On the Origin of Early Solar System Radioactivities: Problems with the Asymptotic Giant Branch and Massive Star Scenarios

Recent improvements in stellar models for intermediate-mass stars and massive stars (MSs) are recalled, together with their expectations for the synthesis of radioactive nuclei of lifetimes τ lesssim 25 Myr, in order to re-examine the origins of now extinct radioactivities that were alive in the solar nebula. The Galactic inheritance broadly explains most of them, especially if r-process nuclei are produced by neutron star merging, according to recent models. Instead, 26Al, 41Ca, 135Cs, and possibly 60Fe require nucleosynthetic events close to the solar formation. We outline the persisting difficulties to account for these nuclei by intermediate-mass stars (2 lesssim M/M ⊙ lesssim 7–8). Models of their final stages now predict the ubiquitous formation of a 13C reservoir as a neutron capture source; hence, even in the presence of 26Al production from deep mixing or hot bottom burning, the ratio 26Al/107Pd remains incompatible with measured data, with a large excess in 107Pd. This is shown for two recent approaches to deep mixing. Even a late contamination by an MS encounters problems. In fact, the inhomogeneous addition of supernova debris predicts nonmeasured excesses on stable isotopes. Revisions invoking specific low-mass supernovae and/or the sequential contamination of the presolar molecular cloud might be affected by similar problems, although our conclusions here are weakened by our schematic approach to the addition of SN ejecta. The limited parameter space that remains to be explored for solving this puzzle is discussed