The cosmic evolution of magnesium isotopes

The abundance of magnesium in the interstellar medium is a powerful probe of star formation processes over cosmological time-scales. Magnesium has three stable isotopes, ²⁴Mg, ²⁵Mg, and ²⁶Mg, which can be produced in both massive and intermediate-mass (IM) stars with masses between 2 and 8 M. In this work, we use constraints on the cosmic star formation rate density (SFRD) and explore the role and mass range of IM stars using the observed isotopic ratios. We compare several models of stellar nucleosynthesis with metallicity-dependent yields and also consider the effect of rotation on the yields of massive stars and its consequences on the evolution of the Mg isotopes. We use a cosmic evolution model updated with new observational SFRD data and new reionization constraints coming from 2018 Planck Collaboration determinations. We find that the main contribution of ²⁴Mg comes from massive stars whereas ²⁵Mg and ²⁶Mg come from IM stars. To fit the observational data on magnesium isotopic ratios, an additional IM SFRD component is preferred. Moreover, the agreement between model and data is further improved when the range of IM masses is narrowed towards higher masses (5-8 M). While some rotation also improves the fit to data, we can exclude the case where all stars have high rotational velocities due to an overproduction of ²⁶Mg.