Peering into the heart of a supernova: Simulation points out how to detect a rapidly spinning stellar core

Jul 12, 2012
This image shows the inner regions of a collapsing, rapidly spinning massive star. The colors indicate entropy, which roughly corresponds to heat: Red regions are very hot, while blue regions are cold. The black arrows indicate the direction of the flow of stellar material. The two white curves with black outlines indicate the neutrino (top) and gravitational-wave (bottom) signals. This frame shows a simulation about 10.5 milliseconds after the stellar core has become a dense proto-neutron star.

Each century, about two massive stars in our own galaxy explode, producing magnificent supernovae. These stellar explosions send fundamental, uncharged particles called neutrinos streaming our way and generate ripples called gravitational waves in the fabric of space-time. Scientists are waiting for the neutrinos and gravitational waves from about 1000 supernovae that have already exploded at distant locations in the Milky Way to reach us. Here on Earth, large, sensitive neutrino and gravitational-wave detectors have the ability to detect these respective signals, which will provide information about what happens in the core of collapsing massive stars just before they explode.

If we are to understand that data, however, scientists will need to know in advance how to interpret the information the detectors collect. To that end, researchers at the California Institute of Technology (Caltech) have found via computer simulation what they believe will be an unmistakable signature of a feature of such an event: if the interior of the is spinning rapidly just before it explodes, the emitted neutrino and gravitational-wave signals will oscillate together at the same frequency.

"We saw this correlation in the results from our simulations and were completely surprised," says Christian Ott, an assistant professor of theoretical astrophysics at Caltech and the lead author on a paper describing the correlation, which appears in the current issue of the journal Physical Review D. "In the gravitational-wave signal alone, you get this oscillation even at slow rotation. But if the star is very rapidly spinning, you see the in the neutrinos and in the , which very clearly proves that the star was spinning quickly—that's your smoking-gun evidence."

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A video of the simulation. Simulation: Christian Ott, Visualization: Steve Drasco

Scientists do not yet know all the details that lead a massive star—one that is at least 10 times as massive as the Sun—to become a supernova. What they do know (which was first hypothesized by Caltech astronomer Fritz Zwicky and his colleague Walter Baade in 1934) is that when such a star runs out of fuel, it can no longer support itself against gravity's pull, and the star begins to collapse in upon itself, forming what is called a proto-neutron star. They also now know that another force, called the strong nuclear force, takes over and leads to the formation of a shock wave that begins to tear the stellar core apart. But this shock wave is not energetic enough to completely explode the star; it stalls part way through its destructive work.

There needs to be some mechanism—what scientists refer to as the "supernova mechanism"—that completes the explosion. But what could revive the shock? Current theory suggests several possibilities. Neutrinos could do the trick if they were absorbed just below the shock, re-energizing it. The proto-neutron star could also rotate rapidly enough, like a dynamo, to produce a magnetic field that could force the star's material into an energetic outflow, called a jet, through its poles, thereby reviving the shock and leading to explosion. It could also be a combination of these or other effects. The new correlation Ott's team has identified provides a way of determining whether the core's spin rate played a role in creating any detected supernova.

It would be difficult to glean such information from observations using a telescope, for example, because those provide only information from the surface of the star, not its interior. Neutrinos and gravitational waves, on the other hand, are emitted from inside the stellar core and barely interact with other particles as they zip through space at the speed of light. That means they carry unaltered information about the core with them.

The ability neutrinos have to pass through matter, interacting only ever so weakly, also makes them notoriously difficult to detect. Nonetheless, neutrinos have been detected: twenty neutrinos from Supernova 1987a in the Large Magellanic Cloud were detected in February 1987. If a supernova went off in the , it is estimated that current neutrino detectors would be able to pick up about 10,000 neutrinos. In addition, scientists and engineers now have detectors—such as the Laser Interferometer Gravitational-Wave Observatory, or LIGO, a collaborative project supported by the National Science Foundation and managed by Caltech and MIT—in place to detect and measure gravitational waves for the first time.

Ott's team happened across the correlation between the neutrino signal and the gravitational-wave signal when looking at data from a recent simulation. Previous simulations focusing on the gravitational-wave signal had not included the effect of after the formation of a proto-neutron star. This time around, they wanted to look into that effect.

"To our big surprise, it wasn't that the gravitational-wave signal changed significantly," Ott says. "The big new discovery was that the neutrino signal has these oscillations that are correlated with the gravitational-wave signal." The correlation was seen when the proto-neutron star reached high rotational velocities—spinning about 400 times per second.

Future simulation studies will look in a more fine-grained way at the range of rotation rates over which the correlated oscillations between the neutrino signal and the gravitational-wave signal occur. Hannah Klion, a Caltech undergraduate student who recently completed her freshman year, will conduct that research this summer as a Summer Undergraduate Research Fellowship (SURF) student in Ott's group. When the next nearby supernova occurs, the results could help scientists elucidate what happens in the moments right before a collapsed explodes.

In addition to Ott, other Caltech authors on the paper, "Correlated Gravitational Wave and Neutrino Signals from General-Relativistic Rapidly Rotating Iron Core Collapse," are Ernazar Abdikamalov, Evan O'Connor, Christian Reisswig, Roland Haas, and Peter Kalmus. Steve Drasco of the California Polytechnic State University in San Luis Obispo, Adam Burrows of Princeton University, and Erik Schnetter of the Perimeter Institute for Theoretical Physics in Ontario, Canada, are also coauthors. Ott is an Alfred P. Sloan Research Fellow.

