as both o-NH2 and o-NH3, in sharp contrast to previous results for dark clouds. We find relatively constant N(NH)/N(o-NH3) and N(o-NH2)/N(o-NH3) ratios with mean values of 3.2 and 1.9 towards W49N, and 5.4 and 2.2 towards G10.6-0.4, respectively. The mean abundance of o-NH3 is ~2x10^-9 towards both sources. The nitrogen hydrides also show linear correlations with CN and HNC towards both sources, and looser correlations with CH. The upper limits on the NH+ abundance indicate column densities < 2 - 14 % of N(NH). Surprisingly low values of the ammonia ortho-to-para ratio are found in both sources, ~0.5 - 0.7 +- 0.1. This result cannot be explained by current models as we had expected to find a value of unity or higher.

研究动机与目标

• 通过观测氮氢化物，研究稀薄和半透明星际气体中含氮物种的形成途径。
• 确定在大质量恒星形成区中，NH、o-NH₂、o-NH₃和para-NH₃在多个径向速度组分中的相对丰度。
• 评估氨的正交-对位比，并与理论预期进行比较。
• 检查氮氢化物与CH、CN、HNC和HCO⁺等其他星际分子的相关性。
• 对NH⁺的丰度设定上限，并与NH的相对丰度进行比较。

提出的方法

• 利用赫歇尔/HIFI仪器对NH、o-NH₂、o-NH₃和para-NH₃的亚毫米波跃迁进行光谱分析。
• 通过速度分辨分解吸收轮廓为多个组分，以分析不同气体相中丰度的变化。
• 基于CH与太阳邻近区分子氢的相关性，使用CH作为H₂的示踪剂来估算丰度。
• 通过Jₖ = 2₁–1₁与Jₖ = 1₀–0₀跃迁的相对强度计算氨的正交-对位比。
• 在速度空间中对氮氢化物与CH、CN、HNC、HCO⁺和H₂O等分子的相关性进行统计分析。
• 基于观测光谱窗口中未检测到NH⁺，推导其柱密度的上限。

实验结果

研究问题

• RQ1 在W49N和G10.6−0.4方向的稀薄和半透明星际气体中，NH、o-NH₂和o-NH₃的相对丰度是多少？
• RQ2 氮氢化物的丰度在前景物质的不同速度组分中如何变化？
• RQ3 稀薄气体中氨的正交-对位比是多少？是否偏离高温平衡值1？
• RQ4 氮氢化物的丰度与CH、CN、HNC和HCO⁺的丰度如何相关？
• RQ5 在这些视线方向上，NH⁺的丰度上限相对于NH是多少？

主要发现

• 在W49N和G10.6−0.4方向，正交-NH₃的平均丰度相对于H₂约为∼2 × 10⁻⁹。
• 在W49N方向，N(NH)/N(o-NH₃)比值为3.2，G10.6−0.4方向为5.4；N(o-NH₂)/N(o-NH₃)比值分别为1.9和2.2。
• 在最强速度组分中，氨的正交-对位比测量值为≈0.5–0.7 ± 0.1，显著低于高温平衡值1。
• 在不同速度组分间未观测到丰度比的显著变化，表明稀薄气体中化学过程一致。
• NH⁺丰度的上限表明N(NH⁺)/N(NH)比值≤2–14%，平均≤6%，表明NH⁺远比NH稀少。
• 氮氢化物与CN和HNC之间存在强线性相关性，与CH的相关性较弱，且所有物种的轮廓均与HCO⁺和H₂O相似，表明它们共同追踪高分子分数气体。