本文的目的是研究AGB星样本的$^{17}$ O/ $^{18}$ O比值，包括M-、S-和C-型星。这些比率的评估关系到基本的恒星演化参数：恒星的初始质量和脉动周期。用各种单碟长波设备对9颗恒星的样本进行了星周$^{13}$ C $^{16}$ O、$^{12}$ C $^{17}$ O和$^{12}$ C $^{18}$ O谱线观测。线强度比示出直接相关的表面$^{17}$ O/ $^{18}$ O丰度比。恒星演化模型预测$^{17}$ O/ $^{18}$ O比值是初始质量的敏感函数，对于初始质量小于5\，$M_{\odot}$的恒星，在整个TP-AGB阶段保持恒定。这使得测量的比率成为初始恒星质量的探针。观测到的$^{17}$ O/ $^{18}$ O的比值被发现是很好的预测范围内的恒星演化模型，不考虑对流过冲。由此，计算出七个源的准确初始质量估计值。对于剩下的两个源，两个质量的解决方案的结果，虽然有更大的概率，低质量的解决方案是正确的。最后，暗示在M/S-和C-型恒星之间可能的分离时，比较的$^{17}$ O/ $^{18}$ O比的恒星脉动周期。
英文摘要：
The aim of this paper is to investigate the $^{17}$O/$^{18}$O ratio for a sample of AGB stars, containing M-, S- and C-type stars. These ratios are evaluated in relation to fundamental stellar evolution parameters: the stellar initial mass and pulsation period. Circumstellar $^{13}$C$^{16}$O, $^{12}$C$^{17}$O and $^{12}$C$^{18}$O line observations were obtained for a sample of nine stars with various single-dish long-wavelength facilities. Line intensity ratios are shown to relate directly to the surface $^{17}$O/$^{18}$O abundance ratio. Stellar evolution models predict the $^{17}$O/$^{18}$O ratio to be a sensitive function of initial mass and to remain constant throughout the entire TP-AGB phase for stars initially less massive than 5\,$M_{\odot}$. This makes the measured ratio a probe of the initial stellar mass. Observed $^{17}$O/$^{18}$O ratios are found to be well in the range predicted by stellar evolution models that do not consider convective overshooting. From this, accurate initial mass estimates are calculated for seven sources. For the remaining two sources two mass solutions result, though with a larger probability that the low-mass solution is the correct one. Finally, hints at a possible separation between M/S- and C-type stars when comparing the $^{17}$O/$^{18}$O ratio to the stellar pulsation period are presented.
Predictions from $s$-process AGB models of the isotopic variations of zirconium and neodymium for comparison to bulk meteorites
Konkoly Observatory, HUN-REN Research Centre for Astronomy and Earth Sciences（孔科利天文台，匈牙利研究与教育网络天文学与地球科学研究中心）；CSFK, MTA Centre of Excellence（匈牙利科学院卓越研究中心）；ELTE Eötvös Loránd University, Institute of Physics（厄特沃什·罗兰大学物理研究所）；School of Physics and Astronomy, Monash University（蒙纳士大学物理与天文学院）；Department of Experimental Physics, Institute of Physics, University of Szeged（塞格德大学物理研究所实验物理系）；Department of Earth and Planetary Sciences, Institute of Geochemistry and Petrology, ETH Zürich（苏黎世联邦理工学院地球与行星科学系地球化学与岩石学研究所）
We report the Sr and Ba isotopic compositions of 18 presolar SiC grains of types Y (11) and Z (7), rare types commonly argued to have formed in lower-than-solar metallicity asymptotic giant branch (AGB) stars. We find that the Y and Z grains show higher 88Sr/87Sr and more variable 138Ba/136Ba ratios than mainstream (MS) grains. According to FRANEC Torino AGB models, the Si, Sr, and Ba isotopic compositions of our Y and Z grains can be consistently explained if the grains came from low mass AGB stars with 0.15 Zsun <= Z < 1.00 Zsun, in which the 13C neutron exposure for the slow neutron-capture…
