An updated picture of pre-solar history from short-lived radioactive isotopes and inferences on the birth of the Sun

Abstract

We examine the origin of the short-lived radionuclides (SLRs, defined as having half-lives between 0.1 and 100 Ma) present in the early Solar System (ESS) by investigating how predictions of their abundances in the interstellar medium (ISM) from steady-state equilibrium relate to their ESS values. For this, we take into account the non-negligible time $t_{\mathrm{iso}}$ elapsed between the isolation of the pre-solar molecular cloud and the formation of the ESS, during which the SLRs decayed freely. We also consider the alternative scenario in which the pre-solar molecular cloud remained partially mixed with the ISM, with a mixing timescale $t_{\mathrm{mix}}$. We find that the ESS abundances of ${}^{107}$Pd and ${}^{182}$Hf produced by \textit{slow} neutron captures (\textit{s}-process), and of ${}^{53}$Mn and ${}^{60}$Fe produced by explosive nucleosynthesis, can be consistently explained within these scenarios. Their required $t_{\mathrm{iso}}$ is 9-12 Ma, and their required $t_{\mathrm{mix}}$ is 11-14 Ma (with one potential exception of $t_{\mathrm{mix}}$ = 38 Ma), depending on galactic uncertainties, such as the galactic star formation history and efficiency and the star-to-gas mass ratio. Another \textit{s}-process SLR, ${}^{205}$Pb has a more uncertain ESS value, and falls within only some of these time values. The same applies to the SLRs produced by the $p$-process (${}^{92}$Nb and ${}^{146}$Sm), depending on the latter's half-life. In agreement with previous studies, we find that the ESS abundances of the \textit{rapid} neutron-capture isotopes (${}^{129}$I, ${}^{244}$Pu, and ${}^{247}$Cm) and of the most short-lived radionuclides (${}^{26}$Al, ${}^{36}$Cl and ${}^{41}$Ca) cannot be explained by assuming steady-state equilibrium in the ISM.

16 pages, 3 figures, 4 tables. Accepted for publication at The Astrophysical Journal