Magnesium Isotopic Detection in Cool Stars: Tracing Nucleosynthetic Signatures from MgH Features

Abstract

Magnesium (Mg) isotopic ratios offer valuable insights into stellar nucleosynthesis and Galactic chemical evolution, particularly in distinguishing contributions from supernovae and asymptotic giant branch (AGB) stars. These isotopes are accessible through MgH molecular features in cool stars, yet their measurement remains challenging across spectral types. We assess the reliability of MgH regions for extracting Mg isotopic ratios (${}^{24}$Mg, ${}^{25}$Mg, ${}^{26}$Mg) in stars from M to G types, and evaluate consistency with nucleosynthetic expectations. Using spectrum synthesis, we applied an analysis pipeline, validated by three well-studied reference stars, to a sample of five additional stars. Individual MgH regions were analysed for their sensitivity to isotopic variation. Europium (Eu) and barium (Ba) abundances were also measured to explore potential correlations with Mg isotopic ratios as $r$ and $s$~process proxies. Of ten previously studied MgH regions, we identify seven as most reliable for isotopic analysis; others showed limited sensitivity across stellar types. Derived Mg isotope ratios (${}^{24}$Mg:${}^{25}$Mg:${}^{26}$Mg) include: HD 11695-81:7:12; HD 18884-81:7:12; HD 18907-69:9:23; HD 22049-71:16:13; HD 23249-66:13:22; HD 128621-67:17:16; HD 10700-78:10:12; and HD 100407-65:10:25. Comparison of Eu abundances with Mg isotopes reveals strong correlations, particularly with ${}^{24}$Mg which is mostly produced by hydrostatic $\alpha$ capture in massive stars, a process that precedes the $r$ process responsible for Eu. In contrast, Ba shows no correlation with ${}^{25}$Mg and ${}^{26}$Mg, despite their shared AGB origin. After removing metallicity effects, results were similar. Our results demonstrate that selected MgH regions can reliably measure Mg isotopes in cool stars, providing a reproducible framework for future studies of nucleosynthesis and galactic chemical evolution.

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