Carbon-enhanced metal-poor stars: a window on AGB nucleosynthesis and binary evolution. I. Detailed analysis of 15 binary stars with known orbital periods
C. Abate, O. R. Pols, A. I. Karakas, R. G. Izzard
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18 pages, 20 figures, accepted for publication on A&A
AGB stars are responsible for producing a variety of elements, including carbon, nitrogen, and the heavy elements produced in the slow neutron-capture process ($s$-elements). There are many uncertainties involved in modelling the evolution and nucleosynthesis of AGB stars, and this is especially the case at low metallicity, where most of the stars with high enough masses to enter the AGB have evolved to become white dwarfs and can no longer be observed. The stellar population in the Galactic halo is of low mass ($\lesssim 0.85M_{\odot}$) and only a few observed stars have evolved beyond the first giant branch. However, we have evidence that low-metallicity AGB stars in binary systems have interacted with their low-mass secondary companions in the past. The aim of this work is to investigate AGB nucleosynthesis at low metallicity by studying the surface abundances of chemically peculiar very metal-poor stars of the halo observed in binary systems. To this end we select a sample of 15 carbon- and $s$-element-enhanced metal-poor (CEMP-$s$) halo stars that are found in binary systems with measured orbital periods. With our model of binary evolution and AGB nucleosynthesis, we determine the binary configuration that best reproduces, at the same time, the observed orbital period and surface abundances of each star of the sample. The observed periods provide tight constraints on our model of wind mass transfer in binary stars, while the comparison with the observed abundances tests our model of AGB nucleosynthesis.
Asymptotic giant branch (AGB) stars are considered to be among the most significant contributors to the fluorine budget in our Galaxy. While at close-to-solar metallicity observations and theory agree, at lower metallicities stellar models overestimate the fluorine production with respect to heavy elements. We present ${}^{19}$F nucleosynthesis results for a set of AGB models with different masses and metallicities in which magnetic buoyancy acts as the driving process for the formation of the ${}^{13}$C neutron source (the so-called ${}^{13}$C pocket). We find that ${}^{19}$F is mainly produc…
Barium (Ba) stars are polluted by material enriched in the slow neutron capture (s-process) elements synthesised in the interior of their former asymptotic giant branch (AGB) companion star, which is now a white dwarf. We compare individual Ba star abundance patterns to AGB nucleosynthesis models to verify if the AGB model mass is compatible with independently derived AGB mass. We selected a sample of 28 Ba stars for which both self-consistent spectroscopic observation and analysis are available and stellar mass determinations, via positioning the star on the HR diagram and comparing with evol…
The NuGrid AGB Evolution and Nucleosynthesis Data Set
University of Hull（赫尔大学）；Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Eötvös Loránd Research Network (ELKH)（孔科利天文台，匈牙利科学院地球科学研究中心，厄特沃什·罗兰研究网络）；Joint Institute for Nuclear Astrophysics, Center for the Evolution of the Elements, Michigan State University（密歇根州立大学核天体物理联合研究所元素演化中心）；University of Edinburgh（爱丁堡大学）；University of Victoria（维多利亚大学）
