Isotopic Compositions of Strontium, Zirconium, Molybdenum, and Barium in Single Presolar SiC Grains and Asymptotic Giant Branch Stars
Isotopic Compositions of Strontium, Zirconium, Molybdenum, and Barium in Single Presolar SiC Grains and Asymptotic Giant Branch Stars
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单日前碳化硅颗粒和渐近巨分支星中锶、锆、钼和钡的同位素组成
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发表时间：
2003
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通讯作者：
F. Käppeler
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作者：
M. Lugaro;A. Davis;R. Gallino;M. Pellin;O. Straniero;F. Käppeler
锶，锆，钼，钡同位素组成预测的质量损失信封的渐近巨星分支（AGB）恒星的太阳金属丰度和质量1.5，3，和5 M的讨论和比较，最近的测量在单个前太阳碳化硅（SiC）颗粒从默奇森陨石。重元素的核合成通过s-过程发生在氦中间层，即氦燃烧层和氢燃烧层之间的区域，产生铁以外的重元素。经过有限数量的热逃逸的氦壳层（热脉冲），在淬火的每个不稳定性，对流信封渗透到顶层的氦intershell（第三次挖掘），混合新合成的12 C和s-过程材料的恒星表面。最终，包层变得富碳（C ≥ O），这是SiC颗粒凝聚的必要条件。在氦中间壳层中，中子分别在脉冲间隔相和热脉冲期间通过与13 C和22 Ne的（α，n）反应释放。一个13 C口袋被认为是形成在一个微小的区域中的氦间壳层的顶层注入少量的质子从信封在每个第三个疏浚情节。然后，13 C在脉冲间阶段辐射燃烧。产生的平均中子密度低，但持续时间长，因此总中子照射量高。我们已经探索了口袋中可能的13 C丰度的大范围。在低质量的AGB星（1.5 M返≤ M返≤ 4 M返）中，第二次小的中子爆发是由热脉冲中边缘的22 Ne燃烧释放的。中子密度达到相当高的高峰值，但持续时间短，因此中子照射量低。在中等质量的AGB星（4 M返< M ≤ 8 M返）中，22 Ne中子源被更有效地激活。中子俘获过程之后，考虑了从4 He到210 Po的所有相关核的后处理代码。预测的锶，锆，钼和钡的同位素组成的包层中的低质量AGB恒星的太阳金属量与个别前太阳SiC颗粒中测得的同位素比一致，而预测的中等质量恒星排除他们作为这些颗粒的来源。太阳周围的金属丰度的低质量AGB恒星的多样性，具有不同的质量和经历不同的中子曝光，需要占单个前太阳SiC颗粒之间的重元素同位素组成的测量传播。平均而言，中子照射的范围对应于比在太阳系中再现s过程主要成分所需的中子照射更低的平均中子照射。
The strontium, zirconium, molybdenum, and barium isotopic compositions predicted in the mass-losing envelopes of asymptotic giant branch (AGB) stars of solar metallicity and mass 1.5, 3, and 5 M☉ are discussed and compared with recent measurements in single presolar silicon carbide (SiC) grains from the Murchison meteorite. Heavy-element nucleosynthesis via the s-process occurs in the helium intershell, the region between the helium-burning and hydrogen-burning shells, producing heavy elements beyond iron. After a limited number of thermal runaways of the helium shell (thermal pulses), at the quenching of each instability, the convective envelope penetrates into the top layers of the helium intershell (third dredge-up), mixing newly synthesized 12C and s-process material to the stellar surface. Eventually, the envelope becomes carbon-rich (C ≥ O), a necessary condition for SiC grains to condense. In the helium intershell, neutrons are released by (α, n) reactions on 13C and 22Ne during interpulse phases and the thermal pulses, respectively. A 13C pocket is assumed to form in a tiny region in the top layers of the helium intershell by injection of a small amount of protons from the envelope during each third dredge-up episode. This 13C then burns radiately during the interpulse phase. The average neutron density produced is low, but of long duration, so the total neutron exposure is high. We have explored a large range of possible 13C abundances in the pocket. In low-mass AGB stars (1.5 M☉ ≤ M ≤ 4 M☉), a second small burst of neutrons is released by marginal 22Ne burning in the thermal pulse. The neutron density reaches quite a high peak value but is of short duration, so the neutron exposure is low. In intermediate-mass AGB stars (4 M☉ < M ≤ 8 M☉), the 22Ne neutron source is more efficiently activated. The neutron capture process has been followed with a postprocessing code that considers all relevant nuclei from 4He to 210Po. The predicted isotopic compositions of strontium, zirconium, molybdenum, and barium in the envelopes of low-mass AGB stars of solar metallicity are in agreement with the isotopic ratios measured in individual presolar SiC grains, whereas predictions for intermediate-mass stars exclude them as the sources of these grains. A multiplicity of low-mass AGB stars with metallicity around solar, having different masses and experiencing different neutron exposures, are required to account for the measured spread in heavy-element isotopic compositions among single presolar SiC grains. The range of neutron exposures corresponds, on average, to a lower mean neutron exposure than that required to reproduce the s-process main component in the solar system.