Scientists study the interior of Cassiopeia supernova remnant

On January 30, 2015 Science published a paper called “The bubble-like interior of the core-collapse supernova remnant Cassiopeia A” about the details of the inner structure and explosion debris of a supernova remnant. We have asked one of the authors of this research, Dr. Dan Milisavljevic from the Harvard-Smithsonian Center for Astrophysics, to comment on this work.

Supernova’s interior

Supernovae are the violent deaths of massive stars. They are believed to involve aspheric explosions initiated by the collapse of an iron core, but the specifics of how this implosion-to-explosion process actually takes place are not well known.

We made a three-dimensional reconstruction of the supernova remnant Cassiopeia A (Cas A), which is the remains of a Galactic supernova that has been expanding for approximately 340 years, to investigate the kinematics of the debris that had been shaped by the explosion.

We obtained spectroscopic observations at near-infrared wavelengths that are less affected by extinction due to dust. Previous observations at longer wavelengths had already found interior emission, but these data gave the impression that the associated material was scattered in random filaments. Our observations, however, which were designed to cover the entire remnant, demonstrated that these earlier works did not go far enough. Indeed, the faint and diffuse emitting material in fact has large-scale coherent structure in the form of cavities or “bubbles” that smoothly connect with the rings of debris previously seen in the bright reverse-shocked material.

The large-scale structure we found tells us important information about how the original supernova explosion proceeded. We believe that the Swiss cheese-like distribution was frozen into the remnant by turbulent mixing processes that encouraged the development of outwardly expanding plumes of radioactive ⁵⁶Ni‑rich ejecta in the first few weeks following core collapse. This ⁵⁶Ni decays to ⁵⁶Co and emits high energy photons that produce a pressure that expands the Ni-rich blobs into the surrounding gas and carve out cavities or “bubbles”. Cas A shares many properties with other supernovae and supernova remnants, so this process is likely a common property of stellar explosions.

Asymmetry in supernova explosions

It has long been believed that massive stars have layered interiors at the time of supernova explosion, and that the explosion will retain this layering – at least roughly. The heaviest elements (like iron) are closest to the core and the lightest elements (like hydrogen) are near the surface. However, in 1987 a spectacular event happened: SN 1987A, the nearest supernova in hundreds of years, was discovered in the Large Magellanic Cloud. Surprisingly, high-energy emission from SN 1987A at gamma‑ray and x‑ray wavelengths was detected before theorists had predicted. This lead to the conclusion that radioactive Ni (the origin of the high-energy emission) had somehow reached the surface faster than anticipated. The only way that this could have happened is if the explosion was NOT spherical and that some asymmetric turbulent process allowed the Ni to speed ahead of lighter elements.

The plaguing issue is to explain how the Ni was turbulently mixed in the first place. The origin of the asymmetry – specifically HOW and WHERE it arises – is not well constrained. There are various mechanisms associated with dynamical instabilities and the influences of rotation and magnetic fields that are capable of breaking symmetry, but it is difficult to constrain their relative contributions.

Future Direction

Our observations of Cas A’s inner ejecta can help unravel which of the competing processes that can introduce asymmetry in core-collapse explosions may be the most significant. The Cas A reconstruction is a map of what the end state of the explosion looks like – both its kinematic and chemical distribution. Now using sophisticated simulations on supercomputers we can attempt to trace back how this arrangement came to be.

One of the major predictions of our paper is that iron should be present inside each of the cavities. So far, we see iron in some cavities but not all of them. People have looked hard with the Hubble Space Telescope (optical), the Spitzer Space Telescope (infra-red) and the Chandra X-ray Observatory (X-ray) but it hasn’t been seen. Unfortunately, the iron may be emitting at wavelengths that we are not sensitive enough to using present observational facilities. This situation may change in the not-too-distant future with deployment of the James Webb Space Telescope. I hope to be one of the first people to see those images – they will not only be beautiful but potentially reveal the “missing” iron we think must exist inside Cas A and represent another major step forward in our understanding of how massive stars explode.