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arXiv 0906.1279astro-ph.SR
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NGC 1978年令人困惑的挖掘模式

The puzzling dredge-up pattern in NGC 1978

Michael T. Lederer, Thomas Lebzelter, Sergio Cristallo, Oscar Straniero, Kenneth H. Hinkle, Bernhard Aringer

备注：
16 pages, 9 figures, 4 tables, accepted for publication in A&A, language revised
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摘要：

低质量恒星是元素工厂，在其演化的最后阶段通过恒星风有效地释放其产品。由于它们的数量很大，它们对宇宙物质循环的贡献很大。为了定量地评估这种贡献，获得恒星内部的详细图像是至关重要的，特别是关于核合成和混合机制。我们寻求基准恒星演化模型的低质量恒星。特别是，我们测量的表面丰度^{12}C的热脉冲AGB星与众所周知的质量和金属丰度，这可以用来推断有关的第三次疏浚的开始和效率的信息。我们记录了大麦哲伦星系团NGC 1978中AGB星的高分辨率近红外光谱。样本包括富氧和富碳恒星，并且在恒星质量，金属丰度和年龄方面受到很好的限制。通过与合成光谱的比较，我们从目标光谱推导出C/O和^{12}C/^{13}C比率。然后，我们比较了恒星演化模型的结果与我们的测量。NGC 1978中的M星显示了C/O和^{12}C/^{13}C的值，这可以用RGB上的适度额外混合以及化学成分中的适度氧增强来最好地解释。这些富氧恒星似乎还没有经历第三次挖掘事件（尚未）。C星显示的碳氧比和碳同位素比值与第三次挖掘的发生一致。我们在这个星团中没有发现S星。我们所考虑的理论方案都不能恰当地再现观测结果。相反，我们讨论了一些非标准的方案来解释NGC 1978年令人困惑的丰度模式。

英文摘要：

Low-mass stars are element factories that efficiently release their products in the final stages of their evolution by means of stellar winds. Since they are large in number, they contribute significantly to the cosmic matter cycle. To assess this contribution quantitatively, it is crucial to obtain a detailed picture of the stellar interior, particularly with regard to nucleosynthesis and mixing mechanisms. We seek to benchmark stellar evolutionary models of low-mass stars. In particular, we measure the surface abundance of ^{12}C in thermally pulsing AGB stars with well-known mass and metallicity, which can be used to infer information about the onset and efficiency of the third dredge-up. We recorded high-resolution near-infrared spectra of AGB stars in the LMC cluster NGC 1978. The sample comprised both oxygen-rich and carbon-rich stars, and is well-constrained in terms of the stellar mass, metallicity, and age. We derived the C/O and ^{12}C/^{13}C ratio from the target spectra by a comparison to synthetic spectra. Then, we compared the outcomes of stellar evolutionary models with our measurements. The M stars in NGC 1978 show values of C/O and ^{12}C/^{13}C that can best be explained with moderate extra-mixing on the RGB coupled to a moderate oxygen enhancement in the chemical composition. These oxygen-rich stars do not seem to have undergone third dredge-up episodes (yet). The C stars show carbon-to-oxygen and carbon isotopic ratios consistent with the occurrence of the third dredge-up. We did not find S stars in this cluster. None of the theoretical schemes that we considered was able to reproduce the observations appropriately. Instead, we discuss some non-standard scenarios to explain the puzzling abundance pattern in NGC 1978.

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