Scientists tap supercomputing to study exotic matter in stars

A dense neutron star (right) pulling matter off a nearby star (left)
Credit: Colby Earles, ORNL Hi-res image

At the heart of some of the smallest and densest stars in the universe lies nuclear matter that might exist in never-before-observed exotic phases. Neutron stars, which form when the cores of massive stars collapse in a luminous supernova explosion, are thought to contain matter at energies greater than what can be achieved in particle accelerator experiments, such as the ones at the Large Hadron Collider and the Relativistic Heavy Ion Collider.

Although scientists cannot recreate these extreme conditions on Earth, they can use neutron stars as ready-made laboratories to better understand exotic matter. Simulating neutron stars, many of which are only 12.5 miles in diameter but boast around 1.4 to two times the mass of our sun, can provide insight into the matter that might exist in their interiors and give clues as to how it behaves at such densities.

A team of nuclear astrophysicists led by Michael Zingale at Stony Brook University is using the Oak Ridge Leadership Computing Facility's (OLCF's) IBM AC922 Summit, the nation's fastest supercomputer, to model a neutron star phenomenon called an X-ray burst—a thermonuclear explosion that occurs on the surface of a neutron star when its gravitational field pulls a sufficiently large amount of matter off a nearby star. Now, the team has modeled a 2D X-ray burst flame moving across the surface of a neutron star to determine how the flame acts under different conditions. Simulating this astrophysical phenomenon provides scientists with data that can help them better measure the radii of neutron stars, a value that is crucial to studying the physics in the interior of neutron stars. The results were published in the Astrophysical Journal.

"Astronomers can use X-ray bursts to measure the radius of a neutron star, which is a challenge because it's so small," Zingale said. "If we know the radius, we can determine a neutron star's properties and understand the matter that lives at its center. Our simulations will help connect the physics of the X-ray burst flame burning to observations."

The group found that different initial models and physics led to different results. In the next phase of the project, the team plans to run one large 3D simulation based on the results from the study to obtain a more accurate picture of the X-ray burst phenomenon.

Switching physics

Neutron star simulations require a massive amount of physics input and therefore a massive amount of computing power. Even on Summit, researchers can only afford to model a small portion of the neutron star surface.

To accurately understand the flame's behavior, Zingale's team used Summit to model the flame for various features of the underlying neutron star. The team's simulations were completed under an allocation of computing time under the Innovative and Novel Computational Impact on Theory and Experiment (INCITE) program. The team varied surface temperatures and rotation rates, using these as proxies for different accretion rates—or how quickly the star increases in mass as it accumulates additional matter from a nearby star.

Alice Harpole, a postdoctoral researcher at Stony Brook University and lead author on the paper, suggested that the team model a hotter crust, leading to unexpected results.

"One of the most exciting results from this project was what we saw when we varied the temperature of the crust in our simulations," Harpole said. "In our previous work, we used a cooler crust. I thought it might make a difference to use a hotter crust, but actually seeing the difference that the increased temperature produced was very interesting."

Massive computing, more complexity

The team modeled the X-ray burst flame phenomenon on the OLCF's Summit at the US Department of Energy's (DOE's) Oak Ridge National Laboratory (ORNL). Nicole Ford, an intern in the Science Undergraduate Laboratory Internship Program at Lawrence Berkeley National Laboratory (LBNL), ran complementary simulations on the Cori supercomputer at the National Energy Research Scientific Computing Center (NERSC). The OLCF and NERSC are a DOE Office of Science user facilities located at ORNL and LBNL, respectively.

With simulations of 9,216 grid cells in the horizontal direction and 1,536 cells in the vertical direction, the effort required a massive amount of computing power. After the team completed the simulations, team members tapped the OLCF's Rhea system to analyze and plot their results.

On Summit, the team used the Castro code—which is capable of modeling explosive astrophysical phenomena—in the adaptive mesh refinement for the exascale (AMReX) library, which allowed team members to achieve varying resolutions at different parts of the grid. AMReX is one of the libraries being developed by the Exascale Computing Project, an effort to adapt scientific applications to run on DOE's upcoming exascale systems, including the OLCF's Frontier. Exascale systems will be capable of computing in the exaflops range, or 10¹⁸ calculations per second.

AMReX provides a framework for parallelization on supercomputers, but Castro wasn't always capable of taking advantage of the GPUs that make Summit so attractive for scientific research. The team attended OLCF-hosted hackathons at Brookhaven National Laboratory and ORNL to get help with porting the code to Summit's GPUs.

"The hackathons were incredibly useful to us in understanding how we could leverage Summit's GPUs for this effort," Zingale said. "When we transitioned from CPUs to GPUs, our code ran 10 times faster. This allowed us to make less approximations and perform more physically realistic and longer simulations."

The team said that the upcoming 3D simulation they plan to run will not only require GPUs—it will eat up nearly all of the team's INCITE time for the entire year.

"We need to get every ounce of performance we can," Zingale said. "Luckily, we have learned from these 2D simulations what we need to do for our 3D simulation, so we are prepared for our next big endeavor."

by Rachel McDowell, Oak Ridge National Laboratory 

Provided by Oak Ridge National Laboratory 

Source: Phys.Org/News

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World's First 3D Simulations Reveal the Physics of Superluminous Supernovae

The nebula phase of the magnetar-powered super-luminous supernova from our 3D simulation. At the moment, the supernova ejecta has expanded to a size similar to the solar system. Large scale mixing appears at the outer and inner region of ejecta. The resulting light curves and spectra are sensitive to the mixing that depends on stellar structure and the physical properties of magnetar. (Image by Ken Chen)

For most of the 20th century, astronomers have scoured the skies for supernovae—the explosive deaths of massive stars—and their remnants in search of clues about the progenitor, the mechanisms that caused it to explode, and the heavy elements created in the process. In fact, these events create most of the cosmic elements that go on to form new stars, galaxies, and life.

