A special class of supernovae is distinguished by anomalous brightness because the shock waves of the explosion collide with a dense shell of gas and dust around the stellar system. New modeling has linked the formation of this cocoon to mass exchange in binary stars during the final stage of their evolution.
The Mystery of Bright Explosions
When a massive star exhausts its thermonuclear fuel, its core collapses, and the shock front blows away the outer layers. Afterward, a neutron star or black hole remains.
In the case of interacting supernovae, the blast wave additionally collides with material that had previously been ejected into the surroundings of the system. The kinetic energy of the collision is converted into radiation, making the outburst much brighter. The origin of this gas-and-dust shell had remained unclear for decades.
Roche Lobe Overflow
Most massive stars exist in binary systems bound by gravity. In the late stages of evolution, one of the partners turns into a red giant and can expand hundreds or even thousands of times compared with its original size.
If the swollen companion fills its Roche lobe — the gravitational region where its attraction dominates over that of its partner — material from its surface begins to flow through the equilibrium point between the two objects onto the neighboring star. That star captures only part of the stream, while the rest forms a gas-and-dust cocoon around both objects.
A Question of Timing
A group of researchers from the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA) conducted hundreds of computer simulations of mass transfer in binary systems, Space.com reports. It turned out that the timing of the start of this process is critical for the emergence of an interacting supernova.
If the transfer of material occurs too early, millions of years before the final collapse, the cocoon has time to disperse into the surrounding space. For the shell to remain dense enough to collide with the shock wave, the exchange must begin only a few thousand years before the supernova explosion.
The Decisive Role of the Companion
According to the simulations, it is the presence of the second object in the system that determines both the course of stellar evolution and the nature of the final explosion. The work was published on June 30 in the peer-reviewed journal The Astrophysical Journal Letters.
By comparison, a single massive star can lose a significant part of its envelope through stellar wind, but this process stretches over millions of years and does not create a dense nearby cocoon. It is the gravitational interaction with a companion that compresses the ejected material into a compact structure, which then becomes the “target” for the supernova shock front. The modeling numerically demonstrated a direct link between the timing of mass transfer in a pair of stars and the formation of an interacting supernova.
