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

TL;DR

This study analyzes neutron-capture element abundances in 62 stars of globular cluster M15, revealing diverse enrichment patterns primarily from the r-process, with implications for understanding nucleosynthesis in early cluster formation.

Contribution

It provides detailed neutron-capture element abundances for 62 M15 stars, identifying their enrichment patterns and suggesting a common r-process origin for Eu-enhanced stars.

Findings

44 stars are highly Eu-enhanced r-II stars

Majority of stars show r-process enrichment patterns

Significant star-to-star abundance spreads in neutron-capture elements

Abstract

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 (M2FS) spectrograph, covering a wavelength range from ~4430-4630 A. 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.