2026/08/02 by S. Orlando, H. -T. Janka, D. Milisavljevic +6
Physics and Astronomy · #astro-ph.HE
The manuscript consists of 17 pages, 11 figures, and 2 Appendices
arxiv created 2026/08/02 · arxiv updated 2026/08/04
JWST observations of Cassiopeia A have revealed the "Green Monster" (GM), a pockmarked region of shocked circumstellar medium (CSM) characterized by circular holes surrounded by bright rings. The origin of these structures remains debated, with proposed mechanisms including post-shock sculpting by ejecta fingers and pre-shock puncturing by fast-moving knots (FMKs). We investigate the physical viability of the FMK-driven scenario using three-dimensional hydrodynamic simulations of FMKs interacting with a dense circumstellar shell, followed by the passage of the supernova remnant forward shock. By exploring a range of knot properties and shell densities, we compare the resulting hole-ring systems with JWST observations. Primary FMKs reproduce the qualitative morphology of the GM but generally produce hole-ring systems larger than the observed 1′′-3′′ (∼0.016-0.048 pc) structures unless the knots are relatively slow (\lesssim8000 km s-1) and the shell is dense (n\rm sh\gtrsim200 cm-3). Moreover, the resulting structures are short-lived, becoming significantly distorted within 30-60 yr after the forward-shock passage. The GM's diverse hole-ring systems and complex kinematics cannot be attributed to a single idealized scenario, but can be instead explained by the simultaneous action of three mechanisms operating within a structured, heterogeneous CSM. While large cavity relics are carved by early-stage primary FMKs, compact pristine rings are produced by recent "first contact" punctures of secondary knots from fragmented ejecta fingers, and older structures are continuously sculpted by long-term post-shock interactions with large-scale ejecta fingers.