The first measurements of carbon isotopic ratios in post-RGB stars: SZ Mon and DF Cyg. E-iSpec: A spectral analysis tool to derive elemental abundances and isotopic ratios for evolved stars
The isotopic compositions of ruthenium (Ru) are measured from presolar silicon carbide (SiC) grains. In a popular scenario, the presolar SiC grains formed in the outskirt of an asymptotic giant branch (AGB) star, left the star as a stellar wind, and joined the presolar molecular cloud from which the solar system formed. The Ru isotopes formed inside the star, moved to the stellar surface during the AGB phase, and were locked into the SiC grains. Following this scenario, we analyze the NuGrid data which provide the abundances of the Ru isotopes in the stellar wind for a set of stars in a wide r…
Recent models for evolved Low Mass Stars (with $M \lesssim 3M_\odot$), undergoing the AGB phase assume that magnetic flux-tube buoyancy drives the formation of $^{13}$C reservoirs in He-rich layers. We illustrate their crucial properties, showing how the low abundance of $^{13}$C generated below the convective envelope hampers the formation of primary $^{14}$N and the ensuing synthesis of intermediate-mass nuclei, like $^{19}$F and $^{22}$Ne. In the mentioned models, their production is therefore of a purely secondary nature. Shortage of primary $^{22}$Ne has also important effects in reducing…
Chemistry in the High expansion-velocity C-rich evolved star AFGL2233. Isotopic ratios, peculiarities and evolutionary status
Instituto de Física Fundamental (IFF-CSIC)（基础物理研究所）；Observatorio Astronómico Nacional (IGN)（国家天文台）；Institut de Radioastronomie Millimetrique（毫米波射电天文学研究所）
Isotope ratios can be measured in presolar SiC grains from ancient Asymptotic Giant Branch (AGB) stars at permil-level (0.1\%) precision. Such precise grain data permit derivation of more stringent constraints and calibrations on mixing efficiency in AGB models than traditional spectroscopic observations. In this paper we compare SiC heavy-element isotope ratios to a new series of FRUITY models that include the effects of mixing triggered by magnetic fields. Based on 2D and 3D simulations available in the literature, we propose a new formulation, upon which the general features of mixing induc…
We present a multi-instrument spectroscopic analysis of the unique Li/Na-rich giant star 25664 in Omega Centauri using spectra acquired with FLAMES-GIRAFFE, X-SHOOTER, UVES and HARPS. Li and Na abundances have been derived from the UVES spectrum using transitions weakly sensitive to non-local thermodynamic equilibrium and assumed isotopic ratio. This new analysis confirms the surprising Li and Na abundances of this star (A(Li) =+2.71+-0.07 dex, [Na/Fe]=+1.00+-0.05 dex). Additionally, we provide new pieces of evidence for its chemical characterisation. The 12C/13C isotopic ratio (15+-2) shows t…
We report NanoSIMS Si and Mg-Al isotopic data (and C, N, and Ti isotopic data when available) for 85 submicron- to micron-sized presolar SiC grains from the CM2 Murchison meteorite, including 60 MS, 8 AB1, 8 X, 7 AB2, and 2 Y grains. The MS and Y grain data demonstrate that (1) C and N contamination mainly appears as surface contamination, and sufficient presputtering is needed to expose a clean grain surface for obtaining intrinsic C and N signals, and (2) Mg and Al contamination appears as adjacent grains and rims, and high-resolution imaging and the choice of small regions of interest durin…
