Catch the wave

Lower left: The simulation is initiated near the endpoint of the white dwarf's lifespan, when a single nuclear flame bubble has been ignited near its center. The buoyancy of the bubble rapidly pushes it up to the surface of the white dwarf.

 

Center: Within one second of physical time, the bubble emerges from the surface of the white dwarf.

 

Upper right: The ash rushes over the surface of the white dwarf, which remains intact and gravitationally-bound, culminating in an intense jet which sets off a detonation on the opposite end of the star about one second later. The resulting detonation releases an amount of energy comparable to the entire luminous output of our sun in a fraction of a second. Image courtesy of Flash Center, University of Chicago

Only in the last few years have sizeable advances in computing power made it possible to study the evolution of a supernova using 3-D first principle simulations.

"These models are so extremely realistic, it's incredible," said Robert Fisher, an astrophysicist at the University of Massachusetts Dartmouth. Fisher and his colleague, Gaurav Khanna, are harnessing the power of high performance computing to do cutting edge simulations of type Ia supernovae. Type Ia supernovae are the result of explosions of white dwarf stars.

"A white dwarf is like a retirement for stars," Fisher explained. "They've had their careers burning nuclear fuels and they go off to the retirement home or retirement hotel, continuing to sort of shine although they are no longer working, no longer burning nuclear fuels. They just are living off their retirement account if you will, giving off whatever remnant heat they have in the form of visible light."

After billions of years of active nuclear burning, the white dwarf heads off to the retirement hotel with its companion, a binary star. And that binary star can donate mass to the white dwarf through a process called accretion, Fisher explains. But the donation can drive the white dwarf to the point of instability and then it explodes, becoming a supernova.

Master's student David Falta assisted with the project, running 3D simulations on the US National Center for Supercomputing Application's now-retired Abe cluster in addition to the Louisiana Optical Network Initiative's QueenBee. The team's results were published earlier this year in Physical Review Letters.

Gravitational waves

Currently, researchers are only able to study the Type Ia supernovae using visible light, which must be measured over weeks and months. This is because, although the real-time white dwarf explosion only lasts about two seconds, the star is so bright that it masks the initial visible light from the explosion.

Gravitational waves, on the other hand, go through anything very freely. That's what makes them so hard to detect but also what makes them amazing sources of information, Fisher said. The actual process of the two-second explosion would be "right there in the gravitational wave."

The team looked at the simulated explosions with a fresh viewpoint. Instead of just trying to simulate the supernova explosion, they explored the consequences of it. By focusing on the consequences, they noticed the explosion doesn't originate from the star's center; it goes off asymmetrically. Recent optical measurements by Keiichi Maeda of the University of Tokyo and colleagues, published in Nature, have independently confirmed that Type Ia supernovae are asymmetric.

Albert Einstein's General Theory of Relativity predicts that if you take a large amount of mass that is asymmetric, it will give off another form of radiation that is not seen by visible light but is actually a distortion in space and time, explained Khanna.

"If you have violent events that are very asymmetric, like a Type Ia supernova, that can cause a ripple in spacetime that travels at the speed of light," he said. "So, very much like you were to throw a pebble on the surface of a pond that causes ripples, you would have a ripple propagating outward. But this would be a ripple in spacetime itself. And this ripple would be what we call gravitational waves, and would eventually come to us on Earth and we would be able to pick it up."

The ability of gravitational waves to pass through matter without corruption of information, or any kind of issue, "and get to us, carrying that information," is one of the motivations for the emerging field of gravitational wave astronomy, Khanna noted. Using gravitational waves supplements what astronomers learn from light.

"And if we can read that information, using gravitational wave detectors, we would have high-quality information about any source we're studying," Khanna said.

Direct detection of cosmic gravitational waves has long been sought. The Laser Interferometer Gravitational-wave Observatory, or LIGO, is a large-scale physics experiment to detect gravitational waves and develop gravitational-wave observations as an astronomical tool. Funded by the National Science Foundation, the project is operated by Caltech and the Massachusetts Institute of Technology. Research is carried out by the LIGO Scientific Collaboration, a group of more than 800 scientists at universities around the United States and in 11 foreign countries. The project has observatories in Hanford, Washington, and Livingston, Louisiana.

But all gravitational waves are not equal. Drawing again on a non-science comparison, Fisher explains that if you could tap a white dwarf star it would ring like a bell, tinkling about every second because of its relatively high density—over a billion times that of water at its center. Compare that to our sun, he says, which would oscillate slowly and ring about once every 30 minutes if tapped. That characteristic ringing frequency is usually about the characteristic frequency of the gravitational wave emission. Extremely dense astrophysical sources, such as neutron stars and black holes, ring at very high frequencies, up to a thousand times a second—right in the range of LIGO. Because Type Ia supernovae are less dense than neutron stars and black holes, their gravitational waves are at a lower frequency than these events, about 1 Hertz, he says.

Catching the wave

Khanna emphasizes that Type Ia gravitational waves are not detectable with current instruments. While LIGO may have advanced instruments capable of detecting neutron stars and white dwarfs by 2014 or 2015, LISA, the Laser Interferometer Space Antenna, would be more likely to capture the Type Ia's lower frequency gravitational waves. LISA will be a more sensitive instrument, thanks to the fact that in space there are far fewer sources of undesired noise such as seismic activity that the experiment must filter out.

LISA was to be a joint project of NASA and the European Space Agency (ESA). Due to budget cuts, NASA pulled out of the project early in 2011. ESA is planning to continue the project, but it most likely will be at a smaller scale, which may limit the antenna's ability to capture the waves from Type Ia supernovae. Future planned spaceborne instruments currently under consideration by NASA, such as the Big-Bang Observer, are more ideally suited to the detection of Type Ia supernovae.

In the meantime, Fisher and Khana are laying the ground work for future research.

"We were able to show Type Ia's have a gravitational wave signature, and make a prediction of what the gravitational wave signature would be. That was a first that came out of this work," Fisher said. "When we first did this study, we thought the chance of a supernova exploding close enough to be easily detectible by future instruments would be quite serendipitous. Then, just as the ink on our paper was drying, astronomers caught the closest Type Ia in half a century, SN 2011fe. This supernova would have been detected in gravitational waves by the proposed Big Bang Observer mission."

In the meantime, modeling and simulation will have to suffice. Hopefully, one day in the next decade, another supernova like SN 2011fe will once again light the skies, and Fisher and Khanna will get to catch the wave.

A version of this article first appeared on the NCSA website here.