Image of supernova remnant with stripped envelope, Cassiopeia A. Photo credit: NASA / CXC / SAO
Massive stars end their lives with energetic explosions known as supernova explosions. Stripped-hulled supernovae show faint or no traces of hydrogen in their ejecta, meaning that the star lost most or all of its hydrogen-rich outer layers before exploding.
Scientists believe that these stars come mainly from binary star systems, in which one of the stars tears off the outer layers of the other star with its gravitational pull. Much searches have been carried out to find the remaining companion star after the stripped-off supernovae. In some searches the companion star was successfully identified, but there are also numerous cases where the companion star could not be found, which is a serious problem for the binary hypothesis. The most famous case is called Cassiopeia A (Cas A): a supernova remnant with the envelope peeled off, which is predicted to have an excellent companion, but whose explosive consequences were nowhere to be found.
In a recently published study by the ARC Center of Excellence for Gravitational Wave Discovery (OzGrav), researchers propose a new scenario for creating these “lonely” stars with stripped hulls.
OzGrav researcher and lead author of the study, Dr. Ryosuke Hirai explains, “In our scenario, the star with the peeled hull had a binary companion with a mass that was very similar to itself. Because the masses are similar, they have very similar lifetimes, which means that the explosion of the first star will take place when the second star is also near death. ”
Snapshots of the hydrodynamic simulation of a supernova hitting a red supergiant star. Photo credit: Dr. Ryosuke Hirai
For the last million years of their lives, massive stars are known as red supergiants, with their outer layers very puffy and unstable. So when the first supernova of the binary system hits the other massive star – while it is this puffy red supergiant – it can easily strip the outer layers and turn it into a star with a stripped hull. The stars interfere after the supernova, so that the secondary star becomes a lonely star widow and a million years later, when it explodes itself, it appears single.
OzGrav scientists performed hydrodynamic simulations of a supernova that collided with a red supergiant to investigate how much mass can be stripped off by this process. They found that when the two stars are close enough, the supernova can remove almost 90% of the “envelope” – the outer layer – from the companion star.
“This is enough for the second supernova of the binary system to become a supernova with a stripped shell, which confirms that our proposed scenario is plausible,” says Hirai. “Even if it isn’t close enough, it can still remove a large part of the outer layers, making the already unstable shell even more unstable, which can lead to other interesting phenomena such as pulsations or eruptions.”
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When the OzGrav scenario occurs, the stripped shell should float as a single-sided shell about 30 to 300 light years from the second supernova location. Recent observations have shown that there is actually a shell of material about 30-50 light years away from the famous Cas A.
Hirai adds, “This may be indirect evidence that Cas A was originally created by our scenario, which explains why it has no binary companion star. Our simulations prove that our new scenario could be one of the most promising ways to explain the origin of one of the most famous supernova remnants, Cas A. ”
The OzGrav scientists also predict that this scenario has a much wider range of possible outcomes – for example, it can produce a similar number of “partially stripped” stars. In the future it will be interesting to study what happens to these partially stripped stars and how they can be observed.
Reference: “Educational pathway for lonely supernova precursors with stripped-off shell: Effects on Cassiopeia A” by Ryosuke Hirai, Toshiki Sato, Philipp Podsiadlowski, Alejandro Vigna-Gómez and Ilya Mandel, 23 September 2020, Monthly announcements from the Royal Astronomical Society.
DOI: 10.1093 / mnras / staa2898
