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.

Figures (3)

• Figure 1: Comparison between models and ESS data performed according to the method described in Sec. \ref{['sec:method']}. The red line represents the abundance ratios of a radioactive isotope relative to stable or long-living isotopes ($Z_R/Z_{\mathrm{ref}}$) divided by the corresponding stellar production ratios ($P_R/P_{\mathrm{ref}}$) as predicted by the steady-state formula (Eq. \ref{['eq:eqSS']}) with a Galactic age of 8.5 Ga, as a function of the mean life. The three panels show the values calculated for the three different values of $K$, indicated at the top of each subplot. The shaded red area represents 1$\sigma$ uncertainties from the Monte Carlo approach of Cote2019b, considering $\delta=1$ Ma. The blue lines represent the results of applying a free-decay time with values indicated in the legend. The downward triangles correspond to the same as the lines above but for $^{247}$Cm and $^{244}$Pu, as these SLRs require a different steady-state equation (see Sec. \ref{['sec:methodsteady']}). The black circles represent the same ratios as above, except that in this case, $Z_R/Z_{\mathrm{ref}}$ is the ESS value for each SLR as indicated by the corresponding label (for their values see Table \ref{['table:big']}). For $^{53}$Mn and $^{60}$Fe, the black and red circles represent, respectively, the GCE results using N13 or R yields. For $^{53}$Mn, the corresponding empty circles represent the GCE models with no $^{53}$Mn ejected by Type Ia SNe.
• Figure 2: Same as Fig. \ref{['fig:fig1']} but with $\tau$ extended down to 0.1 Ma to include the shortest-lived SLRs. Vertical lines represent these three SLRs because we used a range of $P_R/P_{ref}$ ratios (see Section \ref{['sec:methodinput']}) rather than one value only. The red shaded area is not included below $\tau=1$ Ma because the steady-state approximation is invalid below this value Cote2019b.
• Figure 3: Same as Fig. \ref{['fig:fig1']} (and Fig. \ref{['fig:fig1zoomout']} in the bottom right panel), but with the blue lines representing the predictions from the mixing scenario of Clayton1983, described in Sec. \ref{['sec:methodsteady']} for different values of the mixing timescale, $t_{\mathrm{mix}}$.