AGB星周围的高温尘埃凝聚：来自高度原始的前太阳日冕的证据
High-temperature Dust Condensation around an AGB Star: Evidence from a Highly Pristine Presolar Corundum
摘要：
刚玉（$α$ -Al $_{2}$ O $_{3}$）和无定形或亚稳态Al $_{2}$ O $_{3}$是在富氧渐近巨星分支（AGB）恒星周围观察到的星周尘埃的常见成分，并在原始陨石中发现。我们报告了一个详细的同位素和微观结构的调查，一个独特的前太阳能颗粒，QUE 060，确定在酸性残留物的皇后亚历山德拉范围97008（LL3.05）陨石。基于其O和Mg同位素组成，这颗直径为1.4 $μ$ m的颗粒形成于一颗低质量或中等质量的AGB星。它有四个发达的菱形$\{$ 011 $\}$面和一个粗糙的圆形面与空腔。高Mg含量（Mg/Al $>$ 0.004）是由于放射性$^{26}$ Al的衰变所致。未观察到可能从尖晶石中析出的尖晶石（MgAl $_{2}$ O $_{4}$）夹杂物，但存在几个具有调制结构的高Mg域。颗粒QUE 060的半面形是第一个明确的证据，表明在AGB恒星周围的扩展大气中，E2凝结并增长到微米大小。平坦的表面表明，颗粒QUE 060经历了星际介质（ISM）和太阳星云中的气体-颗粒和颗粒-颗粒碰撞的小修改。Mg在其结构中的分布表明，晶粒QUE 060自$^{26}$ Al耗尽后没有经历任何严重的加热事件，但是，它经历了至少一个非常短暂的加热事件以形成高Mg畴。一个可能的机制，产生这种瞬态事件，以及一个粗糙的表面和空腔，是一个单一的晶粒碰撞的ISM。这些结果表明，颗粒QUE 060是迄今为止研究的最原始的星周彗星。
英文摘要：
Corundum ($α$-Al$_{2}$O$_{3}$) and amorphous or metastable Al$_{2}$O$_{3}$ 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 $μ$m 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 $^{26}$Al. No spinel (MgAl$_{2}$O$_{4}$) 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 $^{26}$Al. 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.
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