Evolution, nucleosynthesis and yields of AGB stars at different metallicities (III): intermediate mass models, revised low mass models and the ph-FRUITY interface
Understanding the nucleosynthesis and evolution of Asymptotic Giant Branch (AGB) stars is of primary importance as they are the main producers of some of the key elements in the Universe. They are the predominant sites for the slow neutron-capture nucleosynthesis. The exact physical conditions and nucleosynthetic processes occuring at the interior of AGB stars are not clearly understood, that hinders a better understanding of the contribution of these stars to the Galactic chemical enrichment. The extrinsic stars, which are known to have received products of AGB phase of evolution via binary m…
Evolution and nucleosynthesis of asymptotic giant branch stellar models of low metallicity
我们提出了恒星演化轨迹和核合成预测网格的低和中等质量的渐近巨星分支（AGB）恒星模型在$Z=0.001$（[Fe/H] $=-1.2$）。模型涵盖的初始质量范围为1 M $_{\odot}$至7 M $_{\odot}$。最终的表面丰度和恒星产额计算所有元素从氢铋以及同位素到铁组。我们提出的第一个研究中子俘获核合成的中等质量AGB模型，包括超AGB模型，[Fe/H] = $-1.2$。我们详细研究了一个低质量的2 M $_{\odot}$ AGB模型，其中$^{13}$ C（$α$，$n$）$^{16}$ O反应是中子的主要来源。我们还研究了一个中等质量的AGB模型5 M $_{\odot}$的壳层间温度足够高，以激活$^{22}$ Ne中子源，产生高中子密度高达$\sim 10^{14}$ n cm $^{-3}$。使用$M \geq 3$ M $_{\odot}$的型号可激活热底部燃烧。与3 M $_{\odot}$模型，我们调查的影响，在质量的不同程度的区域，质子从信封混合到intershell在最深的程度，每三个疏浚。我们比较的低质量模型的结果，三个后AGB星的金属丰度[Fe/H] $\simeq -1.2$。的组合物是一个很好的匹配，预测的中子捕获丰度，除了铅，我们确认，所观察到的铅丰度低于AGB模型计算。
We study a sample of post-AGB stars in the Galaxy, with known surface chemical composition and s-process enrichment. The recent determination of the luminosities of these sources, based on Gaia parallaxes, allows for the fist time a deep investigation of Galactic post-AGB stars, with the possibility of characterising the individual objects in terms of mass, chemical composition and age of the progenitors. To this aim we used available evolutionary sequences of AGB stars, extended to the post-AGB phase, complemented by new models, specifically calculated for the present study. The combined know…
The s process in rotating low-mass AGB stars. Nucleosynthesis calculations in models matching asteroseismic constraints
目标。我们调查的S-过程中的AGB阶段的恒星模型，其核心被强制旋转的速率与他们的祖先和继任者的星震学观测一致。方法.我们计算了新的2 M $_{\odot}$，Z=0.01模型，在主序开始时以0，125和250 km/s旋转。人为增加了一种额外的粘性，以增强角动量的传输，从而降低核心旋转速率，使之与星震学观测结果相一致。我们比较了我们的模型的旋转速率与观察到的旋转速率在MS的核心He燃烧结束，和白矮星阶段。结果我们目前的核合成计算这些旋转AGB模型，强制匹配的MS，RGB，He燃烧，和WD星的旋转速率的asteroseisic约束。特别是，我们计算了一个模型，该模型与观测到的核心氦燃烧恒星旋转速率的上限相匹配，我们还包括一个模型，该模型的旋转速度比观测值的上限快一个数量级。在这两个模型中的s-过程生产是可比的非旋转模型。结论.减慢恒星核心的自转速率以满足上述的星震约束，可以将旋转诱导的混合过程减少到对s过程核合成没有影响的程度。这一结果与恒星演化模型的初始自转速率无关。然而，在恒星演化中旋转的处理中仍然存在不确定性，需要减少这些不确定性以确认我们的结论，包括我们降低模型核心旋转速率的方法的物理性质，以及磁过程。
Rubidium and zirconium abundances in massive Galactic asymptotic giant branch stars revisited
发光的银河OH/IR星已被确定为正在产生HBB和Li的大质量（>4-5 M_s）AGB星。它们的Rb丰度和[Rb/Zr]比值来自流体静力学模型大气，是显着高于预测从AGB核合成模型，提出了一个问题，我们的理解AGB的演变和核合成。我们报告新的Rb和Zr丰度的大规模银河系AGB星的完整样本，以前研究的流体静力学模型，通过使用更现实的扩展模型大气。我们使用的频谱合成代码Turbospectrum的修改版本，并考虑存在的拱星信封和径向风。Rb和Zr丰度分别从7800 A Rb I共振线和6474 A ZrO带头确定，并且我们探索导出的丰度对扩展模型中的恒星（Teff）和风（M_loss，beta和vexp）参数变化的敏感性。将从最佳光谱拟合得出的Rb和Zr丰度与最新的AGB核合成理论模型进行比较。新的Rb丰度远低于（甚至1-2 dex）与流体静力学模型得出的，而Zr丰度是相似的。Rb I线剖面和Rb丰度对M_2损失率非常敏感，但对风速律和vexp（OH）的变化不太敏感。我们证实了早期的初步结果的基础上，一个较小的样本的大质量富O AGB星，使用扩展的大气模型可以解决之间的差异AGB核合成理论模型和银河系的大质量AGB星的观测。然而，Rb丰度仍然强烈地依赖于M_loss，这在这些AGB星中是未知的。为了打破模型的简并性，得到这些大质量AGB星的Rb丰度，需要精确的M_损失率。
