Interaction of Supernova Ejecta with Nearby Protoplanetary Disks

The early solar system contained short-lived radionuclides such as ⁶⁰Fe (t_1/2 = 1.5 Myr) whose most likely source was a nearby supernova. Previous models of solar system formation considered a supernova shock that triggered the collapse of the Sun's nascent molecular cloud. We advocate an alternative hypothesis, that the solar system's protoplanetary disk had already formed when a very close (<1 pc) supernova injected radioactive material directly into the disk. We conduct the first numerical simulations designed to answer two questions related to this hypothesis: Will the disk be destroyed by such a close supernova, and will any of the ejecta be mixed into the disk? Our simulations demonstrate that the disk does not absorb enough momentum from the shock to escape the protostar to which it is bound. Only low amounts (<1%) of mass loss occur, due to stripping by Kelvin-Helmholtz instabilities across the top of the disk, which also mix into the disk about 1% of the intercepted ejecta. These low efficiencies of destruction and injection are due to the fact that the high disk pressures prevent the ejecta from penetrating far into the disk before stalling. Injection of gas-phase ejecta is too inefficient to be consistent with the abundances of radionuclides inferred from meteorites. On the other hand, the radionuclides found in meteorites would have condensed into dust grains in the supernova ejecta, and we argue that such grains will be injected directly into the disk with nearly 100% efficiency. The meteoritic abundances of the short-lived radionuclides such as ⁶⁰Fe therefore are consistent with injection of grains condensed from the ejecta of a nearby (<1 pc) supernova, into an already formed protoplanetary disk.