Abundances of Neutron-capture Elements in 62 Stars in the Globular Cluster Messier 15

M15 is a globular cluster with a known spread in neutron-capture elements. This paper presents abundances of neutron-capture elements for 62 stars in M15. Spectra were obtained with the Michigan/Magellan Fiber System spectrograph, covering a wavelength range from ∼4430 to 4630 Å. Spectral lines from Fe i, Fe ii, Sr i, Zr ii, Ba ii, La ii, Ce ii, Nd ii, Sm ii, Eu ii, and Dy ii were measured, enabling classifications and neutron-capture abundance patterns for the stars. Of the 62 targets, 44 are found to be highly Eu-enhanced r-II stars, another 17 are moderately Eu-enhanced r-I stars, and one star is found to have an s-process signature. The neutron-capture patterns indicate that the majority of the stars are consistent with enrichment by the r-process. The 62 target stars are found to show significant star-to-star spreads in Sr, Zr, Ba, La, Ce, Nd, Sm, Eu, and Dy, but no significant spread in Fe. The neutron-capture abundances are further found to have slight correlations with sodium abundances from the literature, unlike what has been previously found; follow-up studies are needed to verify this result. The findings in this paper suggest that the Eu-enhanced stars in M15 were enhanced by the same process, that the nucleosynthetic source of this Eu pollution was the r-process, and that the r-process source occurred as the first generation of cluster stars was forming.

M15是一个在中子俘获元素方面存在已知分布的球状星团。本文给出了M15中62颗恒星的中子俘获元素丰度。光谱是用密歇根/麦哲伦光纤系统光谱仪获得的，覆盖了从约4430到4630埃的波长范围。测量了来自铁Ⅰ、铁Ⅱ、锶Ⅰ、锆Ⅱ、钡Ⅱ、镧Ⅱ、铈Ⅱ、钕Ⅱ、钐Ⅱ、铕Ⅱ和镝Ⅱ的谱线，从而能够对恒星进行分类并确定其中子俘获丰度模式。在62个目标中，发现44颗是铕高度增丰的r - II型恒星，另外17颗是铕中度增丰的r - I型恒星，还有1颗恒星具有s - 过程特征。中子俘获模式表明，大多数恒星与r - 过程的增丰相符。发现62颗目标恒星在锶、锆、钡、镧、铈、钕、钐、铕和镝方面存在显著的星间差异，但在铁方面没有显著差异。还发现中子俘获丰度与文献中的钠丰度有轻微的相关性，这与之前的发现不同；需要后续研究来验证这一结果。本文的研究结果表明，M15中铕增丰的恒星是由相同的过程增丰的，这种铕污染的核合成来源是r - 过程，并且r - 过程源在第一代星团恒星形成时就出现了。