We present the abundance analyses of 7 Carbon enhanced metal-poor (CEMP) stars to understand the origin of carbon in them. We used high-resolution optical spectra to derive abundances of various elements. We also used low-resolution Near-Infrared (NIR) spectra to derive the abundance of O and 12C/13C from the CO molecular band and compared their values with those derived from high-resolution optical spectra. We identified a good agreement between the values. Thus, in cool CEMP stars, the NIR observations complement the high-resolution optical observations to derive the oxygen abundance and the…
Aims. We describe the first s-process post-processing models for asymptotic giant branch (AGB) stars of masses 3, 4 and 5 M at solar metallicity (Z=0.018) computed using the input from the stellar evolutionary code aton. Methods. The models are computed with the new code snuppat(S-process NUcleosynthesis Post-Processing code for aton), including an advective scheme for the convective overshoot that leads to the formation of the main neutron source, 13C. Each model is post-processed with 3 different values of the free overshoot parameter. Included in the code snuppat is the novel Patankar-Euler…
Barium stars are one of the important probes to understand the origin and evolution of slow neutron-capture process elements in the Galaxy. These are extrinsic stars, where the observed s-process element abundances are believed to have an origin in the now invisible companions that produced these elements at their Asymptotic Giant Branch phase of evolution. We have attempted to understand the s-process nucleosynthesis, as well as the physical properties of the companion stars through a detailed comparison of observed elemental abundances of 10 barium stars with the predictions from AGB nucleos…
Metal-Poor Stars Observed with the Magellan Telescope I. Constraints on Progenitor Mass and Metallicity of AGB Stars Undergoing s-Process Nucleosynthesis
本文对3 M $_{\odot}$中的第三次发现进行了系统的研究。该模型从主序列发展到渐近巨分支（AGB）。中等质量恒星是重要的，因为它们通过慢中子捕获核合成做出了重要贡献。这项工作的目的是深入了解AGB星在热脉动过程中的行为。这项调查是基于扩展的数值模拟的演变阶段和完整的，一致的AGB模型计算。特别是，脉动过程中的对流结构将进行研究，特别强调史瓦西边界的稳定性分析，最终将确定第三次疏浚（以下简称TDUP）的发生。我们提供了一个简短的描述，我们更新的进化代码，并主要集中在14热脉冲后获得TDUP。我们详细说明了非标准的治疗对流称为“过冲”，其效率在实现TDUP事件与热脉动（氦壳闪光）。然而，我们对TDUP事件背后的物理知识以及对超调机制的理论描述存在令人尴尬的差距，因此在恒星建模中没有得到适当的处理。这并不奇怪，特别是对于恒星模型中对流等重要混合机制的过度简化描述。然而，由于这样的事件驱动的$^{13}$ C核合成的兴趣，以下的S-过程激活，我们提出了一个全面的评估，这个过程及其影响的计算将进一步进行不同的方向。
s-process nucleosynthesis in low-mass AGB stars by the $^{13}$C($α$,n)$^{16}$O neutron source
University of Granada（格拉纳达大学）；University of Perugia（佩鲁贾大学）；INFN Section of Perugia（意大利国家核物理研究所佩鲁贾分部）；INAF, Osservatorio Astronomico di Roma（意大利国家天体物理研究所罗马天文台）；INAF, Osservatorio Astronomico d’Abruzzo（意大利国家天体物理研究所阿布鲁佐天文台）；INFN, Sezione di Roma（意大利国家核物理研究所罗马分部）
We have calculated the stellar $β$-decay rate of the important s-process branching point ${}^{134}$Cs based on the state of the art shell model calculations. At typical $s$-process temperatures ($T\sim$ 0.2-0.3 GK), our new rate is one order of magnitude lower than the widely-used rate from Takahashi and Yokoi (hereafter TY87). The impact on the nucleosynthesis in AGB stars is investigated with various masses and metallicities. Our new decay rate leads to an overall decrease in the ${}^{134}$Ba/${}^{136}$Ba ratio, and well explains the measured ratio in meteorities without introducing the $i$ …
