Deciphering the Remnants of Core-Collapse Supernovae: Reconstructing Progenitor Star Properties and Explosion Mechanisms

TL;DR

The paper tackles how Cassiopeia A's complex morphology encodes both the explosion mechanism and the progenitor's mass-loss history. It uses a unified 3D modeling pipeline that evolves a $15\,M_\odot$ progenitor's neutrino-driven explosion to $\sim 1000$ yr on a $2048^3$ grid, incorporating $E_k \approx 1.5\times10^{51}$ erg, $M_{\mathrm{ej}} \approx 3.3\,M_\odot$, and $M_{\mathrm{Ni}} \approx 0.1\,M_\odot$, with Ni-bubble heating, RT instabilities, and radiative cooling. Key results reproduce the interior O-rich filament network as a fossil imprint of early explosion dynamics and the Green Monster as a product of ejecta clumps interacting with a dense, asymmetric CSM shell at $\sim 1.5$ pc, yielding holes of $\sim 5''$. This work demonstrates that young SNRs can serve as archaeological records, enabling constraints on neutrino-driven explosion physics and late-stage stellar winds, with broad implications for interpreting other remnants such as SN 1987A.

Abstract

(Abridged) Recent JWST observations of Cassiopeia A (Cas A) reveal unprecedented ejecta substructure, including a web of filaments and the enigmatic "Green Monster" (GM), characterized by nearly circular holes and rings. These features provide new constraints on supernova (SN) explosion physics and ejecta-circumstellar medium (CSM) interactions. We present high-resolution three-dimensional hydrodynamic and magnetohydrodynamic simulations of a neutrino-driven SN explosion tailored to Cas A, following the system from core collapse to an age of $\sim 1000$ yr. The models include key physical processes such as hydrodynamic instabilities, Ni-bubble effects, radiative cooling, non-equilibrium ionization, and electron-ion temperature equilibration. Our results show that the filamentary ejecta network naturally forms during the early explosion due to the interaction of neutrino-driven bubbles and instabilities, retaining a memory of the initial conditions before being progressively modified by the reverse shock. The GM morphology is reproduced by the interaction of dense ejecta clumps with an asymmetric, forward-shocked CSM shell, with radiative cooling enhancing fragmentation and generating the observed holes and rings. Overall, our study demonstrates that Cas A's complex morphology reflects both the imprint of the explosion mechanism and subsequent ejecta-CSM interactions.

Figures (4)

• Figure 1: Distribution of unshocked Ni-rich ejecta (red isosurface; $\rho_{\mathrm{Ni}} > 2 \times 10^{-8}\,\mathrm{g\,cm^{-3}}$) a few hours after shock breakout ($\approx 22$ hr after core collapse) for model W15-IIb-sh-HD+dec-hr. Unshocked O-rich ejecta are shown via volume rendering (blue palette). The upper left panel shows the front view as seen from Earth. The remaining panels show perspectives from arbitrary orientations. An interactive 3D visualization of the O and Ni distributions is available at https://skfb.ly/psXKs.
• Figure 2: Upper panels: Distribution of unshocked O-rich ejecta visualized via volume rendering (blue palette) at the age of Cas A for model W15-IIb-sh-HD+dec-hr. The color scale is shown in the bottom-right corner of each panel. Rendering opacity is proportional to the plasma density, highlighting the denser structures. The upper-left panel shows the front view as seen from Earth, while the upper-right panel shows a side view from a vantage point to the west (positive $x$-axis). Lower panels: Zoom into the central region revealing filamentary O-rich ejecta. In the lower-left panel, the O-rich distribution (blue) is shown together with the unshocked Fe-rich ejecta, displayed as a red isosurface corresponding to Fe densities above $10^{-24}\,\mathrm{g\,cm^{-3}}$. An interactive 3D visualization of the O and Fe spatial distributions at the age of Cas A is available at https://skfb.ly/psXKr.
• Figure 3: 3D visualization of the ejecta-shell interaction in the Cas A SNR from model W15-IIb-sh-MHD+dec-rl-hr. The irregular isosurface represents ejecta with mass density exceeding $10^{-23}\,\mathrm{g\,cm^{-3}}$, with colors indicating the radial velocity in units of $1000\,\mathrm{km\,s^{-1}}$ (color scale shown in the left panels). The green volume rendering depicts the mass density of the shocked shell material. The sequence spans the evolution from the moment the forward shock first encounters the shell at $t \approx 200$ years (upper-left panel) to the later remnant stage at $t \approx 1000$ years (lower-right panel). The panels on the right show the remnant–shell interaction at three representative epochs, with times indicated in the upper-left corner of each panel. The upper-right inset provides a detailed view of the shell structure, showing the progressive development of holes and ring-like features produced by the interaction with the ejecta. For clarity, magnetic field lines (color scale shown in the left panels) are shown only within a selected sub-volume, highlighting the complexity of the magnetic configuration while preserving a clear view of the ejecta structure in other regions of the remnant. Interactive 3D visualizations of the remnant-shell interaction are available at https://skfb.ly/psYpK and https://skfb.ly/pt7wt.
• Figure 4: Formation of holes and ring-like structures during the interaction between the ejecta and the shocked shell in model W15-IIb-sh-MHD+dec-rl-hr. The irregular isosurface traces the ejecta, while the green volume rendering shows the mass density of the shocked shell material, as illustrated in Fig. \ref{['fig3']}. The sequence follows the evolution from the moment the ejecta fingers first encounter the shell at $t \approx 227$ years (left) to a more advanced remnant stage at $t \approx 367$ years (right), corresponding to the age of Cas A. An interactive 3D visualization of the ejecta fingers protruding through the shell is available at https://skfb.ly/ptFHY.