AI 中文总结
研究利用VLT/UVES光谱仪对M 22恒星的Mg同位素丰度比测量，发现s过程富集源不影响Mg同位素比率，确定26Mg/24Mg与轻元素的相关性，通过定制产量得出低质量AGB恒星可重现丰度，排除高质量AGB，限制形成场景并给出年龄差异。
AI 中文摘要
复杂星团M 22（NGC 6656）因其具有不同铁峰和中子俘获元素丰度的两个恒星群，为研究低金属丰度下的慢中子俘获（s过程）核合成提供了独特机会。以往研究将这些丰度差异归因于3 - 6个太阳质量的渐近巨星分支（AGB）恒星的污染，它们产生大量富含中子的Mg同位素25Mg和26Mg。我们报告了首次在球状星团状系统中，利用VLT/UVES光谱仪的超高分辨率和高信噪比光谱（R = 110000，514nm处每像素S/N = 300），对六颗恒星（每个s过程组三颗）在[Fe/H]约为 - 2时的Mg同位素丰度比进行测量。尽管24Mg、25Mg和26Mg存在星与星之间的变化，但我们发现与重元素丰度无相关性，这意味着s过程富集的核合成源一定不会影响Mg同位素比率。相反，这项工作的一个关键结果是我们确定了26Mg/24Mg与一些轻元素之间的相关性。使用一套根据M 22的金属丰度定制的AGB核合成产量，我们发现约1 - 3个太阳质量的低质量AGB恒星能够重现M 22观测到的s过程丰度，并且两个s过程组之间Mg同位素比率没有任何差异排除了质量高于约3个太阳质量的AGB。这对可能的形成场景施加了更严格的限制，并表明这两个群体之间至少存在约280 - 480 Myr的年龄差异，且与等时线拟合无关。
英文摘要
The complex star cluster M 22 (NGC 6656) provides a unique opportunity for studying slow neutron-capture (s-process) nucleosynthesis at low metallicity due to its two stellar groups with distinct iron-peak and neutron-capture element abundances. Previous studies attribute these abundance differences to pollution from 3-6 solar-mass asymptotic giant branch (AGB) stars, which produce significant quantities of the neutron-rich Mg isotopes 25Mg and 26Mg. We report the first-ever measurements of Mg isotopic abundance ratios at [Fe/H] approximately -2 in a globular-cluster-like system using very high-resolution and high signal-to-noise spectra (R = 110000, S/N = 300 per pixel at 514 nm) from the VLT/UVES spectrograph for six stars, three in each s-process group. Despite the presence of star-to-star variations in 24Mg, 25Mg, and 26Mg, we find no correlation with heavy-element abundances, implying that the nucleosynthetic source of s-process enrichment must not influence Mg isotope ratios. Instead, a key result of this work is that we identify correlations between 26Mg/24Mg and some light elements. Using a custom suite of AGB nucleosynthesis yields tailored to the metallicity of M 22, we find that low-mass AGB stars of approximately 1-3 solar masses are capable of reproducing the observed s-process abundances of M 22 and that the absence of any difference in Mg isotope ratios between the two s-process groups precludes AGBs with masses above approximately 3 solar masses. This places tighter constraints on possible formation scenarios and suggests an age difference of at least approximately 280-480 Myr between the two populations that is independent of isochrone fitting.
Comments16 pages, 13 figures, 5 tables. Published in MNRAS
Journal refMNRAS, 527, 7940-7955 (2024)
DOI：10.1093/mnras/stad2999 10.1093/mnras/stad2999
