Short-lived nuclei in the early solar system: A low mass stellar source?

We discuss possible stellar origins of short-lived radioactive nuclei with meanlife (tau) over bar less than or equal to 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 less than or equal to (tau) over bar less than or equal to 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 Mn-53 might be explained by this same scenario. Then we consider the nuclei Pd-107, Al-26, Ca-41 and Fe-60, whose early solar system abundances are too high to have originated in this way. Present evidence favours a stellar origin, particularly for Pd-107, Al-26 and Fe-60, 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 less than or equal to 3M(.)) and with low neutron exposure: this model can provide a solution for Al-26, Ca-41 and Pd-107, with important contributions to 60Fe, which are inside the present uncertainty range of the Fe-60 early solar system abundance. Such a model requires that Al-26 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 Be-10, which must reflect production by later proton bombardment at a low level during early solar history.