High-temperature Dust Condensation around an AGB Star: Evidence from a Highly Pristine Presolar Corundum
High-temperature Dust Condensation around an AGB Star: Evidence from a Highly Pristine Presolar Corundum
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DOI：
10.3847/2041-8213/aad1f5
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发表时间：
2018-07
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影响因子：
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通讯作者：
A. Takigawa;R. Stroud;L. Nittler;C. Alexander;A. Miyake
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作者：
A. Takigawa;R. Stroud;L. Nittler;C. Alexander;A. Miyake
刚玉(α-Al_2O_3)和无定形或亚稳态Al_2O_3是在富氧渐近巨星周围观察到的恒星周围尘埃的常见成分，也存在于原始陨石中。我们报道了一种独特的极前刚玉颗粒QUE060的详细的同位素和显微结构研究，该颗粒在亚历山德拉女王山脉97008(LL3.05)陨石的酸性残留物中被发现。根据它的氧和镁同位素组成，这个直径1.4AGBm的颗粒形成于一颗低质量或中等质量的μ星。它有四个发达的刚玉菱面体{011}面和一个粗糙的圆面和空洞。镁含量高(mg/Al>；0.004)是放射性~(26)Al衰变的结果。没有观察到可能从刚玉中析出的尖晶石(MgAl2O4)包裹体，但存在几个具有调制结构的高镁磁区。QUE060颗粒的亚面体形状是第一个明确的证据，表明刚玉在AGB恒星周围延伸的大气中凝聚并生长到微米大小。平坦的表面表明，在星际介质(ISM)和太阳星云中，QUE060颗粒几乎没有受到气体-颗粒和颗粒-颗粒碰撞的影响。镁在其组织中的分布表明，自26Al耗尽以来，QUE060没有经历过任何严重的加热事件。然而，它至少经历了一次非常瞬时的加热事件来形成高镁磁区。产生这种瞬变事件以及粗糙表面和空穴的一种可能机制是ISM中的单个颗粒-颗粒碰撞。这些结果表明，QUE060颗粒是迄今为止研究过的最原始的星际刚玉。
Corundum (α-Al2O3) and amorphous or metastable Al2O3 are common components of circumstellar dust observed around O-rich asymptotic giant branch (AGB) stars and found in primitive meteorites. We report a detailed isotopic and microstructural investigation of a unique presolar corundum grain, QUE060, identified in an acid residue of the Queen Alexandra Range 97008 (LL3.05) meteorite. Based on its O and Mg isotopic compositions, this 1.4 μm diameter grain formed in a low- or intermediate-mass AGB star. It has four developed rhombohedral {011} faces of corundum and a rough, rounded face with cavities. High Mg contents (Mg/Al > 0.004) are due to the decay of radioactive 26Al. No spinel (MgAl2O4) inclusions that might have exsolved from the corundum are observed, but there are several high-Mg domains with modulated structures. The subhedral shape of grain QUE060 is the first clear evidence that corundum condenses and grows to micrometer sizes in the extended atmospheres around AGB stars. The flat faces indicate that grain QUE060 experienced little modification by gas–grain and grain–grain collisions in the interstellar medium (ISM) and solar nebula. The Mg distribution in its structure indicates that grain QUE060 has not experienced any severe heating events since the exhaustion of 26Al. However, it underwent at least one very transient heating event to form the high-Mg domains. A possible mechanism for producing this transient event, as well as the one rough surface and cavity, is a single grain–grain collision in the ISM. These results indicate that grain QUE060 is the most pristine circumstellar corundum studied to date.