[论文解读] The VLT-FLAMES Tarantula Survey. IX. The interstellar medium seen through Diffuse Interstellar Bands and neutral sodium

J. Th. van Loon, Melanie J. Bailey+12 more|UvA-DARE (University of Amsterdam)|Oct 16, 2012

Astrophysics and Star Formation Studies参考文献 64被引用 47

一句话总结

本研究利用甚大望远镜-FLAMES塔伦图拉星云巡天（VLT-FLAMES Tarantula Survey）对800多颗早型星进行高分辨率光谱观测，绘制了塔伦图拉星云及其周围星际介质中弥散星际带（DIBs）和中性钠（Na i）吸收的分布图。研究发现，4428 Å DIB对强烈辐射具有强韧性，表明其载体为大而中性的物种；而其他DIBs（如5780、5797、6614 Å）在暴露区域逐渐被破坏，提示其载体更小、更脆弱，可能带电。数据还揭示了R 136周围存在壳状结构及气体内流的证据，暗示大麦哲伦云中存在星系喷泉机制。

ABSTRACT

The Tarantula Nebula (30 Dor) is a spectacular star-forming region in the Large Magellanic Cloud, seen through gas in the Galactic Disc and Halo. Diffuse Interstellar Bands offer a unique probe of the diffuse, cool-warm gas in these regions. The aim is to use DIBs as diagnostics of the local interstellar conditions, whilst at the same time deriving properties of the yet-unknown carriers. Spectra of over 800 early-type stars from the VLT Flames Tarantula Survey (VFTS) were analysed. Maps were created, separately, for the Galactic and LMC absorption in the DIBs at 4428 and 6614 Ang and - in a smaller region near the central cluster R136 - neutral sodium (Na I D); we also measured the DIBs at 5780 and 5797 Ang. The maps show strong 4428 and 6614 Ang DIBs in the quiescent cloud complex to the south of 30 Dor but weak absorption in the harsher environments to the north (bubbles) and near the OB associations. The Na maps show at least five kinematic components in the LMC and a shell-like structure surrounding R136, and small-scale structure in the Milky Way. The strengths of the 4428, 5780, 5797 and 6614 Ang DIBs are correlated, also with Na absorption and visual extinction. The strong 4428 Ang DIB is present already at low Na column density but the 6614, 5780 and 5797 Ang DIBs start to be detectable at subsequently larger Na column densities. The relative strength of the 5780 and 5797 Ang DIBs clearly confirm the Tarantula Nebula and Galactic high-latitude gas to represent a harsh radiation environment. The resilience of the 4428 Ang DIB suggests its carrier is large, compact and neutral. Structure is detected in the distribution of cool-warm gas on scales between one and >100 pc in the LMC and as little as 0.01 pc in the Sun's vicinity. Stellar winds from the central cluster R136 have created an expanding shell; some infalling gas is also detected, reminiscent of a galactic "fountain".

研究动机与目标

• 利用弥散星际带（DIBs）和中性钠（Na i）探测塔伦图拉星云及银河系盘面中冷-暖星际介质（ISM）的物理条件。
• 研究不同ISM区域中DIB强度、Na i柱密度、视觉消光及辐射环境之间的关系。
• 通过分析其与星际消光及电离态的相关性，确定DIB载体的本质，尤其关注大质量OB星协附近恶劣环境中的表现。
• 利用DIB与Na i吸收轮廓，绘制大麦哲伦云（LMC）及银河系中星际介质的运动学结构。

提出的方法

• 利用甚大望远镜-FLAMES塔伦图拉星云巡天（VFTS）中800多颗早型星的高分辨率光谱，沿多条视线测量DIB与Na i吸收。
• 光谱分析聚焦于4428 Å、5780 Å、5797 Å与6614 Å的DIBs，以及Na i双线（5890、5896 Å），对每个特征进行等效宽度测量。
• 通过整合巡天区域内的测量结果，构建DIB与Na i吸收的空间分布图，并利用速度分辨分解识别运动学组分。
• 采用统计分析量化DIB强度、Na i柱密度、视觉消光（E(B−V)）及局部辐射环境之间的相关性。
• 将数据与Hα、[O iii]及远红外尘埃发射图进行比对，评估DIB载体与不同ISM相态的关系。
• 通过Na i与DIB轮廓的运动学分解，识别出R 136周围存在壳状结构，提示气体膨胀及可能的内流。

实验结果

研究问题

• RQ1 在不同辐射环境中，不同DIB的强度如何与星际消光及Na i柱密度相关？
• RQ2 DIB载体对辐射的相对抗性暗示其物理尺寸、电荷状态及分子结构为何种特征？
• RQ3 通过DIB与Na i吸收轮廓，IS中塔伦图拉星云的运动学结构（如壳层或内流）如何显现？
• RQ4 DIB载体的分布与大麦哲伦云及银河系盘面中尘埃和电离气体的分布相比如何？
• RQ5 DIB比值（如5780/5797 Å）为大麦哲伦云与银河系之间金属丰度或辐射场差异提供了何种启示？

主要发现

• 4428 Å DIB在低Na i柱密度下即可检测到，表明其载体为大而致密的中性物种，且对电离辐射具有极高抗性。
• 6614、5780与5797 Å DIB仅在较高Na i柱密度下才可检测到，提示其载体更小、更脆弱，可能带弱电荷。
• 5780与5797 Å DIB在塔伦图拉星云及银河系高银纬区域表现出显著较低的相对强度，证实这些区域为强辐射环境。
• 在R 136星团周围探测到一个质量约为~10² M⊙、膨胀速度约40 km s⁻¹的壳状膨胀气体结构，可能由恒星风驱动。
• 观测到气体内流的证据，提示大麦哲伦云中存在星系‘喷泉’循环，物质在热相与冷相之间循环。
• DIB比Na i更能有效追踪冷-暖、中性或弱电离气体，揭示了太阳邻近区达0.01 pc、大麦哲伦云中约1 pc的细结构。