普通球粒陨石中富含铝、钙和钛的氧化物星尘
Aluminum-, Calcium- And Titanium-Rich Oxide Stardust In Ordinary Chondrite Meteorites
摘要：
我们报告的同位素数据，共96 presolar氧化物颗粒中发现的几个不平衡的普通球粒陨石残留物。已确定的颗粒类型包括Al 2 O3、MgAl 2 O 4、黑云母（CaAl 12 O 19）和氧化钛。这一工作极大地增加了太阳前钛铝尖晶石数据库，也是首次报道太阳前钛氧化物。这些颗粒的氧同位素组成跨越了先前观测到的范围，表明大多数颗粒起源于低质量（<2.5 MSun）的红巨星和渐近巨星分支（AGB）恒星。要解释许多颗粒的同位素组成，需要对母AGB星进行冷底处理。黑云母颗粒中的钾-41富集可归因于现已灭绝的41 Ca的原位衰变。推断的初始41 Ca/40 Ca的比例与模型预测的低质量AGB星信封，电离抑制41 Ca衰变。稳定的镁和钙同位素比值的大多数灰榴石颗粒主要反映了母星的初始成分，一般与预期的银河系化学演化一致，但需要一些局部星际化学不均匀性。非常高的17 O/16 O或25 Mg/24 Mg比率表明联星系统中某些颗粒的起源，其中来自进化伴星的质量转移改变了母星的组成。迄今为止神秘的富含18 O的第4族颗粒的超新星起源得到了两种颗粒的多元素同位素数据的有力支持。第4组的数据是一致的起源在一个单一的超新星，其中变量的材料从深16 O丰富的内部混合与一个独特的端元混合物的外层。钛氧化物颗粒主要形成于低质量AGB星。他们是更小，更罕见的比presolar Al 2 O3，反映了较低的丰度钛比铝AGB信封。
英文摘要：
We report isotopic data for a total of 96 presolar oxide grains found in residues of several unequilibrated ordinary chondrite meteorites. Identified grain types include Al2O3, MgAl2O4, hibonite (CaAl12O19) and Ti oxide. This work greatly increases the presolar hibonite database, and is the first report of presolar Ti oxide. O-isotopic compositions of the grains span previously observed ranges and indicate an origin in red giant and asymptotic giant branch (AGB) stars of low mass (<2.5 MSun) for most grains. Cool bottom processing in the parent AGB stars is required to explain isotopic compositions of many grains. Potassium-41 enrichments in hibonite grains are attributable to in situ decay of now-extinct 41Ca. Inferred initial 41Ca/40Ca ratios are in good agreement with model predictions for low-mass AGB star envelopes, provided that ionization suppresses 41Ca decay. Stable Mg and Ca isotopic ratios of most of the hibonite grains reflect primarily the initial compositions of the parent stars and are generally consistent with expectations for Galactic chemical evolution, but require some local interstellar chemical inhomogeneity. Very high 17O/16O or 25Mg/24Mg ratios suggest an origin for some grains in binary star systems where mass transfer from an evolved companion has altered the parent star compositions. A supernova origin for the hitherto enigmatic 18O-rich Group 4 grains is strongly supported by multi-element isotopic data for two grains. The Group 4 data are consistent with an origin in a single supernova in which variable amounts of material from the deep 16O-rich interior mixed with a unique end-member mixture of the outer layers. The Ti oxide grains primarily formed in low-mass AGB stars. They are smaller and rarer than presolar Al2O3, reflecting the lower abundance of Ti than Al in AGB envelopes.
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