Presolar Grains
Presolar Grains
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DOI：
10.1093/acrefore/9780190647926.013.265
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发表时间：
2021-12
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影响因子：
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通讯作者：
Nan Liu
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作者：
Nan Liu
这是即将发表在《牛津行星科学研究百科全书》上的一篇文章的预摘要。请稍后查看全文。前太阳颗粒是在地球太阳系形成之前死亡的恒星所产生的尘埃。星尘颗粒从低质量恒星表面的恒星风以及恒星爆炸所损失的冷却气体中凝结出来，并成为星际介质（ISM）的组成部分。大约46亿年前，星际介质中的一个分子云坍缩形成太阳系，在此期间，来自星际介质的一些原始星尘颗粒得以幸存，并被纳入太阳系早期形成的小天体中。这些太阳系小天体中的一些，包括小行星和彗星，逃脱了行星形成过程，并且几乎没有发生变化，从而保留了它们最初包含的前太阳颗粒。小行星和彗星的碎片作为行星际尘埃颗粒（IDPs）和陨石在地球上被收集。在前太阳颗粒已在原始行星际尘埃颗粒和球粒陨石（未因母体的熔化或分异而发生改变的石质陨石）中被发现。前太阳颗粒通常小于几微米，通过其独特的同位素特征在原始地外物质中被识别出来，揭示了银河系化学演化（GCE）、恒星核合成以及宇宙射线照射的影响。将前太阳颗粒的同位素数据与恒星观测以及核合成模型计算进行比较表明，前太阳颗粒主要来源于渐近巨星分支恒星和核心坍缩超新星，尽管在确定某些颗粒群的恒星类型方面仍然存在模糊性。到目前为止，已经识别出了各种前太阳相，如刚玉、橄榄石和碳化硅，反映了不同类型恒星中多样的凝结环境。原始地外物质中不同前太阳相的丰度差异很大，从前太阳硅酸盐的百分之几到前太阳氧化物的百万分之几不等。前太阳颗粒研究依赖于天文学、天体物理学、核物理学和宇宙化学之间的协同作用。为了了解前太阳颗粒的恒星来源，将前太阳颗粒的同位素数据与不同类型恒星天体的天文观测进行比较是很重要的。当没有此类天文观测时，就必须依靠恒星核合成模型，这需要输入（a）根据太阳系核素丰度估计的初始恒星成分，（b）恒星演化模型，以及（c）由理论和实验室实验确定的核反应速率。一旦确定了一组前太阳颗粒的恒星来源，从这些颗粒中提取的同位素信息就可以用来约束恒星混合过程、核反应速率、银河系化学演化以及这些颗粒在星际介质中的停留时间。此外，前太阳颗粒的晶体结构和化学成分可以提供信息来推断其母恒星中的尘埃凝结条件，而原始球粒陨石中前太阳颗粒的丰度可以帮助约束其宿主球粒陨石的母体小行星所经历的次生过程。自20世纪80年代在陨石中发现前太阳颗粒以来，通过对它们的研究已经收集到了关于恒星和银河系化学演化的各种各样的信息。技术进步可能会使未来能够发现更多类型的前太阳颗粒，并对更小、更典型的前太阳颗粒进行分析。
This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Planetary Science. Please check back later for the full article. Presolar grains are dust produced by stars that died before the formation of the Earth’s solar system. Stardust grains condense out of cooling gas lost via stellar winds from the surface of low-mass stars and stellar explosions and become a constituent of interstellar medium (ISM). About 4.6 Ga, a molecular cloud in the ISM collapsed to form the solar system, during which some primordial stardust grains from the ISM survived and were incorporated into small bodies formed in the early solar system. Some of these small solar system bodies, including asteroids and comets, escaped planet formation and have remained minimally altered, thus preserving their initially incorporated presolar grains. Fragments of asteroids and comets are collected on Earth as interplanetary dust particles (IDPs) and meteorites. Presolar grains have been found in primitive IDPs and chondrites—stony meteorites that have not been modified by either melting or differentiation of their parent bodies. Presolar grains, typically less than a few μm, are identified in primitive extraterrestrial materials by their unique isotopic signatures, revealing the effects of galactic chemical evolution (GCE), stellar nucleosynthesis, and cosmic ray exposure. Comparisons of presolar grain isotope data with stellar observations and nucleosynthesis model calculations suggest that presolar grains were dominantly sourced from asymptotic giant branch stars and core-collapse supernovae, although there are still ambiguities in assigning the type of star to some groups of grains. So far, various presolar phases have been identified such as corundum, olivine, and silicon carbide, reflecting diverse condensation environments in different types of stars. The abundances of different presolar phases in primitive extraterrestrial materials vary widely, ranging from a few percent for presolar silicates to a few parts per million for presolar oxides. Presolar grain studies rely on the synergy between astronomy, astrophysics, nuclear physics, and cosmochemistry. To understand the stellar sources of presolar grains, it is important to compare isotope data of presolar grains to astronomical observations for different types of stellar objects. When such astronomical observations are unavailable, stellar nucleosynthesis models must be relied upon, which require inputs of (a) initial stellar composition estimated based on solar system nuclide abundances, (b) stellar evolution models, and (c) nuclear reaction rates determined by theories and laboratory experiments. Once the stellar source of a group of presolar grains is ascertained, isotope information extracted from the grains can then be used to constrain stellar mixing processes, nuclear reaction rates, GCE, and the ISM residence times of the grains. In addition, crystal structures and chemical compositions of presolar grains can provide information to infer dust condensation conditions in their parent stars, while abundances of presolar grains in primitive chondrites can help constrain secondary processing experienced by the parent asteroids of their host chondrites. Since the discovery of presolar grains in meteorites in 1980s, a diverse array of information about stars and GCE has been gleaned by studying them. Technological advances will likely allow for the discovery of additional types of presolar grains and analysis of smaller, more typical presolar grains in the future.