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. Outwardly protruding iron-rich fingers in the galactic supernova remnant Cassiopeia A are uniquely suggestive of this picture. Detecting signatures of specific elements synthesized in the high-entropy nuclear burning regime (i.e., $\alpha$-rich freeze out) would be among the strongest substantiating evidence. Here we report the discovery of such elements, stable Ti and Cr, at a confidence level greater than 5$\sigma$ in the shocked high-velocity iron-rich ejecta of Cassiopeia A. We found the observed Ti/Fe and Cr/Fe mass ratios require $\alpha$-rich freeze out, providing the first observational demonstration for the existence of 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. These results support the operation of the convective supernova engine via neutrino heating in the supernova that produced Cassiopeia A.