Galactic Chemical Evolution of Radioactive Isotopes

Benoit Côté, Maria Lugaro, Rene Reifarth, Marco Pignatari, Blanka Világos, Andrés Yagüe, and Brad K. Gibson

The Astrophysical Journal, Volume 878, Number 2


“The presence of short-lived (~Myr) radioactive isotopes in meteoritic inclusions at the time of their formation represents a unique opportunity to study the circumstances that led to the formation of the solar system. To interpret these observations, we need to calculate the evolution of radioactive-to-stable isotopic ratios in the Galaxy. We present an extension of the open-source galactic chemical evolution codes NuPyCEE and JINAPyCEE that enable the decay of radioactive isotopes in the interstellar medium to be tracked. We show how the evolution of the isotopic ratio depends on the star formation history and efficiency, star-to-gas mass ratio, and galactic outflows. Given the uncertainties in the observations used to calibrate our model, our predictions for isotopic ratios at the time of formation of the Sun are uncertain by a factor of 3.6. At that time, to recover the actual radioactive-to-stable isotopic ratios predicted by our model, one can multiply the steady-state solution (see Equation (1)) by ${2.3}_{-0.7}^{+3.4}$. However, in the cases where the radioactive isotope has a half-life longer than ~200 Myr, or the target radioactive or stable isotopes have mass- and/or metallicity-dependent production rates, or they originate from different sources with different delay-time distributions, or the reference isotope is radioactive, our codes should be used for more accurate solutions. Our preliminary calculations confirm the dichotomy between radioactive nuclei in the early solar system with r- and s-process origin, and that 55Mn and 60Fe can be explained by galactic chemical evolution, while 26Al cannot.”