A radioactive element produced near the heart of collapsing stars hints at the mechanism behind Cassiopeia A’s supernova explosion.

Cassiopeia A in X-rays

NuSTAR's observations of titanium-44's radioactive glow (blue) complement Chandra's previous X-ray observations of hot, shocked gas (red, yellow, and green). The titanium was born close to the collapsing star's core, so carries with it the echoes of the explosion's asymmetries.

NASA / JPL-Caltech / CXC / SAO

Each massive star plays the leading role in its own tragedy. It tries desperately to stave off collapse by burning first hydrogen, then helium, then heavier elements arrayed in rainbow-like shells around a dead core. Yet the end remains inevitable: a spectacular demise as a supernova.

But unlike Shakespeare’s plays, mystery shrouds this story’s moment of truth. The star’s core implodes in a matter of seconds, a crucial time that is nearly impossible to observe directly. So astronomers are grateful for the messy aftermath of discarded gas, which provides a window into the invisible seconds of collapse.

Now Brian Grefenstette (California Institute of Technology) and colleagues have published a brand new view of the discarded gas nearest the core of a well-studied supernova remnant in the February 20 issue of Nature, observations that might hold the key for theoretical models trying to replicate reality.

“When we try to make stars explode in computer simulations, we tend to get duds,” says Greffenstette. The shock wave that should rip a spherical star apart ‘stalls out,’ he explains, restrained by circumstellar material. Stars are generally pretty close to perfect spheres so if the shock wave is to break free, something must break the symmetry. But what?

Stellar Jet or Sloshing Star

Cassiopeia A is a beautiful cascade of gases surrounding a neutron star that imploded some 300 years ago. Dozens of telescopes on the ground and in space have trained their eyes on this nearby remnant, imaging the hot, shocked gas that originated in the star’s outer layers. These layers heated up as they shot outward through the material surrounding the star. But cooler gas originating closer to the star’s collapsed core has so far remained invisible.

Fortunately, material doesn’t have to be hot to glow. Radioactive titanium-44, which glows with high-energy X-rays, forms right at the boundary that divides the imploding stellar core from its exploding outer layers. Previous X-ray telescopes have seen this element, but haven’t had the resolution to map its distribution.

Enter NuSTAR. This school-bus-size satellite stared at Cas A for 13.8 days straight to map out the faint glow of titanium-44. Grefenstette and colleagues found these X-rays radiate from clumps scattered unevenly around the remnant’s center. And surprisingly, they don’t align with a jet-like feature seen in previous optical and X-ray observations.

Views of Cassiopeia A

Surprisingly, the map of radioactive titanium-44 forged deep in the explosion (blue) doesn't align with the jet-like feature that appears in maps of silicon and magnesium (which are produced in the star's outer layers), nor does it align with the distribution of iron, which ought to be forged in the same place as titanium-44.

NASA / JPL-Caltech / CXC / SAO

Some models had suggested that a jet might have ripped Cas A’s star apart — a pretty good theory since evidence suggests jets are involved in more bombastic supernova explosions called long gamma-ray bursts. But if a jet emanated from the neutron star itself, titanium-44 should have clumped along the same axis.

Instead Grefenstette and colleagues argue that titanium’s uneven distribution points to a subtler asymmetry: sloshing within the star itself. The star’s explosion releases neutrinos, the by-products of fusion, and those tiny particles could reheat gas traveling behind the shock wave. If the star were sloshing before the explosion, that heating would be uneven, producing bubbles that poke through the material holding the shock wave back. Once the symmetry is broken, the shock wave breaks through, blowing the star apart.

The first 150 milliseconds (about two blinks of an eye) of the sloshing-star scenario would look something like this:

And what about the jet? Grefenstette and colleagues argue that what looks like a jet is actually just holes in the material surrounding the star. The star’s outer layers would have poked their way through these holes first, producing a jet-like feature. The team simulates two extremes to bolster their reasoning: the first models a bipolar collapse that sends a narrow jet ripping through the star, and the second models a roughly spherical collapse. Neither explains the observations, so the truth clearly lies somewhere in-between. “One of the difficulties here is that the simulations in three dimensions are incredibly computer intensive, costing millions of hours of computer time,” Grefenstette notes. Though the sloshing-star model hasn’t been worked out for Cas A yet, simulations are underway. “It is striking that they can make these sort of resolved measurements of the titanium-44 distribution,” says Stanley Woosley (University of California, Santa Cruz, and Lick Observatory), a supernova expert not involved in the current study. “We can expect to see a number of papers claiming to explain the result.” Rethinking Supernova Physics So far so good, but then the team compared NuSTAR’s titanium map with Chandra X-ray Observatory’s previous map of shocked iron. Surprisingly, the radioactive titanium and hot iron don’t align — even though standard theory says iron is produced in essentially the same location in the exploding star. Since Chandra’s exquisitely detailed map of iron in the remnant only sees those iron atoms shocked to temperatures high enough to emit X-rays, there might be more iron present that simply isn’t hot enough to be seen, but that explanation remains to be tested.

Kepler supernova

The remnant of the Type Ia supernova Johannes Kepler spotted in 1604.

X-ray: NASA / CXC / NCSU / M.Burkey & others; Infrared: NASA / JPL-Caltech
As theorists continue to hack away at the problem with million-computer-hour simulations, the NuSTAR team is moving on to the next observations: mapping titanium in the Kepler, Tycho, and G1.9+0.3 supernova remnants, as well as the supernova recently spotted in nearby galaxy M82. These are all remains of Type Ia supernovae, where a white dwarf obliterated itself in a thermonuclear blast. Just as for the core-collapse supernova Cas A, mapping the titanium will trace the nature of these explosions.


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