The evolution of isotope ratios in the Milky Way Galaxy

Abstract

Isotope ratios have opened a new window into the study of the details of stellar evolution, supernovae and galactic chemical evolution. We present the evolution of the isotope ratios of elemental abundances (from C to Zn) in the solar neighbourhood, bulge, halo and thick disc, using chemical evolution models with updated yields of asymptotic giant branch (AGB) stars and core-collapse supernovae. The evolutionary history of each element is different owing to the effects of the initial progenitor mass and metallicity on element production. In the bulge and thick disc the star formation time-scale is shorter than in the solar neighbourhood, leading to higher [α/Fe] ratios. Likewise, the smaller contribution from Type Ia supernovae in these regions leads to lower [Mn/Fe] ratios. Also in the bulge, the abundances of [(Na, Al, P, Cl, K, Sc, Cu, Zn)/Fe] are higher because of the effect of metallicity on element production from core-collapse supernovae. According to our predictions, it is possible to find metal-rich stars ([Fe/H] ≳−1) that formed in the early Universe as a result of rapid star formation. The chemical enrichment time-scale of the halo is longer than in the solar neighbourhood, and consequently the ratios of [(C, F)/Fe] and 12C/13C are higher owing to a significant contribution from low-mass AGB stars. While the [α/Fe] and [Mn/Fe] ratios are the same as in the solar neighbourhood, the [(Na, Al, P, Cl, K, Sc, Cu, Zn)/Fe] ratios are predicted to be lower. Furthermore, we predict that isotope ratios such as 24Mg/25, 26Mg are larger because of the contribution from low-metallicity supernovae. Using isotopic ratios, it is possible to select stars that formed in a system with a low chemical enrichment efficiency such as the satellite galaxies that were accreted on to our own Milky Way Galaxy.