Supernovae. The bubble-like interior of the core-collapse supernova remnant Cassiopeia A

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.

Spectropolarimetry of stripped-envelope supernovae: observations and modelling

Spectropolarimetry is one of the most powerful methods to study the multi-dimensional geometry of supernovae (SNe). We present a brief summary of the spectropolarimetric observations of stripped-envelope core-collapse SNe. Observations indicate that stripped-envelope SNe generally have a non-axisymmetric ion distribution in the ejecta. Three-dimensional clumpy geometry nicely explains the observed properties. A typical size of the clumps deduced from observations is relatively large: [Formula: see text]25% of the photosphere. Such a large-scale clumpy structure is similar to that observed in Cassiopeia A, and suggests that large-scale convection or standing accretion shock instability takes place at the onset of the explosion.This article is part of the themed issue 'Bridging the gap: from massive stars to supernovae'.