The Gaia -ESO Survey: a new approach to chemically characterising young open clusters II. Abundances of the neutron-capture elements Cu, Sr, Y, Zr, Ba, La, and Ce
The Gaia -ESO Survey: a new approach to chemically characterising young open clusters II. Abundances of the neutron-capture elements Cu, Sr, Y, Zr, Ba, La, and Ce
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盖亚 -ESO 巡天：一种对年轻疏散星团进行化学表征的新方法 II。
DOI：
10.1051/0004-6361/202141069
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发表时间：
2021
影响因子：
6.5
通讯作者：
Baratella M
中科院分区：
文献类型：
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作者：
Baratella M
背景年轻疏散星系团(年龄小于200Myr)在化学成分上表现出几个特点。这些异常包括略低于太阳的铁含量、某些原子物种(如电离铬)的超太阳丰度，以及[Ba/Fe]的非典型增强，其值高达~0.7Dex。在这项工作中，我们对五个年轻的疏散星系团(IC2391，IC2602，IC4665，NGC2516和NGC2547)和一个恒星形成区(NGC2264)的样本进行了扩展，目的是测定不同中子俘获元素的丰度，主要是Cu I，Sr I，Sr II，Y II，Zr II，Ba II，NgC2547La II和Ce II。对于NGC2264和NGC2547，我们首次给出了这些元素的测量结果。经过仔细的选择，我们通过光谱合成计算得到了孤立谱线和清洁线的丰度，并对太阳进行了严格的微分。结果我们发现我们的星系团在不确定的范围内有太阳的[Cu/Fe]，而我们确认[Ba/Fe]是超太阳的，取值范围从+0.22到+0.64Dex。我们的分析还指出，Y略有增强，[Y/Fe]比率在0到+0.3Dex之间。对于其他过程元素，我们发现[X/Fe]值在任何年龄都是太阳的。结论不可能用标准恒星产额和银河系化学演化模型预测来调和年轻时Ba和Y的异常行为。我们探讨了与谱线行为相关的不同可能的情景，从对不同电离阶段的依赖、对磁场存在的敏感性(通过Landé因子)到第一电离势效应。我们还研究了它们可能是由于恒星活动水平的增加而引起的恒星光球层结构的变化，从而影响了谱线的形成，从而得到了丰度。这些影响似乎在年龄小于~100 Myr的恒星中更强。然而，我们仍然无法解释这些增强，巴之谜仍然没有解开。通过目前的研究，我们认为其他元素，如锶、锆、镧和铈，可能是年轻时这一过程的更可靠的示踪，我们强烈鼓励进一步的批判性观察。
ContextYoung open clusters (ages of less than 200 Myr) have been observed to exhibit several peculiarities in their chemical compositions. These anomalies include a slightly sub-solar iron content, super-solar abundances of some atomic species (e.g. ionised chromium), and atypical enhancements of [Ba/Fe], with values up to ~0.7 dex. Regarding the behaviour of the others-process elements like yttrium, zirconium, lanthanum, and cerium, there is general disagreement in the literature: some authors claim that they follow the same trend as barium, while others find solar abundances at all ages.AimsIn this work we expand upon our previous analysis of a sample of five young open clusters (IC 2391, IC 2602, IC 4665, NGC 2516, and NGC 2547) and one star-forming region (NGC 2264), with the aim of determining abundances of different neutron-capture elements, mainly Cu I, Sr I, Sr II, Y II, Zr II, Ba II, La II, and Ce II. For NGC 2264 and NGC 2547 we present the measurements of these elements for the first time.MethodsWe analysed high-resolution, high signal-to-noise spectra of 23 solar-type stars observed within theGaia-ESO survey. After a careful selection, we derived abundances of isolated and clean lines via spectral synthesis computations and in a strictly differential way with respect to the Sun.ResultsWe find that our clusters have solar [Cu/Fe] within the uncertainties, while we confirm that [Ba/Fe] is super-solar, with values ranging from +0.22 to +0.64 dex. Our analysis also points to a mild enhancement of Y, with [Y/Fe] ratios covering values between 0 and +0.3 dex. For the others-process elements we find that [X/Fe] ratios are solar at all ages.ConclusionsIt is not possible to reconcile the anomalous behaviour of Ba and Y at young ages with standard stellar yields and Galactic chemical evolution model predictions. We explore different possible scenarios related to the behaviour of spectral lines, from the dependence on the different ionisation stages and the sensitivity to the presence of magnetic fields (through the Landé factor) to the first ionisation potential effect. We also investigate the possibility that they may arise from alterations of the structure of the stellar photosphere due to the increased levels of stellar activity that affect the spectral line formation, and consequently the derived abundances. These effects seem to be stronger in stars at ages of less than ~ 100 Myr. However, we are still unable to explain these enhancements, and the Ba puzzle remains unsolved. With the present study we suggest that other elements, for example Sr, Zr, La, and Ce, might be more reliable tracer of thes-process at young ages, and we strongly encourage further critical observations.