(ORDO NEWS) — X-ray images show the complex dynamics of shock waves and plasma in the supernova remnant Cassiopeia A. The expansion of the plasma cloud is not uniform, and some sections of the shock wave move towards the center of the explosion, that is, in the opposite direction.
Supernova remnant Cassiopeia A is located 11,000 light-years from Earth and is one of the closest and youngest known objects of its kind.
The outburst itself in the constellation Cassiopeia could have been observed in the second half of the 17th century, but due to the powerful absorption in the dust disk of the Galaxy, it did not impress earthly observers. It was probably seen by the English astronomer Flamsteed in 1680 as a dim sixth-magnitude star at the limit of visibility with the naked eye.
Interstellar absorption makes it difficult to study many objects in the Milky Way, but with the development of multiwavelength astronomy, the remnant of this supernova has become a convenient object for study.
In other ranges of electromagnetic radiation, the absorption is weaker than in the optical one, and the resulting nebula, like many other objects in the Galaxy, can be seen and studied in all details.
In a recent study , scientists led by Jakko Vinck of the University of Amsterdam studied the supernova remnant Cassiopeia A in X-rays.
To do this, they compared and processed a series of archival images of the Chandra space observatory, taken over a period of 19 years. The original article with the results of the study was published on the preprint site arxiv.org .
It is easy to assume that plasma clouds generated by such a powerful explosion as a supernova should expand uniformly in all directions, like an inflated balloon. However, this notion turns out to be far from the truth.
Supernova remnant Cassiopeia A consists of a cloud of plasma expanding unevenly at an average speed of 5800 kilometers per second, and two concentric shock waves can be distinguished in it.
The outer one approximately coincides with the outer edge of the nebula, while the inner shock wave propagates against the direction of the plasma flow itself at an average speed of 3000 kilometers per second.
In the western section of the shock wave, this speed reaches 8000 kilometers per second and exceeds the speed of the flow itself in magnitude.
Thus, this area itself moves towards the center of the nebula at speeds up to 2000 kilometers per second, similar to how waves can rise up against the current of a river.
A careful analysis of a series of images revealed another unusual fact – the acceleration of an external wave in the same area, reaching 0.5 kilometers per second per year. Usually, shock waves from supernova explosions only slow down with time, meeting the resistance of previously ejected shells and the interstellar medium.
Scientists offer the following mechanism for the observed picture. The exploded star was born as a massive luminary with a mass of 15-25 solar masses. Shortly before the explosion, she completely shed the outer shell, in which a seal formed in the western section.
In the case of the progenitor Cassiopeia A, it is not yet known what caused this loss – it is possible that a nearby component helped to remove the shell, which was later absorbed by it or exploded.
But the shedding of the shell turned it into a Wolf-Rayet class star with a mass of only four to six solar masses, with enormous luminosity and a strong stellar wind. This wind cleared the immediate surroundings of the dropped material, and then the star exploded.
The incandescent material ejected in the supernova explosion at high speed collided with the inner edge of the previously ejected shell, and caused an explosive jump in temperature and pressure. He provoked both observed effects.
First, a second shock wave formed, propagating against the oncoming flow into the rarefied inner regions of the supernova remnant. Then the initial wave passed through the shell and, probably, was focused in the compacted section of the shell, which led to its acceleration.
It should be noted that the complex dynamics of the motion of matter in the remnants of supernova explosions is a well-known phenomenon.
It is caused by the complexity of the processes occurring at the late stages of the evolution of massive stars: both the ejection of the shells and the explosion itself are characterized by significant asymmetry. Many details of these complex and varied phenomena still remain unexplored.
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