[论文解读] CO ro-vibrational lines in HD100546: A search for disc asymmetries and the role of fluorescence

R. P. Hein Bertelsen, I. Kamp+6 more|Nov 21, 2013

Thermodynamic properties of mixtures被引用 16

一句话总结

本研究利用甚大望远镜/CRIRES在八个位置角上的高分辨率光谱，分析了HD100546原行星盘中CO转振发射。结果表明，发射主要源自盘壁区域（10–13 AU）。模型在因子两倍以内再现了谱线轮廓和v=1–0的辐射通量，但对更高振动态带通量的低估达约10倍，表明紫外荧光激发不足；内盘边缘处较低的气体体积密度可改善与观测带比和振动温度（~3000 K vs. 模型的1700 K）的一致性。

ABSTRACT

We have studied the emission of CO ro-vibrational lines in the disc around the Herbig Be star HD100546 with the final goal of using these lines as a diagnostic to understand inner disc structure in the context of planet formation. High-resolution IR spectra of CO ro-vibrational emission at eight different position angles were taken with CRIRES at the VLT. From these spectra flux tables, CO ro-vibrational line profiles, and population diagrams were produced. We have investigated variations in the line profile shapes and line strengths as a function of slit position angle. We used the thermochemical disc modelling code ProDiMo based on the chemistry, radiation field, and temperature structure of a previously published model for HD100546. Comparing observations and the model, we investigated the possibility of disc asymmetries, the excitation mechanism (UV fluorescence), the geometry, and physical conditions of the inner disc. The observed CO ro-vibrational lines are largely emitted from the inner rim of the outer disc at 10-13 AU. The line shapes are similar for all v levels and line fluxes from all vibrational levels vary only within one order of magnitude. All line profile asymmetries and variations can be explained with a symmetric disc model to which a slit correction and pointing offset is applied. Because the angular size of the CO emitting region (10-13 AU) and the slit width are comparable the line profiles are very sensitive to the placing of the slit. The model reproduces the line shapes and the fluxes of the v=1-0 lines as well as the spatial extent of the CO ro-vibrational emission. It does not reproduce the observed band ratios of 0.5-0.2 with higher vibrational bands. We find that lower gas volume densities at the surface of the inner rim of the outer disc can make the fluorescence pumping more effcient and reproduce the observed band ratios.

研究动机与目标

• 确定HD100546盘中CO转振发射的空间起源及其物理条件。
• 评估紫外荧光在激发CO转振跃迁中的作用。
• 研究盘的不对称性及光谱孔径位置对观测谱线轮廓的影响。
• 检验ProDiMo热化学模型是否能再现观测到的CO谱线通量与带比。
• 评估CO转振谱线在探测内盘结构与行星形成方面的诊断潜力。

提出的方法

• 利用甚大望远镜上的CRIRES获取了HD100546在八个位置角上的高光谱分辨率红外光谱，观测CO转振谱线。
• 基于观测光谱构建了谱线轮廓、通量表与布居图，以分析轮廓形状与通量变化。
• 采用针对HD100546的ProDiMo热化学盘模型，其中包含具有7个振动态和60个转动态的CO分子。
• 将模型计算的谱线通量与轮廓与观测结果进行比较，并应用光谱孔径修正以考虑光束弥散与指向偏移的影响。
• 通过在内盘边缘降低气体体积密度，对模型进行修改，以测试荧光激发效率与带比再现能力。
• 对空间轮廓进行点扩散函数（PSF）卷积以提高比较的保真度，尽管完整PSF卷积仍为正在进行的工作。

实验结果

研究问题

• RQ1 CO转振发射在盘中的主要起源位置为何？其激发受何种物理条件控制？
• RQ2 观测谱线轮廓的不对称性在多大程度上反映了真实的盘不对称性，而非仪器或光谱孔径几何效应？
• RQ3 仅靠紫外荧光是否足以解释观测到的CO转振带比，特别是v=2–1与v=1–0通量比偏低的现象？
• RQ4 气体密度与垂直结构的变化如何影响盘内边缘紫外荧光激发的效率？
• RQ5 光谱孔径的位置与宽度对空间分辨CO转振光谱中观测谱线轮廓有何影响？

主要发现

• CO转振发射主要来自外盘内缘（10–13 AU）的盘壁结构，与盘壁结构一致。
• 所有振动态与转动态的谱线轮廓相似，通量变化在十倍以内，表明激发环境均匀。
• 观测到的谱线不对称性与变化可完全由对称盘模型结合光谱孔径修正与指向偏移解释，无需引入内在不对称性。
• 模型在因子两倍以内再现了观测谱线轮廓与v=1–0通量，证实了发射区域的空间范围。
• 更高振动态带（如v=2–1）的通量在模型中被低估约十倍，表明标准模型中紫外荧光激发不足。
• 在盘内缘降低气体体积密度可改善与观测带比及振动温度（~3000 K）的一致性，表明垂直方向延伸且密度较低的层可增强荧光激发效率。