Stardust grains that originated in ancient stars and supernovae are recovered from meteorites and carry the detailed composition of their astronomical sites of origin. We present evidence that the majority of large ($μ$m-sized) meteoritic silicon carbide (SiC) grains formed in C-rich asymptotic giant branch (AGB) stars that were more metal-rich than the Sun. In the framework of the slow neutron-captures (the s process) that occurs in AGB stars the lower-than-solar 88Sr/86Sr isotopic ratios measured in the large SiC grains can only be accompanied by Ce/Y elemental ratios that are also lower tha…
M1-92: AGB interruption and isotopic ratio paradox. Chemistry and morpho-kinematics from improved shapemol modelling
Observatorio Astronómico Nacional (OAN-IGN)（国家天文台（OAN-IGN））；Centro de Astrobiología (CAB), CSIC-INTA（天体生物学中心（CAB），西班牙国家研究委员会-西班牙航空航天技术研究所）；Institut de Radioastronomie Millimétrique（毫米波射电天文学研究所）；Ilumbra, AstroPhysical MediaStudio（Ilumbra 天体物理媒体工作室）；Department of Physics and Astronomy, University of Calgary（卡尔加里大学物理与天文学系）
The effect of including molecular opacities of variable composition on the evolution of intermediate-mass AGB stars
从恒星演化模型的低和中等质量的渐近巨星分支（AGB）恒星的计算提供了元素丰度和产量的预测与观测比较。然而，有许多不确定性降低了这些预测的准确性。一个这样的不确定性涉及低温分子不透明度的处理，占表面丰度变化的C，N和O。一些先前的计算的中间质量AGB恒星模型，将有效的第三次疏浚和热底部燃烧包括分子不透明度的处理，不考虑消耗的C和O由于热底部燃烧。在这里，我们更新的分子不透明度的处理和调查的效果，这种改进对中等质量AGB恒星模型的计算。我们对两种质量（5 M $_{\odot}$和6 M $_{\odot}$）和两种金属丰度（$Z~=~0.001$和$Z~=~0.02$）进行测试，以量化两种混浊处理之间的变化。我们发现，几个进化属性（例如半径，$T_{\rm eff}$和$T_{\rm bce}$）是依赖于不透明处理。与$Z~=~0.02$模型相比，$Z~=~0.001$模型出现更大的结构差异，表明不透明处理在较低金属丰度下具有更显著的效果。由于结构的变化，同位素产量的预测略有影响，大多数同位素经历的变化高达60%的$Z~=~0.001$模型和20%的$Z~=~0.02$模型。尽管这种温和的影响，我们的结论是，它是更适合使用可变的分子不透明度进行热底部燃烧的模型。
A transmission electron microscopy study of presolar hibonite
我们报告的Krymka LL3.1普通球粒陨石的酸残留物中确定的五个presolar黑云母颗粒的同位素和显微结构数据。二次离子质谱法（SIMS）的同位素测量证实了这些颗粒的太阳系前拱星起源。通过用聚焦离子束扫描电子显微镜原位切片和剥离，能够对晶粒的晶体结构和化学进行透射电子显微镜（TEM）检查。同位素组成与模型的比较表明，五颗颗粒中有四颗形成于通过红巨星/渐近巨星分支演化的低质量恒星，而一颗颗粒形成于II型超新星的喷出物。选区电子衍射图表明，所有的晶粒是黑云母的单晶。一些晶粒包含堆垛层错和小的取向扩展，这可以归因于生长缺陷和通过晶粒-晶粒碰撞的机械加工的组合。超新星颗粒的结构与RGB/AGB星的结构相似，表明其形成条件相似。辐射损伤，如果存在的话，发生在我们的检测极限以下。在我们研究的五种颗粒中，只有一种具有CaAl 12 O 19的纯黑云母成分。所有其他的都含有少量的Mg、Si、Ti和Fe。微结构数据通常与理论预测一致，理论预测将拱星冷凝温度限制在1480 K至1743 K的范围内，假设相应的总气压在1 x 10-3和1 x 10-6 atm之间。TEM数据被用来开发一个校准SIMS测定氧化物颗粒中的Ti含量。与极端的18 O耗尽，表明深混合发生在其母星AGB恒星的颗粒，是轻微的钛富集相比，从没有深混合的恒星的颗粒，最有可能反映在颗粒凝聚条件的差异。
Results from observations report a growing number of metal-poor stars showing an abundance pattern midway between the s- and r-processes. These so-called r/s-stars raise the need for an intermediate neutron capture process (i-process), which is thought to result from the ingestion of protons in a convective helium-burning region, but whose astrophysical site is still largely debated. We investigate whether an i-process during the asymptotic giant branch (AGB) phase of low-metallicity low-mass stars can develop and whether it can explain the abundances of observed r/s-stars. At the beginning of…
