The shocks in Cas A reveal important information on the supernova explosion and the progenitor
Supernova remnants (expanding clouds produced by the explosion of massive stars) are fascinating objects. Their study, in fact, can unveil the physical processes working during supernova explosions and even the properties of the stellar progenitors. To these aims, of particular importance is the analysis of the physical and chemical properties of the ejecta (which are the fragments of the dying star ejected during the explosion at velocities even larger than 10000 km/sec), and the morphology of the shocks within the remnants: both the forward shock produced during the explosion which is always expanding, and the shock produced by the interaction between the forward shock and the surrounding material, called reverse shock. The direction of propagation of the reverse shock, as measured by a rest observer, depends on its velocity compared with that of the expanding ejecta. In the frame of the ejecta, in fact, the reverse shock always move inward, impacting and heating the ejecta. A rest observer, instead, will observe the reverse shock moving inward if the reverse shock is moving within the ejecta faster than the velocity of the expansion of the ejecta in the remnant. This may occur, for instance, as soon as the ejecta are slowed down by the expansion of the remnant or by the interaction with the surrounding material.
Cassiopeia A is one of the most interesting supernova remnants because of the asymetric chemical and physical properties of its ejecta. As shown in a paper published in 2021 (link) by astronomers of INAF – Astronomical Observatory of Palermo, these asimmetries are intrinsic of the supernova explosion. The forward shock, instead, is almost completely homogeneous and it is expanding with a velocity of about 5500 km/sec. On the other hand, the reverse shock is strongly dishomogeneous: while it is observed to expand with a velocity of 2000-4000 km/sec in the east and north sectors of the remnant, in the other parts it is observed to be stationary or even moving inward with a velocity of about 2000 km/sec.
The team of researchers led by the astronomer S. Orlando (INAF – Astronomical Observatory of Palermo) has recently presented hydrodynamic and magnetohydrodynamic models that succed in explaining most of the properties of Cas A as a consequence of the interaction between the reverse shock in the remnant and a pre-existing shell of circumstellar material, with a mass of about 2 solar masses and a disomogeneous density. This shell has been created likely by the progenitor star during its last evolutive phases. Giant and supergiant stars, in fact, are characterized by intense mass loss episodes, mainly when they belong to interacting binary systems. In particular, the progenitor of Cas A is thought to have lost from 9 to 14 solar masses of material during its late evolution. The shell may have been produced in a violent espulsion occurred 10000-100000 years before the supernova explosion, if the star was part of a binary system, or from the wind ejected with a velocity of about 1000 km/sec during a short Wolf-Rayet phase (the Wolf-Rayet stars are evolved stars which have lost their hydrogen-rich outer envelope and show a helium-rich outer layer). The interaction between this thin (with a section of 0.07 light years) and disomogeneous shell occurred in the first 300 years of evolution of Cas A, and it produced a reflected shock that interacted with the reverse shock. The latter has been strongly affected by this interaction, mainly in those regions where the shell is more dense. This resulted in the homogeneous forward shock and disomogeneous reverse shock that we observe in Cas A today. The research is described in the paper: “Evidence for past interaction with an asymmetric circumstellar shell in the young SNR Cassiopeia A“, recently appeared on Astronomy & Astrophysics. Together with S. Orlando, the team counts other 10 astronomers, among whom F. Bocchino of INAF – Astronomical Observatory of Palermo and M. Miceli and G. Peres from the University of Palermo.
The figure (click here to visualize the entire image) shows a section of the material heated up by the shock in Cas A in different time intervals, as shown in the upper left corner of each panel. The dashed circle mark the position of the circumstellar shell (please note the increasing spatial scale). while the white cross marks the center of the explosion.
Mario Giuseppe Guarcello ( follow mguarce) ( youtube)
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