The slow neutron-capture process (s-process) efficiency in low-mass AGB stars (1.5 < M/M${}_{\odot }$ < 3) critically depends on how mixing processes in stellar interiors are handled, which is still affected by considerable uncertainties. In this work, we compute the evolution and nucleosynthesis of low-mass AGB stars at low metallicities using the MESA stellar evolution code. The combined data set includes models with initial masses M${}_{ini}$/M${}_{\odot }=2$ and 3 for initial metallicities $Z=0.001$ and $0.002$. The nucleosynthesis was calculated for all relevant isotopes by post-processing with the NuGrid mppnp code. Using these models, we show the impact of the uncertainties affecting the main mixing processes on heavy element nucleosynthesis, such as convection and mixing at convective boundaries. We finally compare our theoretical predictions with observed surface abundances on low-metallicity stars. We find that mixing at the interface between the He-intershell and the CO-core has a critical impact on the s-process at low metallicities, and its importance is comparable to convective boundary mixing processes under the convective envelope, which determine the formation and size of the ¹³C-pocket. Additionally, our results indicate that models with very low to no mixing below the He-intershell during thermal pulses, and with a ¹³C-pocket size of at least ∼3 × 10${}^{-4}$ M${}_{\odot }$, are strongly favored in reproducing observations. Online access to complete yield data tables is also provided.

中文翻译：

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低金属AGB恒星的混合不确定性：对恒星结构和核合成的影响

低质量AGB星的慢中子俘获过程（s-过程）效率（1.5 <M / M${}_{\odot }$<3）关键取决于如何处理恒星内部的混合过程，但仍然受到相当大的不确定性的影响。在这项工作中，我们使用MESA恒星演化代码计算了低金属原子质量的低质量AGB恒星的演化和核合成。组合数据集包括初始质量为M的模型${}_{\mathrm{一世}ñ\mathrm{一世}}$/米${}_{\odot }=2$ 初始金属性为3 $ž=0.001$ 和 $0.002$。通过使用NuGrid mppnp代码进行后处理，可以计算所有相关同位素的核合成。使用这些模型，我们显示了影响主要混合过程的不确定性对重元素核合成的影响，例如对流边界处的对流和混合。最后，我们将理论预测与低金属度恒星上观测到的表面丰度进行比较。我们发现，在低金属含量的情况下，在He壳层和CO核之间的界面处的混合对s过程具有至关重要的影响，其重要性可与对流包络下的对流边界混合过程相比较，后者决定了形成和形成。^13的大小C型口袋。此外，我们的结果表明，在热脉冲期间，在He壳层以下的混合非常低甚至没有混合的模型，并且¹³ C型腔尺寸至少约为3×10${}^{-4}$ 中号${}_{\odot }$，在重现观察结果时极受青睐。还提供对完整产量数据表的在线访问。