[论文解读] Squeezed between shells? On the origin of the Lupus I molecular cloud. - II. APEX CO and GASS HI observations

B. Gaczkowski, V. Roccatagliata+8 more|UTAS Research Repository|Oct 20, 2017

Astrophysics and Star Formation Studies参考文献 50被引用 4

一句话总结

本研究通过分析高分辨率APEX CO和GASS HI观测数据，探究了路西斯I分子云的起源，发现该云的运动学特征与上天蝎座H I壳层及上半夏蝎座-路西斯风泡紧密关联。结果支持一种形成机制：在上述两个大尺度结构的相互作用区域，由外部压缩引发的壳层不稳定性导致云的形成，非热速度弥散度表明存在动力压缩。

ABSTRACT

[Abridged] The Lupus I cloud is found between the Upper-Scorpius and the Upper-Centaurus-Lupus sub-groups, where the expanding USco HI shell appears to interact with a bubble currently driven by the winds of the remaining B-stars of UCL. We investigate if the Lupus I molecular could have formed in a colliding flow, and how the kinematics of the cloud might have been influenced by the larger scale gas dynamics. We performed APEX 13CO and C18O observations of three parts of Lupus. We compare these results to the atomic hydrogen data from the GASS HI survey and our dust emission results presented in the previous paper. Based on the velocity information, we present a geometric model for the interaction zone between the USco shell and the UCL wind bubble. We present evidence that the molecular gas of Lupus I is tightly linked to the atomic material of the USco shell. The CO emission in Lupus I is found mainly at velocities in the same range as the HI velocities. Thus, the molecular cloud is co-moving with the expanding USco atomic Hi shell. The gas in the cloud shows a complex kinematic structure with several line-of-sight components that overlay each other. The non-thermal velocity dispersion is in the transonic regime in all parts of the cloud and could be injected by external compression. Our observations and the derived geometric model agree with a scenario where Lupus I is located in the interaction zone between the USco shell and the UCL wind bubble. The kinematics observations are consistent with a scenario where the Lupus I cloud formed via shell instabilities. The particular location of Lupus I between USco and UCL suggests that counter-pressure from the UCL wind bubble and pre-existing density enhancements, perhaps left over from the gas stream that formed the stellar subgroups, may have played a role in its formation.

研究动机与目标

• 确定位于天蝎座-半人马OB协会的上天蝎座（USco）和上半夏蝎座-路西斯（UCL）亚群之间的路西斯I分子云的起源。
• 调查该云是否通过碰撞流机制形成，特别是上天蝎座H I壳层与上半夏蝎座-路西斯风泡之间的相互作用。
• 利用高分辨率CO和HI数据分析路西斯I的运动学结构，评估外部压缩在云形成中的作用。
• 检验该云的形成是否由上半夏蝎座-路西斯风泡的反压力及先前存在的密度增强所触发的假设。

提出的方法

• 对路西斯I内三个不同区域执行APEX $^{13}$CO (2–1) 和 $^{18}$CO (2–1) 线观测，以获得高空间分辨率和光谱分辨率的运动学数据。
• 将CO发射速度与GASS调查的原子氢（H I）数据进行比较，以评估分子相与原子相气体之间的共动性与相互作用。
• 使用辐射转移模型，从CO谱线中推导光学厚度 ($\tau_{18}$)、激发温度 ($T_{\mathrm{ex}}$) 和H$_2$柱密度。
• 应用LTE分析与线型拟合技术，对视线方向上的复杂速度成分进行分解，采用高斯拟合处理光谱。
• 基于分子气体与原子气体在速度和空间上的一致性，构建上天蝎座壳层与上半夏蝎座-路西斯风泡相互作用区的几何模型。
• 采用 $\tau_{18}/(1 - \exp(-\tau_{18}))$ 校正方法，以校正 $^{18}$CO发射线中的光学厚度展宽效应。

实验结果

研究问题

• RQ1 路西斯I分子云是否与膨胀的上天蝎座H I壳层存在运动学关联？
• RQ2 上半夏蝎座-路西斯风泡在塑造路西斯I的运动学结构方面起什么作用？
• RQ3 路西斯I中观测到的非热速度弥散是否表明来自周围大尺度流场的外部压缩？
• RQ4 路西斯I中复杂的视线速度成分能否通过上天蝎座壳层与上半夏蝎座-路西斯泡在相互作用区的碰撞流情景加以解释？
• RQ5 先前存在的密度增强在路西斯I形成过程中的意义是什么？

主要发现

• 路西斯I中的分子气体与上天蝎座H I壳层共动，CO发射主要集中在径向速度 $v_{\mathrm{LSR}} = 3$ 至 $6\,\mathrm{km\,s^{-1}}$ 范围内，与H I速度范围一致。
• 该云表现出复杂的运动学结构，存在多个重叠的视线速度成分，表明存在动态相互作用及可能的碎片化。
• 路西斯I中的非热速度弥散处于跨音速范围，表明外部压缩是湍流的驱动力。
• 上天蝎座壳层与上半夏蝎座-路西斯风泡相互作用区的几何模型与观测到的分子气体和原子气体的速度与空间分布一致。
• 路西斯I的形成最合理的解释是：由上半夏蝎座-路西斯风泡的反压力与星际介质中预先存在的密度增强共同触发的壳层不稳定性。
• 结果支持一种形成情景：路西斯I在两个大尺度膨胀结构的界面处通过压缩形成，与碰撞流模型一致。