Short-Lived Nuclei in the Early Solar System: A Low Mass Stellar Source?

Abstract

We discuss possible stellar origins of short-lived radioactive nuclei with meanlife τ ≤ 100 Myr, which were shown to be alive in the Early Solar System (ESS). We first review current ideas on the production of nuclides having 10 ≤ τ ≤ 100 Myr, which presumably derive from the continuous interplay of galactic astration, nucleosynthesis from massive supernovae and free decay in the interstellar medium. The abundance of the shorter lived ⁵³Mn might be explained by this same scenario. Then we consider the nuclei ¹⁰⁷Pd, ²⁶Al, ⁴¹Ca and ⁶⁰Fe, whose early solar system abundances are too high to have originated in this way. Present evidence favours a stellar origin, particularly for ¹⁰⁷Pd, ²⁶Al and ⁶⁰Fe, rather than an in situ production by energetic solar particles. The idea of an encounter (rather close in time and space) between the forming Sun and a dying star is therefore discussed: this star may or may not have also triggered the solar formation. Recent nucleosynthesis calculations for the yields of the relevant short-lived isotopes and of their stable reference nuclei are discussed. Massive stars evolving to type II supernovae (either leaving a neutron star or a black hole as a remnant) seem incapable of explaining the four most critical ESS radioactivities in their observed abundance ratios. An asymptotic giant branch (AGB) star seems to be a viable source, especially if of relatively low initial mass (M ≤ 3 M_o) and with low neutron exposure: this model can provide a solution for ²⁶Al, ⁴¹Ca and ¹⁰⁷Pd, with important contributions to ⁶⁰Fe, which are inside the present uncertainty range of the ⁶⁰Fe early solar system abundance. Such a model requires that ²⁶Al is produced substantially on the AGB by cool bottom processing. The remaining inventory of short-lived species in the solar nebula would then be attributed to the continuous galactic processing, with the exception of ¹⁰Be, which must reflect production by later proton bombardment at a low level during early solar history.