[论文解读] Molecular gas in the Galactic center region III. Probing shocks in molecular cores

S. Huettemeister, G. Dahmen+4 more|arXiv (Cornell University)|Mar 5, 1998

Astrophysics and Star Formation Studies参考文献 5被引用 11

一句话总结

本研究利用SEST、JCMT和HHT望远镜对银河系中心分子核中的C18O和SiO同位素体进行多线观测，探究冲击效应。研究发现，SiO辐射源自高密度、低温气体（n(H₂) > 10⁴ cm⁻³，T_KIN ~ 25 K），SiO丰度约为10⁻⁹，且在预测存在云-云碰撞的区域显著增强，表明冲击活动由大尺度运动学驱动。

ABSTRACT

Multiline observations of C18O and SiO isotopomers toward 33 molecular peaks in the Galactic center region, taken at the SEST, JCMT and HHT telescopes, are presented. The C18O presumably traces the total H_2 column density, while the SiO traces gas affected by shocks and high temperature chemistry. The J =2-1 line of SiO is seen only in few regions of the Galactic disk. This line is easily detected in all Galactic center sources observed. A comparison of the strength of the rare isotopomers 29SiO and 30SiO to the strength of the main isotopomer 28SiO implies that the J = 2-1 transition of 28SiO is optically thick. The 29Si/30Si isotope ratio of 1.6 in the Galactic center clouds is consistent with the terrestrial value. Large Velocity Gradient models show that the dense component (n_(H_2) > 10^4cm-3) in typical molecular cores in the Galactic center is cool (TKIN ~ 25K), contrary to what is usually found in Giant Molecular Clouds in the disk, where the densest cores are the hottest. High kinetic temperatures, > 100K, known to exist from NH_3 studies, are only present at lower gas densities of a few 10^3cm-3, where SiO is highly subthermal. Assuming that C18O traces all of the molecular gas, it is found that in all cases but one, SiO emission is compatible with arising in gas at higher density that is (presently) relatively cool. The relative abundance of SiO is typically 10^-9, but differs significantly between individual sources. It shows a dependence on the position of the source within the Galactic center region. High abundances are found in those regions for which bar potential models predict a high likelihood for cloud-cloud collisions. These results can be used to relate the amount of gas that has encountered shocks within the last ~ 10^6 years to the large scale kinematics in the inner ~500pc of the Galaxy.

研究动机与目标

• 利用高 spectral 分辨率谱线观测，探测银河系中心分子核中的冲击激发机制。
• 确定SiO辐射气体的物理条件（密度、温度），并评估其在冲击过程中的起源。
• 研究SiO丰度的空间分布特征，并将其与大尺度运动学及云-云碰撞联系起来。
• 检验28SiO J=2-1线的光学厚度，并约束银河系中心的同位素比值。
• 比较银河系中心分子核与银盘巨分子云的激发条件。

提出的方法

• 利用SEST、JCMT和HHT望远镜对银河系中心33个分子峰进行C18O和SiO同位素体（28SiO、29SiO、30SiO）的多线观测。
• 将C18O用作总H₂柱密度的示踪剂，假设其能追踪所有分子气体。
• 将SiO用作冲击示踪剂，因其对高温化学和非-LTE激发敏感。
• 通过29SiO和30SiO同位素体的相对强度分析28SiO J=2-1线的光学厚度。
• 应用大速度梯度（LVG）模型，通过线强度比推导出气体的激发温度和密度。
• 将观测到的SiO丰度与条形势场模型的预测进行比较，以评估云-云碰撞的可能性。

实验结果

研究问题

• RQ1 银河系中心分子核中SiO辐射气体的物理条件（密度和温度）是什么？
• RQ2 28SiO J=2-1线是否光学厚？这如何影响丰度测定？
• RQ3 SiO丰度在银河系中心如何变化？是否与预测的云-云碰撞区域存在相关性？
• RQ4 为何28SiO J=2-1跃迁在银河系中心可被探测到，但在银河系盘面却极为罕见？
• RQ5 银河系中心分子核的激发条件与银盘巨分子云有何不同？

主要发现

• 28SiO J=2-1线光学厚，由29SiO和30SiO同位素体的相对强度表明。
• 银河系中心云中29Si/30Si同位素比值为1.6，与地球值一致。
• 银河系中心分子核中的致密组分具有约25 K的激发温度，显著低于银盘巨分子云中的温度。
• 仅在较低密度（~几×10³ cm⁻³）区域观测到较高激发温度（>100 K），此时SiO处于亚热平衡状态。
• SiO丰度通常约为10⁻⁹，且显著变化，与云碰撞概率高的区域相关。
• 观测到的SiO辐射与高密度、低温气体一致，而非高温、低密度气体，表明冲击激发发生在高密度环境。