High-entropy ejecta plumes in Cassiopeia A from neutrino-driven convection

Recent multi-dimensional simulations suggest that high-entropy buoyant plumes help massive stars to explode^1,2. Outwardly protruding iron (Fe)-rich fingers of gas in the galactic supernova remnant^3,4 Cassiopeia A seem to match this picture. Detecting the signatures of specific elements synthesized in the high-entropy nuclear burning regime (that is, α-rich freeze out) would constitute strong substantiating evidence. Here we report observations of such elements-stable titanium (Ti) and chromium (Cr)-at a confidence level greater than 5 standard deviations in the shocked high-velocity Fe-rich ejecta of Cassiopeia A. We found that the observed Ti/Fe and Cr/Fe mass ratios require α-rich freeze out, providing evidence of the existence of the high-entropy ejecta plumes that boosted the shock wave at explosion. The metal composition of the plumes agrees well with predictions for strongly neutrino-processed proton-rich ejecta^2,5,6. These results support the operation of the convective supernova engine via neutrino heating in the supernova that produced Cassiopeia A.