The Cosmic Evolution of Magnesium Isotopes

The abundance of magnesium in the interstellar medium is a powerful probe of star formation processes over cosmological timescales. Magnesium has three stable isotopes, 24Mg, 25Mg, 26Mg, which can be produced both in massive and intermediate-mass (IM) stars with masses between 2 and 8 M_\odot. In this work, we use constraints on the cosmic star formation rate density (SFRD) and explore the role and mass range of intermediate mass 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 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 intermediate mass stars. To fit the observational data on magnesium isotopic ratios, an additional intermediate mass 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_\odot). While some rotation also improves the fit to data, we can exclude the case where all stars have high rotational velocities due to an over-production of 26Mg.
Comment: 12 pages, 17 figures