[论文解读] HIFISTARS Herschel/HIFI observations of VY Canis Majoris. Molecular-line inventory of the envelope around the largest known star

J. Alcolea, V. Bujarrabal+17 more|UvA-DARE (University of Amsterdam)|Oct 9, 2013

Astrophysics and Star Formation Studies参考文献 43被引用 13

一句话总结

本研究利用赫歇尔/HIFI仪器对猎犬座VY星（VY Canis Majoris）进行了高分辨率亚毫米波与远红外波段的观测，检测到CO、H₂O、NH₃、SiO、SO等分子的众多谱线，其跃迁能级范围广泛（10–1000 K）。主要发现为：VY CMa的包层由多个分离的组分构成，强发射线来自振动态激发态及稀有同位素取代分子，表明其温暖内区存在极端物理条件，并且分子丰度显著高于其他演化后期恒星。

ABSTRACT

To gain insight into the physical conditions and kinematics of the warm (100-1000 K) gas around the red hyper-giant VY CMa, we performed sensitive high spectral resolution observations of molecular lines in the sub-mm/FIR using the HIFI instrument of the Herschel Space Observatory. We observed CO, H2O, and other molecular species, sampling excitation energies from a few tens to a few thousand K. These observations are part of the Herschel Guaranteed Time Key Program HIFISTARS. We detected the J=6-5, J=10-9, and J=16-15 lines of 12CO and 13CO at about 100, 300, and 750K above the ground state (and the 13CO J=9-8 line). These lines are crucial for improving the modelling of the internal layers of the envelope around VY CMa. We also detected 27 lines of H2O and its isotopomers, and 96 lines of species such as NH3, SiO, SO, SO2 HCN, OH and others, some of them originating from vibrationally excited levels. Three lines were not unambiguously assigned. Our observations confirm that VY CMa's envelope must consist of two or more detached components. The molecular excitation in the outer layers is significantly lower than in the inner ones, resulting in strong self-absorbed profiles in molecular lines that are optically thick in this outer envelope, for instance, low-lying lines of H2O. Except for the most abundant species, CO and H2O, most of the molecular emission detected at these sub-mm/FIR wavelengths arise from the central parts of the envelope. The spectrum of VY CMa is very prominent in vibrationally excited lines, which are caused by the strong IR pumping present in the central regions. Compared with envelopes of other massive evolved stars, VY CMa's emission is particularly strong in these vibrationally excited lines, as well as in the emission from less abundant species such as H13CN, SO, and NH3.

研究动机与目标

• 研究猎犬座VY星（目前已知最大恒星）周围包层的物理与化学结构。
• 探测内包层中温度为100–1000 K的温暖分子气体，该区域无法通过亚毫米波观测手段探测。
• 确定关键分子物种（包括稀有同位素取代分子与振动态激发态）的激发条件、运动学特征与分子丰度。
• 将VY CMa的分子辐射与其它演化后期恒星进行比较，以理解其异常的质量流失与化学复杂性。

提出的方法

• 利用赫歇尔HIFI仪器在亚毫米波与远红外波段进行高 spectral 分辨率观测。
• 在激发能从数十至数千开尔文的范围内，检测到12CO、13CO、H₂O、H₂O同位素取代分子、NH₃、SiO、SO、SO₂、HCN、H13CN、OH等分子的谱线。
• 通过分析谱线轮廓，推导出光学厚度、激发温度与柱密度，自吸收特征表明外层包层组分具有高光学厚度。
• 通过归一化积分强度，将数据与其它演化后期恒星（NML Cyg、IRC +10420、参宿四、AFGL 2343）进行比较，以评估相对分子辐射强度。
• 以13CO J=6–5线作为归一化参考，比较不同物种与不同源的辐射强度。
• 分析振动态跃迁与脉泽线，评估红外泵浦在激发高能级中的作用。

实验结果

研究问题

• RQ1 猎犬座VY星包层中温暖分子气体的激发结构如何？
• RQ2 VY CMa的物理条件与分子丰度与其它演化后期恒星相比如何，特别是在稀有同位素取代分子与振动态激发态方面？
• RQ3 为何振动态激发谱线表现出强发射？这与中心区域的红外泵浦有何关联？
• RQ4 为何VY CMa在H13CN、SO与NH₃等稀少物种的谱线中为最强发射体，而其他超巨星则较弱？
• RQ5 哪些证据支持VY CMa存在多个分离的包层组分？

主要发现

• HIFI观测检测到12CO、13CO、H₂O等物种的96条谱线，包括高激发跃迁（如12CO的J=16–15跃迁，其能级高于基态约750 K）。
• 检测到13CO的J=10–9与J=16–15谱线，证实内包层存在高激发温度，这对建模温暖气体结构至关重要。
• VY CMa在振动态激发谱线（特别是H₂O与SiO）中表现出极强的发射，表明中心区域存在强烈的红外泵浦。
• 该源在稀有物种（如H13CN、SO、NH₃）的谱线中为最强发射体，表明其丰度高于其他演化后期恒星。
• para-H₂¹⁸O 1₁,₁–0₀,₀谱线在VY CMa中显著弱于NML Cyg，表明该同位素取代分子的丰度较低。
• C¹⁸O尚未检测到，与¹⁸O丰度较低或主同位素取代分子光学厚度较高一致。