Stellar Pollution Mystery Deepens: Magnesium Isotopes Challenge Ancient Star Theory
Astronomers studying an ancient star cluster found that magnesium isotope ratios don't track with heavy element enrichment—contradicting a leading model for how old stars chemically evolve. The discovery forces a rethink of nucleosynthesis models used to understand stellar evolution and element formation, with implications for astrophysics research programs and computational modeling validation.
Originaltitel: The complex stellar system M 22: constraining the chemical enrichment from AGB stars using magnesium isotope ratios
<p>The complex star cluster M 22 (NGC 6656) provides a unique opportunity for studying the slow neutron capture (s-)process nucleosynthesis at low metallicity due to its two stellar groups with distinct iron-peak and neutron capture element abundances. Previous studies attribute these abundance differences to pollution from 3 − 6 M<img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Codot" data-classname="equation" data-title="" /> asymptotic giant branch (AGB) stars which produce significant quantities of the neutron-rich Mg isotopes <sup>25</sup>Mg and <sup>26</sup>Mg. We report the first-ever measurements of Mg isotopic abundance ratios at [Fe/H] ∼ −2 in a globular cluster-like system using very high-resolution and signal-to-noise spectra (R = 110 000, S/N = 300 per pixel at 514 nm) from the VLT/UVES spectrograph for six stars; three in each s-process group. Despite the presence of star-to-star variations in <sup>24</sup>Mg, <sup>25</sup>Mg, and <sup>26</sup>Mg, we find no correlation with heavy element abundances, implying that the nucleosynthetic source of s-process enrichment must not influence Mg isotope ratios. Instead, a key result of this work is that we identify correlations between <sup>26</sup>Mg/<sup>24</sup>Mg and some light elements. Using a custom suite of AGB nucleosynthesis yields tailored to the metallicity of M 22, we find that low mass (∼ 1−3 M<img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Codot" data-classname="equation" data-title="" />) AGB stars are capable of reproducing the observed s-process abundances of M 22 and that the absence of any difference in Mg isotope ratios between the two s-process groups precludes AGBs with masses above ∼ 3 M<img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Codot" data-classname="equation" data-title="" />. This places tighter constraints on possible formation scenarios and suggests an age difference of at least ∼ 280−480 Myr between the two populations which is independent of isochrone fitting.</p>