[论文解读] The EDIBLES survey III. C2-DIBs and their profiles

M. Elyajouri, R. Lallement+7 more|arXiv (Cornell University)|May 29, 2018

Spectroscopy and Laser Applications参考文献 34被引用 9

一句话总结

本研究利用EDIBLES巡天获得的高分辨率、高信噪比光谱，识别出与高C₂柱密度相关的C₂-DIBs（与C₂分子相关的弥散星际带）。研究发现，至少14个C₂-DIB表现出与分子载体未分辨转动能级跃迁一致的亚结构，表明存在具有不同转动常数的多种分子载体，并可能伴随温度依赖的轮廓变化。

ABSTRACT

The so-called C2-DIBs are a class of very weak bands that fall in the blue part of the optical spectrum and are associated with high column densities of the C2 molecule. DIB profile structures constrain potential molecular carriers, but their measurement requires high S/N and spectra and the use of sightlines without Doppler splitting, as typical for a single-cloud situation. Spectra from the ESO Diffuse Interstellar Bands Large Exploration Survey (EDIBLES) conducted at the VLT (ESO/Paranal) were explored to identify single-cloud and high C2 column sightlines, extract the corresponding C2-DIBs and study their strengths and profiles, and to investigate in detail any sub-structures. The target selection was made based on profile-fitting of the Sodium doublets and the detection of C2 lines. The C2 (2-0) Phillips system was fitted using a physical model of the host cloud. C2 column densities, temperatures as well as gas densities were derived for each sightline. 18 known C2-DIBs and 8 strong non-C2 DIBs were extracted towards 8 targets, comprising 7 single-cloud and one multi-cloud line-of-sights. Correlational studies revealed a tight association of the former group with the C2 columns, whereas the non-C2 DIBs are primarily correlated with reddening. We report three new weak diffuse band candidates. We show for the first time that at least 14 C2-DIBs exhibit spectral sub-structures which are consistent with unresolved rotational branches of molecular carriers. The variability of their peak separations among the bands for a given sightline implies that their carriers are different molecules with quite different sizes. We also illustrate how profiles of the same DIB vary among targets and as a function of physical parameters and provide tables defining the sub-structures to be compared with future models and experimental results.

研究动机与目标

• 利用EDIBLES巡天的高信噪比、高分辨率光谱识别并表征C₂-DIBs。
• 分离出具有高C₂柱密度的单云视线，以最小化多普勒分裂效应，实现轮廓的精确分析。
• 研究C₂-DIB轮廓中的亚结构，作为分子载体转动能级跃迁的诊断工具。
• 考察DIB等效宽度与星际参数（如C₂柱密度和消光）之间的相关性。
• 评估C₂-DIBs在不同视线及物理条件（包括温度效应）下的轮廓变异性。

提出的方法

• 对欧洲南方天文台甚大望远镜获取的光谱进行大气和恒星特征校正，并进行叠加以提高信噪比。
• 利用物理模型拟合C₂ (2-0) Phillips系统（8750–8849 Å），以推导C₂柱密度、径向速度、动能温度和气体密度。
• 通过Na I双线（3303和5895 Å）的轮廓拟合，识别出多普勒分裂最小的单云视线。
• 在八个目标中测量了18个已知C₂-DIBs和八个强非C₂-DIBs的C₂-DIB等效宽度，其中包括一个多重云视线。
• 亚结构检测依赖于高分辨率轮廓分析，通过叠加三个视线的轮廓以增强弱特征。
• 应用线性回归分析DIB等效宽度与C₂柱密度及消光的相关性，以对DIB家族进行分类。

实验结果

研究问题

• RQ1 C₂-DIB是否表现出与分子载体未分辨转动能级分支一致的亚结构？
• RQ2 C₂-DIB轮廓在具有不同物理条件的星际视线中如何变化？
• RQ3 C₂-DIB等效宽度与C₂柱密度及消光之间的关系是什么？
• RQ4 是否存在与温度相关的DIB轮廓形状变化，如轮廓展宽或次峰间距改变？
• RQ5 能否在高质量的EDIBLES光谱中识别出新的弱弥散带候选体？

主要发现

• 在18个C₂-DIB中，至少14个表现出具有两个或三个显著次峰的亚结构，与分子载体未分辨转动能级跃迁一致。
• C₂-DIB的等效宽度与C₂柱密度高度相关（r² > 0.9），表明其与C₂分子存在直接物理关联。
• 在4737.5、5547.4和5769.8 Å处发现了三个新的弱弥散带候选体，其等效宽度均低于10 mÅ。
• 同一视线中C₂-DIB轮廓的变异性表明存在不同的分子载体或电子跃迁对称性，表现为次峰间距和振幅的差异。
• 对于某些DIB，星际温度升高与轮廓展宽及次峰间距略微增加相关，与转动能级布居的布居分布变化一致。
• 5850 Å DIB似乎属于一个独立家族，因其与C₂柱密度或消光无显著相关性，而其他C₂-DIB则表现出此类相关性。