Magnesium isotopic detection in cool stars: Tracing nucleosynthetic signatures from MgH features

University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand

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Received: 20 October 2025
Accepted: 14 February 2026

Abstract

Context. Magnesium isotopic ratios offer valuable insights into stellar nucleosynthesis and Galactic chemical evolution, particularly in distinguishing between contributions from supernovae and asymptotic giant branch (AGB) stars. These isotopes are accessible through MgH molecular features in cool stellar atmospheres, yet their measurement remains challenging across a range of spectral types.

Aims. We aim to assess the reliability of MgH spectral regions for extracting magnesium isotopic ratios (²⁴Mg, ²⁵Mg, and ²⁶Mg) in stars spanning spectral types from M to G, and to evaluate the consistency of these measurements with nucleosynthetic expectations.

Methods. We applied an analysis pipeline using spectrum synthesis to derive isotopic ratios, validated using three well-studied reference stars, to a sample of five additional dwarf and giant stars. Individual MgH molecular band regions were analysed to determine their sensitivity to isotopic variation. Europium (Eu) and barium (Ba) abundances were also measured to explore potential correlations with magnesium isotopic ratios as r and s process proxies, respectively.

Results. There are ten wavelength regions for which MgH have previously been investigated. Our study determined that seven of these regions were the most reliable for extracting isotopic information. Other regions exhibited limited sensitivity between stellar type and parameters. The Mg isotope ratios (²⁴Mg:²⁵Mg:²⁶Mg) obtained in this work 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 europium (Eu) abundances with the three magnesium isotopes revealed strong correlations. The strongest correlation was with ²⁴Mg. Note that ²⁴Mg is predominantly produced by hydrostatic α capture in massive stars, a process that precedes the r process responsible for Eu production. In contrast, barium (Ba) abundances showed no significant correlation with ²⁵Mg and ²⁶Mg, despite their shared AGB origin. With removal of the effect of metallicity similar results were found.

Conclusions. Our results demonstrate that selected MgH regions can reliably measure magnesium isotopes in cool stars, providing a reproducible framework for future studies of stellar nucleosynthesis and galactic chemical evolution.