[论文解读] Ammonia observations in the LBV nebula G79.29+0.46. Discovery of a cold ring and some warm spots

J. R. Rizzo, Aina Palau+2 more|arXiv (Cornell University)|Jan 23, 2014

Astrophysics and Star Formation Studies参考文献 44被引用 9

一句话总结

本研究首次通过埃菲尔伯格100米射电望远镜对NH₃ (1,1)和(2,2)跃迁的探测，实现了对明亮蓝变星（LBV）G79.29+0.46周围冷氨（NH₃）气体相干环状结构的检测。该环状结构与红外弥漫星云相关，氨丰度极低（~10⁻¹⁰–10⁻⁹），温度较高（>30 K），表明受到紫外辐射驱动的光致离解作用；相比之下，邻近的IRDC区域表现出均匀低温气体（T_rot ~10 K）和高氨丰度（~10⁻⁸），且团块边缘氨丰度增强，提示冰质尘粒包膜因激波作用而蒸发。

ABSTRACT

The surroundings of Luminous Blue Variable (LBV) stars are excellent laboratories to study the effects of their high UV radiation, powerful winds, and strong ejection events onto the surrounding gas and dust. The LBV G79.29+0.46 powered two concentric infrared rings which may interact with the infrared dark cloud (IRDC) G79.3+0.3. The Effelsberg 100m telescope was used to observe the NH_3 (1,1), (2,2) emission surrounding G79.29+0.46 and the IRDC. In addition, we observed particular positions in the (3,3) transition toward the strongest region of the IRDC. We report here the first coherent shell-like structure of dense NH_3 gas associated with an evolved massive star. The shell, two or three orders of magnitude more tenuous than the IRDC, is well traced in both ammonia lines, and surrounds the ionized nebula. The NH_3 emission in the IRDC is characterized by a low and uniform rotational temperature (T_rot ~ 10 K) and moderately high opacities in the (1,1) line. The rest of the observed field is spotted by warm or hot zones (T_rot > 30 K) and characterized by optically thin emission of the (1,1) line. The NH_3 abundances are about 10^{-8} in the IRDC, and 10^{-10}-10^{-9} elsewhere. The warm temperatures and low abundances of NH_3 in the shell suggest that the gas is being heated and photo-dissociated by the intense UV field of the LBV star. An outstanding region is found to the south-west (SW) of the LBV star within the IRDC. The NH_3 (3,3) emission at the centre of the SW region reveals two velocity components tracing gas at temperatures > 30K. The northern edge of the SW region agrees with the border of the ring nebula and a region of continuum enhancement; here, the opacity of the (1,1) line and the NH_3 abundance do not decrease as expected in a typical clump of an isolated cold dark cloud. This strongly suggests some kind of interaction between the ring nebula and the IRDC.

研究动机与目标

• 确定LBV候选体G79.29+0.46周围致密气体的物理条件。
• 研究LBV强烈的紫外辐射和质量流失事件在塑造分子气体及改变氨丰度方面的作用。
• 探索LBV驱动的星云与邻近红外暗云（IRDC）G79.3+0.3之间的相互作用。
• 评估该区域激波诱导化学反应及尘埃颗粒包膜分子蒸发的潜在机制。

提出的方法

• 利用埃菲尔伯格100米射电望远镜，在以G79.29+0.46为中心的7′×7′区域范围内，对NH₃ (1,1)和(2,2)转动能级进行高灵敏度映射观测。
• 针对IRDC西南区域NH₃ (1,1)发射最强的区域，开展NH₃ (3,3)跃迁的定向观测。
• 通过多个氨反转跃迁的旋转温度诊断法，推导出该区域的气体动能温度和柱密度分布。
• 通过比较NH₃柱密度与1.2 mm连续辐射发射，估算H₂柱密度，进而计算氨丰度。
• 分析速度分辨的发射特征，识别不同成分，并与红外及射电连续辐射结构的空间关联性。
• 应用光学厚度诊断法，评估光学厚度效应，推断团块状致密气体的物理条件。

实验结果

研究问题

• RQ1 是否存在与LBV星云G79.29+0.46相关的致密氨气相干扩展结构？其物理特性如何？
• RQ2 在IRDC和LBV环状星云中，旋转温度与氨丰度如何变化？其变化的驱动力是什么？
• RQ3 在SW区域，有哪些证据表明LBV的质量流失喷射与IRDC发生相互作用？
• RQ4 为何IRDC团块边缘的氨丰度高于其中心区域？何种物理过程可解释此现象？

主要发现

• 检测到NH₃ (1,1)和(2,2)发射的相干环状结构，其空间位置与已知的红外环状星云一致，这是首次与演化中的大质量恒星相关的此类结构。
• IRDC中的旋转温度均匀偏低，为7–11 K，(1,1)线光学厚度较高，氨丰度约为10⁻⁸，表明气体处于冷、密、静止状态。
• 相比之下，环状星云及附近热点区域的旋转温度高于30 K，氨丰度为10⁻¹⁰至10⁻⁹，表明受LBV强烈紫外场加热并发生光致离解。
• IRDC西南区域的北部边缘表现出(1,1)线高光学厚度，且团块边缘氨丰度增强，违背典型团块行为，提示存在活跃的分子释放过程。
• SW区域NH₃ (3,3)发射揭示了三个速度成分，包括两个温度较高的核心（>30 K）和一个冷成分（11 K），支持激波诱导化学反应及动态相互作用。
• 观测到的空间分布与速度特征强烈表明，LBV的质量流失事件正在与IRDC相互作用，可能通过激波从冰质尘粒包膜中释放NH₃，与该区域先前观测到的低速激波证据一致。