Probing the Site for r-Process Nucleosynthesis with Abundances of Barium and Magnesium in Extremely Metal-poor Stars
Probing the Site for r-Process Nucleosynthesis with Abundances of Barium and Magnesium in Extremely Metal-poor Stars
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探索极其贫金属恒星中富含钡和镁的 r 过程核合成场所
DOI：
10.1086/312514
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发表时间：
2000
期刊：
影响因子：
--
通讯作者：
Y. Yoshii
中科院分区：
文献类型：
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作者：
T. Tsujimoto;T. Shigeyama;Y. Yoshii
我们认为，如果早期银河系r过程核合成的天体物理位置被限制在II型超新星(SN II)前体的狭窄质量范围内，质量下限为Mms=20M☉，则可以充分再现极端贫金属星的[Ba/mg]与[mg/H]分布的独特特征。我们将这一特征与两个截然不同的过程联系起来，即在镁丰度区间-3.7≤[Mg/H]≤-2.3中观测到的元素比分成两个分支。第一个分支，我们称之为y分支，与单个大质量超新星产生的巴和镁有关。当Mms=20M☉时，在SNE II中合成的Ba的导数质量为8.5×10-6M☉；当Mms=25M☉时，在SNE II中合成的Ba导出质量为4.5×10-8M☉。我们的结论是，具有MMS≈20M☉的SNE II是银河系早期r-过程核合成的主要来源。具有这种依赖于MMS的BA产量的SN诱导的化学演化模型创建了y-分支，反映了具有MMS≳20M☉的每个SN II的不同的核合成产量。第二个分支，我们称为I-分支，与恒星的元素丰度比有关，这些恒星是由SNE II用Mms<；20M☉卷起的星际介质的致密壳层中形成的，不合成r-过程元素，它适用于观测到的镁丰度在[mg/H]<；-2.7范围内的恒星。这些恒星中的Ba丰度反映了它们形成时(后来)星际气体的丰度。[Ba/mg]I分支的存在强烈地表明，与前身质量为Mms的恒星≤20M☉有关的SneII是产生r过程元素的贫瘠来源。我们预测，对于其他r-过程元素，如Eu(Eu)，只要它们的生产地点与Ba相同，就会存在这种i-分支。
We suggest that if the astrophysical site for r-process nucleosynthesis in the early Galaxy is confined to a narrow mass range of Type II supernova (SN II) progenitors, with a lower mass limit of Mms = 20 M☉, a unique feature in the observed distribution of [Ba/Mg] versus [Mg/H] for extremely metal-poor stars can be adequately reproduced. We associate this feature, a bifurcation of the observed elemental ratios into two branches in the Mg abundance interval -3.7 ≤ [Mg/H] ≤ -2.3, with two distinct processes. The first branch, which we call the y-branch, is associated with the production of Ba and Mg from individual massive supernovae. The derived mass of Ba synthesized in SNe II is 8.5 × 10-6 M☉ for Mms = 20 M☉ and 4.5 × 10-8 M☉ for Mms = 25 M☉. We conclude that SNe II with Mms ≈ 20 M☉ are the dominant source of r-process nucleosynthesis in the early Galaxy. An SN-induced chemical evolution model with this Mms-dependent Ba yield creates the y-branch, reflecting the different nucleosynthesis yields of [Ba/Mg] for each SN II with Mms ≳ 20 M☉. The second branch, which we call the i-branch, is associated with the elemental abundance ratios of stars which were formed in the dense shells of the interstellar medium swept up by SNe II with Mms < 20 M☉ that do not synthesize r-process elements, and it applies to stars with observed Mg abundances in the range [Mg/H] < -2.7. The Ba abundances in these stars reflect those of the interstellar gas at the (later) time of their formation. The existence of a [Ba/Mg] i-branch strongly suggests that SNe II that are associated with stars of progenitor mass Mms ≤ 20 M☉ are infertile sources for the production of r-process elements. We predict the existence of this i-branch for other r-process elements, such as europium (Eu), to the extent that their production site is in common with Ba.