Sulfur in presolar silicon carbide grains from asymptotic giant branch stars
Sulfur in presolar silicon carbide grains from asymptotic giant branch stars
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DOI：
10.1111/maps.12449
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发表时间：
2015-06-01
影响因子：
2.2
通讯作者：
Fujiya, Wataru
中科院分区：
文献类型：
--
作者：
Hoppe, Peter;Lodders, Katharina;Fujiya, Wataru
我们研究了14个太阳前SiC主流颗粒的C、Si、S同位素组成和S元素丰度。10粒颗粒S污染程度较低，CI球粒陨石标准化S/Si比值在2x10(-5) ~ 2x10（-4）之间。所有颗粒的s同位素组成在太阳值的2西格玛范围内兼容。它们的平均S同位素组成与太阳的偏差最多只有几个百分点，与碳星IRC+10216的观测值一致，碳星IRC+10216被认为是颗粒和星际介质的代表源星。同位素数据也与低质量渐近巨支（AGB）恒星的恒星模型预测一致。在S-33和S-34的对比图中，数据沿一条斜率为1.8 +/- 0.7的直线拟合，表明这是星系化学演化的印记。观测到的S丰度低于在压力和温度条件下由SiC中更多难熔元素的丰度推断出的固溶体中CaS与SiC的平衡缩聚。钙与S的丰度比通常高于1，这与晶粒中化学计量CaS溶液的预期相反，可能是由于CaC2缩合成SiC。我们观察到镁和硫丰度之间的相关性，表明镁在SiC中存在固溶体。主流晶粒中S的低丰度支持了这样一种观点，即在某些AB型SiC晶粒中发现的过量S-32丰度明显较高，这是来自再生AGB恒星的放射性Si-32在原位衰变后凝聚成AB晶粒的结果。
We studied 14 presolar SiC mainstream grains for C-, Si-, and S-isotopic compositions and S elemental abundances. Ten grains have low levels of S contamination and CI chondrite-normalized S/Si ratios between 2x10(-5) and 2x10(-4). All grains have S-isotopic compositions compatible within 2 sigma of solar values. Their mean S isotope composition deviates from solar by at most a few percent, and is consistent with values observed for the carbon star IRC+10216, believed to be a representative source star of the grains, and the interstellar medium. The isotopic data are also consistent with stellar model predictions of low-mass asymptotic giant branch (AGB) stars. In a S-33 versus S-34 plot the data fit along a line with a slope of 1.8 +/- 0.7, suggesting imprints from galactic chemical evolution. The observed S abundances are lower than expected from equilibrium condensation of CaS in solid solution with SiC under pressure and temperature conditions inferred from the abundances of more refractory elements in SiC. Calcium to S abundance ratios are generally above unity, contrary to expectations for stoichiometric CaS solution in the grains, possibly due to condensation of CaC2 into SiC. We observed a correlation between Mg and S abundances suggesting solid solution of MgS in SiC. The low abundances of S in mainstream grains support the view that the significantly higher abundances of excess S-32 found in some Type AB SiC grains are the result of insitu decay of radioactive Si-32 from born-again AGB stars that condensed into AB grains.