Injection of Radioactive Nuclides from the Stellar Source that Triggered the Collapse of the Presolar Nebula

Abstract

We examine the gravitational capture of supersonic gas and dust as it impacts and triggers the collapse of a molecular cloud core. We use two techniques to track the triggering material in two dimensions, a set of tracer particles and a color field, much like a dye, computed in the same manner as the hydrodynamic density variable. The two tracking techniques produce very similar results. We find that about 10% to 20% of the supersonic material with an initial impact parameter less than the molecular cloud core's initial radius is captured by the collapsing cloud. This fraction is less than the 100% capture estimate often used to constrain the distance to possible stellar sources of radioactive isotopes, and hence may require these stars to be closer than would otherwise be the case. Rayleigh-Taylor instabilities occur and aid in the mixing of the shock material with the target cloud. The impacting material is injected into the outer layers of the collapsing protostar roughly one free-fall time (2 × 105 yr) after the first contact of the triggering material with the cloud, and injection continues for approximately two more free-fall times. These time intervals are substantially less than the mean life of one of the radioactive nuclides of interest, 1.1 × 106 yr for 26Al, and are comparable to the mean life (1.5 × 105 yr) of another short-lived nuclide,41Ca. Evidence for live 26Al and 41Ca in the early solar system is thus consistent with a scenario involving supersonic triggering and injection of freshly synthesized radioactive nuclides into the presolar cloud. Because injection proceeds at a steady pace, it does not appear to be a significant source of temporal heterogeneity in the distribution of 26Al, though the outer layers of the presolar cloud are preferentially enriched in the injected material.