Look! A lopsided supernova may be hiding a mysterious void

Cassiopeia A isn't behaving quite like it should.

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An unseen obstacle in the cloud of gas around a supernova remnant 11,000 light years away is making the supernova’s blast waves lopsided, according to two recent studies.

Two teams of astrophysicists recently examined 19 years’ worth of data from the Chandra X-ray telescope on Cassiopeia A.

One, led by Jacco Vink of the University of Amsterdam, realized that on one side of the nebula slowly spreading out from the dead star, blast waves from the supernova seem to have bumped into something in space, which changed their speed and direction. That paper has been accepted by the Astrophysical Journal.

The other involves a team led by Salvatore Orlando, of Italy’s National Institute of Astrophysics, created computer simulations of the supernova and suggested a possible suspect in the case of the lopsided blast waves. That paper has been submitted to Astronomy & Astrophysics and is awaiting peer review.

According to both groups, the behavior of the blast waves from supernova Cassiopeia A could help physicists reconstruct important details about the life and death of the star that produced it.

The arrows represent where certain areas of Cassiopeia A are moving outward as well as where it’s not quite behaving as it should.


What’s new — In 19 years’ worth of images from the Chandra X-ray Observatory, Vink and his colleagues noticed that one side of the nebula Cassiopeia A looked a bit wonky. The astrophysicists expected to see shock waves from the star’s explosive death rippling outward in two concentric rings through the surrounding cloud of gas. Instead, on one side of the nebula, the blast waves looked lopsided, as if they’d run into something that disrupted their smooth passage through the nebula.

Strangely enough, two most likely cosmic obstacles disrupting the wave are opposites. Either the shock waves ran into a big cosmic clump of something, most likely gas blasted out by the star in the last few hundred thousand years of its life, or they passed through a big cosmic clump of nothing, a void carved out of the gas cloud by the hot wind of a dying star. Either answer could tell us something interesting about the star and its fiery demise.

Orlando and his colleagues simulated the life and death of a star like the one believed to have produced the slowly spreading debris cloud of Cassiopeia A. In their models, the cosmic obstacle that produced lopsided shock waves like the ones Chandra observed was a shell of gas particles, which the dying star had coughed outward into space on a blast of stellar wind about 10,000 to 100,000 years before the supernova. Because the shock waves from the blast travel so much faster than expanding clouds of gas, the waves caught up with the ejected gas by about 180 to 240 years after the supernova.

"So, the shell encounter is now my favorite," Vink tells Inverse, because it explains details of the shock waves' behavior (more on that below).

But he acknowledges another possibility: if Cassiopeia A had briefly evolved into what's called a Wolf-Rayet star (a type of star that's rapidly shedding mass toward the end of its life), its very strong stellar wind could have carved a hole in the surrounding gas cloud. If the blast wave from the supernova passed through a void carved by Wolf-Rayet stellar wind, then encountered a wall of denser material on the other side, it could behave in exactly the way Chandra observed.

"To some extent, the Orlando model and the Wolf-Rayet have some similarities, and both are worth investigating further as it may reveal more information about what type of star created the supernova," Vink says.

A close breakdown of what’s happening at this strange star.

Castro et al

Here’s the background — Cassiopeia A exploded as a supernova about 11,000 years ago, and its light finally reached Earth around 1670, although the telescopes at the time weren’t powerful enough to detect it. But astronomers have made many observations in the last few decades, and supernovae are among the few objects in space that change quickly enough for astronomers to watch on a human timescale.

Physicists expect a supernova like Cassiopeia A to produce two shock waves. One, called the “forward shock,” is created by the force of the explosion, and it travels outward through the gas and debris of the star’s death. The further the wave travels from the center of the blast, the sparser the gas and dust it’s passing through become, so the wave slows down and eventually peters out.

Along the way, the forward wave heats up the gas it’s running into. This heated gas piles up in a dense, hot bow wave ahead of the blast front — picture the bow wave at the front of a speeding boat. As this heated material collides with gas which has been slowly cooling since it was blasted out from the dying star, the temperature difference creates a new shock wave, which also radiates outward from the center of the nebula. Eventually, the wave is slowed down and then turned backward by the density of the material piled up ahead of it.

That’s called the “reverse shock,” and it usually takes a few thousand years for it to slow down and turn back toward the center of the nebula. Most of Cassiopeia A’s reverse shock is still on its way outward, driving heated gas before it. But in one quadrant (which astronomers have labeled as the west), the reverse shock wave is already retreating.

Meanwhile, the western front of the forward shock is speeding up, when it should normally be slowing down.

It’s what astrophysicsts would expect to see if the pair of shock waves encountered something, like a denser patch of gas, that would push the reverse shock back, while causing the forward shock to speed up; keep in mind that waves move faster through a denser medium, which is why sound here on Earth travels faster at sea level than at high altitudes. And if the gas cloud around Cassiopeia A is clumpy in places, that could offer some interesting clues about how the former star evolved during its final few hundred thousand years.

Why it matters — The structure of a nebula, and the behavior of the blast waves from a supernova, are shaped by all the matter they pass through. That matter, in turn, is a physical record of the last phase of a star’s life, if physicists can just work backward to understand why today’s nebula looks like it does.

“Supernova remnants keep a memory of the density structure of the medium through which they expanded. This information, however, is encoded in the intricate structure of the supernova remnant,” Orlando tells Inverse. “Models like those we have developed are the key to decipher the observations and extract relevant information about the progenitor star.”

In particular, Orlando, Vink, and their colleagues are interested in figuring out how much mass Cassiopeia ejected on its dying breaths of solar wind in the millennia before its explosive death scene.

Figuring out whether there's a clump of denser gas, or perhaps a strangely empty void, on the west side of the nebula could reveal details about how much matter the star ejected during the final phase of its life. That, in turn, could offer clues about the mass, size, and type of star that produced Cassiopeia A — something astrophysicists have been debating for years.

“Our favorite explanation is one in which the blast wave encountered the dense shell,” Vink says, referring to Orlando’s research. “This means that the last few hundred thousands year of the star was not governed by a steady evolution, but had some violent eruptions.” The cloud of gas surrounding Cassiopeia A is dense and contains about 10 solar masses of gas. Since the star probably began life with about 18 solar masses of material to its name, it lost more than half its mass before exploding.

But other questions will be harder to answer. For instance, astrophysicists still debate whether Cassiopeia A once had a companion star, with which it eventually merged, or if the doomed star lived and died alone. The lopsided shock waves, according to Vink, are “only a piece of the puzzle.”

What’s next — Observations from the James Webb Space Telescope (JWST) later this year may help shed more light on Cassiopeia A. The infrared telescope will give scientists a better look at the parts of the nebula that are still cold, unheated by the reverse shock.

“If there are large holes in there that would give more credence to the [empty bubble] hypothesis,” Vink says.

New X-ray observations from Chandra will also contribute to more precise models. And when the X-ray satellite XRISM launches, Vink hopes it will help measure the velocity of X-ray-bright material in the supernova remnant, which will provide even more information.

Meanwhile, Orlando and his colleagues will also use the upcoming JWST images to refine their models of the star’s life, death (as a supernova), and afterlife (as a nebula). Their goal will be to reconstruct the distribution of gas around the star prior to the supernova, identify major mass ejections in the star’s final phase of life, and eventually to draw conclusions about the structure of the star just before its core collapsed into a supernova. Comparing the new data with the models may also shed light on whether Cassiopeia A was once a single star or part of binary system.

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