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

TL;DR

This work develops a robust MgH-based isotopic analysis for cool stars (from M to G types) using a MOOG-based pipeline to derive $^{24}$Mg, $^{25}$Mg, and $^{26}$Mg. Seven MgH regions are identified as reliable for isotopic extraction, with careful treatment of line blends, continuum placement, and uncertainties via a Hessian-based method. The results show $^{24}$Mg dominates, with meaningful but smaller $^{25}$Mg and $^{26}$Mg fractions, and reveal strong correlations with Eu (an r-process proxy) but weak or absent correlations with Ba (an s-process proxy), consistent with nucleosynthetic expectations and Galactic chemical evolution. The methodology is validated against Gaia Benchmark stars and can be applied to broader stellar populations, providing a reproducible framework for probing stellar nucleosynthesis and chemical evolution in the Galaxy.

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 via MgH molecular features in cool stellar atmospheres, though their measurement remains challenging across spectral types. We assess the reliability of MgH spectral regions for extracting magnesium 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 dwarf and giant stars. Individual MgH band regions were analysed for sensitivity to isotopic variation. Europium (Eu) and barium (Ba) abundances were also measured to explore correlations with Mg isotopic ratios as r- and s-process proxies. Of ten previously studied MgH wavelength 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; HD 100407-65:10:25. Comparison of Eu abundances with Mg isotopes reveals strong correlations, particularly with ($^{24}$Mg, which is predominantly produced by hydrostatic $α$-capture in massive stars, a process preceding the r-process responsible for Eu. In contrast, Ba shows no significant correlation with $^{25}$Mg or $^{26}$Mg, despite their shared s-process origin. Our results demonstrate that selected MgH regions can reliably measure Mg isotopes in cool stars, providing a reproducible framework for future studies of stellar nucleosynthesis and Galactic chemical evolution.

Figures (6)

• Figure 1: Kiel Diagram showing the distribution of ISAM stars in temperature space (circled), coloured by metallicity. The background points are the stars from GBS V3 soubiran_gaia_2024 to show the main sequence more clearly.
• Figure 2: Individual and combined contributions of Mg isotopes to Region 1 of HD 18907. The top left panel shows the synthetic spectral feature from $^{24}$Mg only (no contribution from $^{25}$Mg or $^{26}$Mg). The top right panel shows the feature from $^{25}$Mg only, and the bottom left panel shows the $^{26}$Mg contribution. The bottom right panel with the fitting region in the blue box, presents the combined synthetic spectrum including all three isotopes, providing the best fit solution for this region.
• Figure 3: Regions 1 through 10 showing the MgH lines and the fitting region (blue shaded rectangle) for all stars colour coded by effective temperature. Vertical lines mark the positions of the three magnesium isotopes: dot–dash for $^{24}\mathrm{Mg}$, dashed for $^{25}\mathrm{Mg}$, and solid for $^{26}\mathrm{Mg}$.
• Figure 4: Mean isotopic percentage as a function of region wavelength for $^{24}$Mg (red), $^{25}$Mg (blue), and $^{26}$Mg (green). Error bars indicate the standard deviation of the ISAM sample for each region.
• Figure 5: Comparison of line by line abundances of europium and barium with the isotopic magnesium abundances, [${^{24}\mathrm{Mg}}$/H], [${^{25}\mathrm{Mg}}$/H], and [${^{26}\mathrm{Mg}}$/H].