Most of the computations were completed on the Zwicky Cluster in the Caltech Center for Advanced Computing Research. Ott built the cluster with a grant from the National Science Foundation. It is supported by the Sherman Fairchild Foundation.

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A2G
1 / 5 (7) Jul 12, 2012
Quote from this article, "There needs to be some mechanismwhat scientists refer to as the "supernova mechanism"that completes the explosion."

Same thing these guys found out in this article:
http://phys.org/n...ova.html

What we call Supernova remnants are misnamed. We have seen flashes of light in the sky and then find these supernova remnants. But no one has seen a star explode. Ever. Show me the proof. The proof is not the remnants. The proof would be watching the star explode. A distance flash of light is not proof. The best in the field cannot even get a star to explode in their computer sims. Shouldn't that tell you something?

Ventilator
not rated yet Jul 12, 2012
Last I checked, current verification of any given star going supernova is when a satellite sees a star, then a bright flash centered from where the star was, then empty space.

Supernovae exist: that they are the final act of a star capable of going supernova is pretty well known. Most of the time, however, a telescope is necessary to observe the event itself, rather than the naked eye.

yyz
4.5 / 5 (8) Jul 12, 2012
"But no one has seen a star explode."

On February 23, 1987 a 12th magnitude blue supergiant star in Dorado named Sanduleak -69 202 was observed to spike in emissions across the electromagnetic spectrum (aka SN 1987A: http://en.wikiped...2%B0_202 ). A burst of neutrinos was simultaneously recorded. Subsequent spectroscopic observations detected a debris front ejected from the star that would collide with a circumstellar nebula surrounding the star around the year 2000 (and this was subsequently observed in multiwavelength observations, i.e.: http://upload.wik...tion.gif )

So given numerous observations of this event:

1) Where is Sanduleak 69 202? No star exists at that location today.

2) What was the cause of the spike in EM emissions/neutrinos from Sand 69 202 in February of 1987?

3) What initiated the high speed debris front that was observed to collide with the circumstellar nebular ring?
packrat
1.7 / 5 (6) Jul 12, 2012
I don't understand something... How can a star made up of neutrons which I've always understood to have no electrical charge create a magnetic field that would produce a jet from the poles even when spinning?
jsdarkdestruction
1 / 5 (1) Jul 13, 2012
Quote from this article, "There needs to be some mechanismwhat scientists refer to as the "supernova mechanism"that completes the explosion."

Same thing these guys found out in this article:
http://phys.org/n...ova.html

What we call Supernova remnants are misnamed. We have seen flashes of light in the sky and then find these supernova remnants. But no one has seen a star explode. Ever. Show me the proof. The proof is not the remnants. The proof would be watching the star explode. A distance flash of light is not proof. The best in the field cannot even get a star to explode in their computer sims. Shouldn't that tell you something?


what evidence would it take you to reconsider your *hunch*(to be nice about it.) that supernovae dont happen? what proof do you have that they do not exist? can that info explain supernovae and the observations we've made up till now better than the current theory? can you make any sort of predictions that are better w/it?
A2G
1 / 5 (6) Jul 13, 2012
what evidence would it take you to reconsider your *hunch*(to be nice about it.) that supernovae dont happen? what proof do you have that they do not exist? can that info explain supernovae and the observations we've made up till now better than the current theory? can you make any sort of predictions that are better w/it?

Yup sure do, but we are not releasing that for about another two months. Final touches being put on it all now. BE patient, but you will see. Easily explain those rings on 1987A and the signal there as well. No prob. Vote me down if you want. But you will see.
A2G
1 / 5 (7) Jul 13, 2012
BTW why doesn't the ring on 1987a change size seeing as it was "blown" away from the initial "explosion. But the second "blast" catches the first blast that has "stalled?" Even the best in the field are puzzled by this formation.
A2G
1 / 5 (7) Jul 13, 2012
Since the posters here are smarter than the experts, maybe some of you need to offer your services to program the computer sim so that the star does indeed "explode". Because it doesn't work. They say so and they are trying to get it so, still can't.
roboferret
5 / 5 (2) Jul 13, 2012
Since the posters here are smarter than the experts, maybe some of you need to offer your services to program the computer sim so that the star does indeed "explode". Because it doesn't work. They say so and they are trying to get it so, still can't.


And yet stars keep on exploding. Science still hasn't worked out all the details why. It's a complex, dynamic system, but if we knew everything, we WOULDN'T NEED SCIENCE. Its a process of discovery, not a body of knowledge.
We know far more about supernovas than you think. Those "flashes of light" carry a large amount of information. We can tell the composition of the source from its spectrum. We can tell its velocity from its red/blue shift (doppler effect) and its distance from its brightness. Supernova are unerringly made of rapidly expanding stellar material, complete with shock wave, sometimes with a neutron star at the centre - all of which is exactly what would be expected from a supernova. Research the Crab Pulsar.
jyro
1 / 5 (5) Jul 14, 2012
compare this spin to a black hole event horizon.