XSEDE visualization expert helps researchers show colliding white dwarfs
The Universe is full of explosions. These explosions, called supernovae, come about for many reasons but they're foundational to the beginning of man including the prevalence of iron and calcium in our bodies.
One specific type of supernovae, type Ia (pronounced "type one-a"), is also useful in another way: measuring distances in the Universe. Type Ia supernovae, which results from explosions of at least one white dwarf, are extremely bright allowing researchers to monitor them as they get further away, proving the accelerating expansion of the Universe. This concept won the 2011 Nobel prize in physics for "discovery of the accelerating expansion of the Universe through observations of distant supernovae."
Researchers using XSEDE try to answer a long-standing question: what causes type Ia supernovae?
"I believe that it is our task to understand what causes stars to explode and allow life as we know it," said Doron Kushnir, researcher at the Weizmann Institute of Science.
Kushnir, along with Subo Dong from Peking University, Boaz Katz of the Weizmann Institute of Science and Jose L. Preto from the Millennium Institute of Astrophysics in Chile, utilized XSEDE resources, including Extended Collaboration Support Services (ECSS), to help publish "Type Ia Supernovae with Bi-Modal Explosions Are Common – Possible Smoking Gun for Direct Collisions of White Dwarfs" in the Royal Astronomical Society. This paper proposed that colliding white dwarfs "leads to the ignition of a detonation wave that explodes the two white dwarfs," said Kushnir. This collision produces two spheres of a specific isotope (Ni56) in the ejecta from that explosion, detectable in a form called a "double peak spectra"; as Kushnir said, "an observation of a double peak spectra is a ‘smoking gun' for colliding white dwarfs."
Kushnir and his team examined data from multiple international programs that compiled information about recorded supernovae, identifying three supernovae "with clear evidence of doubly-peaked velocity profiles."
The double peak spectra were reproduced in a three-dimensional simulation of colliding white dwarfs, which was done using an XSEDE allocation. Specifically, he used 4 million CPU hours over the course of two years on the Stampede supercomputer located at the Texas Advanced Computing Center (TACC) to simulate white dwarf collisions.
So what are white dwarfs?
Simply put, white dwarfs are stars of a certain size and mass that, once their internal energy supply has run out, shrink to approximately the size of Earth but maintain a mass comparable to the sun and are therefore much denser than the sun.
"While white dwarfs are stable, their exceptional high density makes them very powerful thermonuclear bombs. If properly ignited, white dwarfs are capable of powerful thermonuclear explosions with a sufficient energy release to account for a supernova," said Kushnir.
Meanwhile, much more massive dying stars become neutron stars or black holes. The real difference is in the objects' "escape velocity" or the power it would take to escape the object's gravitational pull. It's easier to leave the Earth's gravity than that of a black hole, for instance, which holds on to anything slower than the speed of light.
Because of the distance between Earth and any observed collisions, as well as the amount of "noise" from Earth, scientifically detecting gravitational waves from colliding white dwarfs is not yet possible – the Laser Inferometer Space Antenna (LISA), a space orbit facility should be able to do so, much as the LIGO project detects merging neutron stars and black holes on the Earth's surface, but a paper from the new "evolved" LISA—or eLISA—collaboration doesn't expect readings back until the 2030s.
Until then, Kushnir's team believes Ni56 spectra to provide the best proof of white dwarf collisions.
ECSS shows the way
Dave Bock, an award-winning visualization expert from the National Center for Supercomputing Applications who is a member of XSEDE's ECSS team, helped Kushnir and his team make a series of animations that show what a collision of white dwarfs looks like.
"Presenting the simulations in a clear and professional way is a major step in helping experts and the public grasp the complicated process that we are simulating, a direct collision of two white dwarfs. For example, the movie clearly shows the ignition of the detonation wave that results from the collision, and the synthesis of Ni56 in two different blobs," said Kushnir."Preparing such a movie requires a specific knowledge and visualization expertise that my team lacked. Making the movie was a dialogue between David, who knows the capabilities of presenting data, and between us, as we tried to emphasize specific features of the simulation and of the science.
"XSEDE is the simplest way to use high performance computing. The proposal process is simple and the technical support makes the use of the high performance machines smooth. In addition, the ECSS help is valuable, such as the expertise not routinely found in scientific groups, and are sometimes essential to properly communicate the results."
Learn more soon!
Bock will soon hold a symposium talk in regards to his visualization work with Kushnir's group on Tuesday, June 20, 2017. This online presentation is free and open to the public at the websites and phone numbers listed below.