Hard X-ray irradiation of cosmic silicate analogs: structural evolution and astrophysical implications
Hard X-ray irradiation of cosmic silicate analogs: structural evolution and astrophysical implications
复制标题
宇宙硅酸盐类似物的硬 X 射线照射：结构演化和天体物理意义
DOI：
10.1051/0004-6361/201527708
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发表时间：
2016
影响因子：
6.5
通讯作者：
G. Martínez
中科院分区：
文献类型：
--
作者：
L. Gavilan;C. Jager;A. Simionovici;J. Lemaire;T. Sabri;E. Foy;S. Yagoubi;T. Henning;D. Salomon;G. Martínez
上下文原行星盘、星际云和活动星系核都包含X射线主导的区域。X射线与这种环境中存在的灰尘和气体相互作用。虽然一些实验室的X射线辐照实验已经在冰上进行，但到目前为止，对裸露的宇宙尘埃类似物的X射线辐照实验还很少见。目标。我们的目标是通过原位X射线衍射研究硬X射线对宇宙尘埃类似物的影响。通过使用硬X射线同步加速器纳米束，我们试图模拟在这些天体物理环境中的尘埃颗粒在其寿命期间的累积X射线暴露，并提供硬X射线对尘埃颗粒结构的影响的上限。方法.我们通过熔融淬火技术制备了顽火辉石（MgSiO3）纳米颗粒，类似于宇宙硅酸盐。然后将这些非晶颗粒退火以获得多晶颗粒。这些特征在于通过扫描电子显微镜（SEM）和高分辨率透射电子显微镜（HRTEM）照射前。粉末样品在X射线透明基底中制备，并用由欧洲同步辐射设施（Grenoble）的束线ID 16 B提供的硬X射线纳米束（29.4keV）照射。以透射模式记录X射线衍射图像，并将随后的衍射图作为总X射线暴露时间的函数进行分析。结果我们检测到嵌入在有机基质中的多晶硅酸盐的非晶化后，累积的X射线曝光为6.4 × 10 27 eV cm −2。纯结晶硅酸盐颗粒（无树脂）没有表现出非晶化。无定形硅酸盐样品（纯的和嵌入树脂中的）都没有经历结晶。我们分析了嵌入在有机基质中的多晶样品作为X射线曝光的函数的演变。结论.衍射峰强度的损失、峰加宽以及离散斑点和弧的消失揭示了树脂嵌入的（最初多晶的）硅酸盐样品的非晶化。我们探索这个实验室结果的天体物理学意义，作为X射线对宇宙硅酸盐结构影响的上限。
Context. Protoplanetary disks, interstellar clouds, and active galactic nuclei, contain X-ray dominated regions. X-rays interact with the dust and gas present in such environments. While a few laboratory X-ray irradiation experiments have been performed on ices, X-ray irradiation experiments on bare cosmic dust analogs have been scarce up to now. Aims. Our goal is to study the effects of hard X-rays on cosmic dust analogs via in-situ X-ray diffraction. By using a hard X-ray synchrotron nanobeam, we seek to simulate cumulative X-ray exposure on dust grains during their lifetime in these astrophysical environments, and provide an upper limit on the effect of hard X-rays on dust grain structure. Methods. We prepared enstatite (MgSiO 3) nanograins, analogs to cosmic silicates, via the melting-quenching technique. These amorphous grains were then annealed to obtain polycrystalline grains. These were characterized via scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) before irradiation. Powder samples were prepared in X-ray transparent substrates and were irradiated with hard X-rays nanobeams (29.4 keV) provided by beamline ID16B of the European Synchrotron Radiation Facility (Grenoble). X-ray diffraction images were recorded in transmission mode and the ensuing diffractograms were analyzed as a function of the total X-ray exposure time. Results. We detected the amorphization of polycrystalline silicates embedded in an organic matrix after an accumulated X-ray exposure of 6.4 × 10 27 eV cm −2. Pure crystalline silicate grains (without resin) did not exhibit amorphization. None of the amorphous silicate samples (pure and embedded in resin) underwent crystallization. We analyzed the evolution of the polycrystalline sample embedded in an organic matrix as a function of X-ray exposure. Conclusions. Loss of diffraction peak intensity, peak broadening, and the disappearance of discrete spots and arcs, revealed the amorphization of the resin embedded (originally polycrystalline) silicate sample. We explore the astrophysical implications of this laboratory result as an upper-limit to the effect of X-rays on the structure of cosmic silicates.