The puzzling dredge-up pattern in NGC 1978
The puzzling dredge-up pattern in NGC 1978
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NGC 1978 中令人费解的挖掘模式
DOI：
10.1051/0004-6361/200911857
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发表时间：
2009
影响因子：
6.5
通讯作者：
B. Aringer
中科院分区：
文献类型：
--
作者：
M. Lederer;T. Lebzelter;S. Cristallo;O. Straniero;K. Hinkle;B. Aringer
语境。低质量恒星是元素工厂，在演化的最后阶段通过恒星风有效地释放其产品。由于它们数量众多，因此对宇宙物质循环做出了重大贡献。为了定量评估这种贡献，获得恒星内部的详细图像至关重要，特别是在核合成和混合机制方面。目标。我们寻求对低质量恒星的恒星演化模型进行基准测试。特别是，我们测量了具有已知质量和金属丰度的热脉冲 AGB 恒星中 12 C 的表面丰度，这可以用来推断有关第三次挖掘的起始和效率的信息。方法。我们记录了 LMC 星团 NGC 1978 中 AGB 恒星的高分辨率近红外光谱。该样本包含富氧和富碳恒星，并且在恒星质量、金属丰度和年龄方面受到良好约束。通过与合成光谱的比较，我们从目标光谱中得出了 C/O 和 12 C/ 13 C 比率。然后，我们将恒星演化模型的结果与我们的测量结果进行了比较。结果。 NGC 1978 中的 M 星显示了 C/O 和 12 C/ 13 C 值，这可以通过 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。这些富含氧的恒星似乎还没有经历第三次疏浚事件。 C星显示的碳氧比和碳同位素比与第三次疏浚事件的发生一致。我们在这个星团中没有发现S星。我们考虑的理论方案都无法适当地重现观察结果。相反，我们讨论一些非标准情景来解释 NGC 1978 中令人费解的丰度模式。
Context. Low-mass stars are element factories that effi ciently release their products in the final stages of their ev olution by means of stellar winds. Since they are large in number, they contri bute significantly to the cosmic matter cycle. To assess this contribution quantitatively, it is crucial to obtain a detailed picture o f the stellar interior, particularly with regard to nucleos ynthesis and mixing mechanisms. Aims. 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 ons et and effi ciency of the third dredge-up. Methods. 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-constraine d in terms of the stellar mass, metallicity, and age. We deriv ed the C/O and 12 C/ 13 C ratio from the target spectra by a comparison to synthetic s pectra. Then, we compared the outcomes of stellar evolutionary models with our measurements. Results. 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 o f 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.