Galactic Chemical Evolution and the Abundances of Short-lived Radio- Nuclides Inherited by the Solar System from the Interstellar Medium

Introduction: In order to understand the origin of the short-lived radionuclides (SLRs) in the early solar system, it is necessary to have a clear, astrophysically based picture of galactic chemical evolution and the resulting SLR abundances in the interstellar medium (ISM). A discussion of the origin of the SLRs in the early solar system, including a detailed model of galactic chemical evolution as it relates to SLRs, is given in [1]. In [2], we present the galactic evolution model from [1]. Here we present some implications of that model for the SLRs in the early solar system. The main features of this discussion are summarized in Fig. 1, which compares the data for the early solar system to predictions of steadystate abundances in the ISM and two classes of models for the abundances that could have been inherited by the early solar system from the ISM. Steady-State Abundances: The straight solid line represents the steady-state abundances of the SLRs at a galactic age of 7.5 Ga, when the solar system formed. For times that are much longer than the mean lifetimes of the SLRs, the slope of this line is given by the mean life, !, divided by the galactic age, t. As the mean life approaches t, the steady-state abundance falls below the !/t line and the trend asymptotically approaches the effective production ratio (y = 1 on this plot). The vertical position of this line and the curves derived from it on Fig. 1 depend on our choice of t. Newly synthesized elements are injected into the interstellar medium from dying stars in discrete events. The abundances of SLRs in a particular region of the ISM depend on the mean lives of the nuclides, the mixing time for that region, and on the repeat time of injection of newly synthesized material. For longer-lived nuclides, such as 146 Sm (! = 149 Ma) and 244 Pu (! = 115 Ma), the repeat time and mixing time are short compared to the mean lives, so the abundances should be relatively homogeneous over an annulus of the galaxy. However, the SLRs with very short mean lives (e.g., 41 Ca, ! = 0.14 Ma; 36 Cl, ! = 0.43 Ma; 26 Al, ! = 1.05 Ma), mixing and repeat times are long compared to the mean lives and abundances probably vary widely around the steady-state abundances. Nuclides with intermediate lifetimes will have intermediate behavior. Free-Decay Interval: Beginning with the discovery that 129 I was present in the early solar system [3], cosmochemists have used the concept of a free-decay interval between the last input of nucleosynthetic material and the formation of the solar system. The curved solid lines in Fig. 1 show how free-decay intervals of 50 Ma and 100 Ma affect abundances. A free-decay interval of ~100 Ma provides a reasonable match to the 129 I/ 127 I in the early solar system [4], but the presence of 26 Al in the early solar system is inconsistent with such a long period of free decay. A late addition of newly synthesized material is required [e.g., 5]. However, a free-decay model does not describe the real astrophysical situation.