Chemical abundance study of two strongly s-process enriched post-AGB stars in the LMC: J051213.81-693537.1 and J051848.86-700246.9

K. De Smedt; H. Van Winckel; D. Kamath; P. R. Wood

摘要

Context. This paper is part of a larger project in which we systematically study the chemical abundances of extra-galactic post-asymptotic giant branch (post-AGB) stars. The aim of our programme is to derive chemical abundances of stars covering a large range in luminosity and metallicity with the ultimate goal of testing, constraining, and improving our knowledge of the poorly understood AGB phase, especially the third dredge-up mixing processes and associated s-process nucleosynthesis. Aims. Post-AGB photospheres are dominated by atomic lines and indicate the effects of internal chemical enrichment processes over the entire stellar lifetime. In this paper, we study two carefully selected post-AGB stars: J051213.81-693537.1 and J051848.86-700246.9 in the Large Magellanic Cloud (LMC). Both objects show signs of s-process enhancement. The combination of favourable atmospheric parameters for detailed abundance studies and their known distances (and hence luminosities and initial masses) make these objects ideal probes of the AGB third dredge-up and s-process nucleosynthesis in that they provide observational constraints for theoretical AGB models. Methods. We use high-resolution optical UVES spectra to determine accurate stellar parameters and subsequently perform detailed elemental abundance studies of post-AGB stars. Additionally, we use available photometric data covering optical and IR bands to construct spectral energy distributions for reddening and luminosity determinations. We then estimate initial masses from theoretical post-AGB tracks. Results. We obtained accurate atmospheric parameters for J051213.81-693537.1 (Teff = 5875 ± 125 K, log? g = 1.00 ± 0.25 dex, [Fe/H] = ?0.56 ± 0.16 dex) and J051848.86-700246.9 (Teff = 6000 ± 125 K, log? g = 0.50 ± 0.25 dex, [Fe/H] = ?1.06 ± 0.17 dex). Both stars show extreme s-process enrichment associated with relatively low C/O ratios of 1.26 ± 0.40 and 1.29?±?0.30 for J051213-693537.1 and J051848-700246.9, respectively. We could only derive upper limits of the lead (Pb) abundance. These upper limits show a possible very slight Pb overabundance with respect to heavy s-elements for J051213-693537.1, while J051848-700246.9 shows an upper limit of the Pb abundance similar to [hs/Fe]. A comparison with theoretical post-AGB evolutionary tracks in the HR-diagram reveals that both stars have low initial masses between 1.0 and 1.5 M⊙. Conclusions. This study adds to the results obtained so far on a very limited number of s-process enriched stars in the Magellanic Clouds. With the addition of the two stars in this study, we find an increasing discrepancy between observed and predicted Pb abundances towards lower metallicities of the studied s-process rich post-AGB stars in the Magellanic Clouds. The more metal-rich J051213-693537.1 fits the theoretical Pb abundance predictions well, while the five other objects with [Fe/H] < 1, including J051848-700246.9, have much lower Pb overabundances than predicted. In all objects found so far, including the objects in this study, the C/O ratio is very moderate because of the enhancement of O as well as C. We find that all s-process rich stars in the LMC and SMC studied so far, cluster in the same region of the HR-diagram and are associated with low-mass stars with a low metallicity on average. We corroborate the published lack of correlation between the metallicity and the neutron irradiation, while the neutron exposure ([hs/ls]) is strongly correlated with the third dredge-up efficiency ([s/Fe]). These correlations seem to hold in our Galaxy as well as in the Magellanic Clouds.

机译：上下文。本文是一个较大项目的一部分，在该项目中，我们系统地研究了银河外渐近巨星分支（post-AGB）星的化学丰度。我们计划的目的是获得涵盖大范围光度和金属度的恒星化学丰度，其最终目标是测试，限制和提高我们对AGB相位知之甚少的知识，尤其是第三次挖泥混合过程及其相关的知识s-过程核合成。目的AGB后的光球以原子线为主，并指示内部化学富集过程在整个恒星生命周期中的影响。在本文中，我们研究了两个精心挑选的AGB后恒星：大麦哲伦星系（LMC）中的J051213.81-693537.1和J051848.86-700246.9。这两个对象都显示出s过程增强的迹象。用于详细丰度研究的有利大气参数及其已知距离（以及因此的光度和初始质量）的组合使这些对象成为AGB三次疏edge和s过程核合成的理想探针，因为它们为理论AGB模型提供了观测约束。方法。我们使用高分辨率光学UVES光谱确定准确的恒星参数，然后对AGB后的恒星进行详细的元素丰度研究。此外，我们使用覆盖光学和红外波段的可用光度数据来构建光谱能量分布，以进行变红和发光度测定。然后，我们从理论上的AGB后轨道估算初始质量。结果。我们获得了J051213.81-693537.1（Teff = 5875±125 K，log？g = 1.00±0.25 dex，[Fe / H] = 0.56±0.16 dex）的准确大气参数和J051848.86-700246.9（Teff = 6000） ±125 K，log？g = 0.50±0.25 dex，[Fe / H] =？1.06±0.17 dex）。两颗星均显示出极端的S过程富集，分别对应于J051213-693537.1和J051848-700246.9的相对较低的C / O比（分别为1.26±0.40和1.29？±？0.30）。我们只能得出铅（Pb）丰度的上限。对于J051213-693537.1，这些上限显示相对于重元素而言，Pb过量可能非常轻微，而J051848-700246.9显示出类似于[hs / Fe]的Pb过量上限。与HR图中理论上的AGB后演化轨迹进行比较后发现，两颗恒星的初始质量都较低，介于1.0和1.5M⊙之间。结论。这项研究增加了迄今为止对麦哲伦星云中数量非常有限的S过程富集恒星获得的结果。通过在本研究中增加两颗恒星，我们发现，在麦哲伦星云中，所研究的s过程丰富的AGB后恒星的观测到的和预测的Pb丰度之间的差异越来越小，金属含量较低。金属含量更高的J051213-693537.1与理论上的Pb丰度预测非常吻合，而其他[Fe / H] <1的五个对象（包括J051848-700246.9）的Pb丰度比预期的要低得多。在迄今为止发现的所有天体中，包括本研究中的天体，由于O和C的增强，C / O比率都非常适中。我们发现，到目前为止，LMC和SMC中所有富含S过程的恒星，聚集在HR图的同一区域，并且与平均质量较低的低质量恒星有关。我们证实了已发表的金属性和中子辐照之间缺乏相关性，而中子暴露量[[hs / ls]）与第三次挖泥效率（[s / Fe]）密切相关。这些相关性似乎在我们的银河系以及麦哲伦星系中都存在。

Chemical abundance study of two strongly s-process enriched post-AGB stars in the LMC: J051213.81-693537.1 and J051848.86-700246.9

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