Shallow Water May Shed Light on Supernovae

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Although supernovae — the violent death of massive stars — are a well-known stellar event, the underlying mechanisms remain a mystery to astrophysicists. But with the help of a spinning cylinder of shallow water, one physicist thinks he may have spotted the phenomenon that gives these stellar explosions their "kick."

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In 1987, astronomers spotted one of the brightest supernovae seen in 400 years, dubbed 1987A. Seventeen years later, the Hubble Space Telescope snapped the above image showcasing a new kind of light display from 1987A: a cosmic "ring of pearls" made up of several bright spots strewn along a ring of gas.

Those glowing pearls were the result of the original supersonic shock wave from the explosion colliding into the gassy ring at high speeds. The ring itself likely formed from ejected material before the original star exploded — probably a by-product of it consuming another smaller star.

I bring it up because (a) it's such a pretty image, and (b) it's still possible for supernovae to surprise us. We have a lot to learn about stellar explosions, like, just what is the underlying mechanism that causes them to explode in the first place?

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Yes, we understand the basics: It starts when the iron core of a massive star collapses and then stops collapsing abruptly, forming a neutron star. That sudden stop sends a giant shock wave ricocheting out into space.

But to get the explosions astrophysicists observe, something needs to re-energize that shock wave suddenly for it push through the star and separate the outer layers. We just don't know what that "something" might be.

And we don't understand why neutron stars rotate extremely rapidly right after their birth. Sure, it's because angular momentum is conserved: the slower rotation of the parent star starts to speed up as it collapses into the smaller, denser neutron progeny — just like an ice skater will pull in his or her arms close to the body to achieve a faster spin. But what gives it that initial "kick"?

Enter French physicist Thierry Foglizzo, who worked with colleagues at CEA-Saclay to create an experimental analog of what might be going on in these stellar explosions using something a bit closer to home: shallow water.

He is not the first to notice some striking similarities in the behavior of hydrodynamical systems with supernova shock waves. There have been several large-scale numerical simulations modeling those processes.

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For instance, it's now known that the shock wave itself is unstable. Tiny ripples or perturbations gradually become amplified so that you get a "sloshing" effect, similar to how water behaves when it's perturbed and starts sloshing against the sides of a container.

The hypothesis is that this underlying instability — called a standing accretion shock instability (SASI) — is the critical factor in stellar explosions, possibly even that "kick" that sets the neutron star's spin in motion.

But a simulation is still an abstract model. Foglizzo et al. came up with an ingenious way to test that hypothesis, in a set up they call the shallow water analog of a shock instability (SWASI). In true physicist fashion, they simplified matters, recreating just the primary components of a SASI: an infalling flow, and something akin to advective and sound waves. (Advective flow is what happens when water's motion carries silt or pollutants downstream.)

Surface gravity waves behave a lot like sound waves, while a good analog for shocks could be hydraulic jumps. The latter are common in river rafting or kayaking, for example, arising when a fast-flowing liquid, like water, slows suddenly and piles up on top of itself — pretty much how a shock wave forms.

You can see the experimental set-up in the image above. At the top is what it looks like when everything is stable. Then the researchers injected water toward the central cylinder, and the resulting hydraulic jump destabilizes the flow, giving it a distinctive rotation in the process. The results matched very well with the simulations.

The next step is to add angular momentum into the experimental mix to look more closely at how a star's spin factors into the phenomenon. And hopefully astronomers will have the good fortune to witness another supernova explosion in the coming years, perhaps a bit closer to our Milky Way galaxy. Should they then detect more neutrinos, or the first gravitational waves, that, too, could shed a bit more light on the underlying mechanism of a supernova.

Images: Top: Hubble image of supernova 1987A (NASA/STScI). Middle: The apparatus of the experiment (Foglizzo et al.)

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