Because no one can actually see a supernova up close, researchers rely on supercomputer simulations to give them insights into the physics that ignites and drives the event. Now for the first time ever, an international team of astrophysicists simulated the three-dimensional (3D) physics of superluminous supernovae—which are about a hundred times more luminous than typical supernovae. They achieved this milestone using Lawrence Berkeley National Laboratory’s (Berkeley Lab’s) CASTRO code and supercomputers at the National Energy Research Scientific Computing Center (NERSC). A paper describing their work was published in Astrophysical Journal.

Astronomers have found that these superluminous events occur when a magnetar—the rapidly spinning corpse of a massive star whose magnetic field is trillions of times stronger than Earth’s—is in the center of a young supernova. Radiation released by the magnetar is what amplifies the supernova’s luminosity. But to understand how this happens, researchers need multidimensional simulations.

“To do 3D simulations of magnetar-powered superluminous supernovae, you need a lot of supercomputing power and the right code, one that captures the relevant microphysics,” said Ken Chen, lead author of the paper and an astrophysicist at the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA), Taiwan.

He adds that the numerical simulation required to capture the fluid instabilities of these superluminous events in 3D is very complex and requires a lot of computing power, which is why no one has done it before.

The turbulent core of a magnetar bubble inside the superluminous supernovae. Color coding shows densities. The magnetar is located at the center of this image and two bipolar outflows are emitted from it. The physical size of the outflow is about 10,000 km. (Image by Ken Chen)

Fluid instabilities occur all around us. For instance, if you have a glass of water and put some dye on top, the surface tension of the water will become unstable and the heavier dye will sink to the bottom. Because two fluids are moving past each other, the physics of this instability cannot be captured in one dimension. You need a second or third dimension, perpendicular to height to see all of the instability. At the cosmic scale, fluid instabilities that lead to turbulence and mixing play a critical role in the formation of cosmic objects like galaxies, stars, and supernovae.

“You need to capture physics over a range of scales, from very large to really tiny, in extremely high-resolution to accurately model astrophysical objects like superluminous supernovae. This poses a technical challenge for astrophysicists. We were able to overcome this issue with a new numerical scheme and several million supercomputing hours at NERSC,” said Chen.

For this work, the researchers modeled a supernova remnant approximately 15-billion kilometers wide with a dense 10-kilometer wide magnetar inside. In this system, the simulations show that hydrodynamic instabilities form on two scales in the remnant material. One instability is in the hot bubble energized by the magnetar and the other occurs when the young supernova’s forward shock plows up against ambient gas.

“Both of these fluid instabilities cause more mixing than would normally occur in a typical supernova event, which has significant consequences for the light curves and spectra of superluminous supernovae. None of this would have been captured in a one-dimensional model,” said Chen.

They also found that the magnetar can accelerate calcium and silicon elements that were ejected from the young supernova to velocities of 12,000 kilometers per second, which account for their broadened emission lines in spectral observations. And that even energy from weak magnetars can accelerate elements from the iron group, which are located deep in the supernova remnant, to 5,000 to 7,000 kilometers per second, which explains why iron is observed early in core-collapse supernovae events like SN 1987A. This has been a long-standing mystery in astrophysics.

Turbulent core of magnetar bubble inside the superluminous supernovae. Color coding shows the densities. The magnetar is located at the center of this image. Strong turbulence is caused by the radiation from the central magnetar. (Image by Ken Chen)

“We were the first ones to accurately model a superluminous supernova system in 3D because we were fortunate to have access to NERSC supercomputers,” said Chen. “This facility is an extremely convenient place to do cutting-edge science.”

In addition to Chen, other authors on the paper are Stan Woosley (University of California, Santa Cruz) and Daniel Whalen (University of Portsmouth and University of Vienna). The team also received technical support from staff at NERSC and Berkeley Lab’s Center for Computational Sciences and Engineering (CCSE).

Chen started using NERSC as a graduate student at the University of Minnesota in 2011, then as the IAU-Gruber Fellow in the Department of Astrophysics at UC Santa Cruz before taking positions at the National Astronomical Observatory of Japan, and his current role at ASIAA.

Written by Linda Vu
Contact: CScomms@lbl.gov

Source: Berkeley Lab Comuting Science/News

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Computer simulation confirms supernova mechanism in three dimensions

Fig. 1: History of the explosion in the stellar interior: within fractions of a second, the core inflates to many times its volume. The snapshots impressively show that the explosion is far from symmetric and that convective buoyancy and turbulence play an important role. The colour code indicates the speed of the ejected material. The thin bluish line shows the position of the shock front. 

Fig. 2: This diagram shows the progression of the stellar explosion; the thick red line traces the position of the shock front. The shock forms when a neutron star is born in the center, begins to first move outwards rapidly, and then stalls before it is revived by neutrino heating. The red areas indicate regions with strong turbulent motions of the stellar matter.

Massive stars explode as supernovae at the end of their lives, but how exactly does the explosion begin and what is the role of different physical processes? For the first time, scientists at the Max Planck Institute for Astrophysics have been able to simulate such a stellar explosion in all three dimensions with detailed physical input. The results show that the energetic neutrinos radiated by the newly formed neutron star indeed trigger the explosion by heating the stellar matter. Turbulent flows support this process and lead to an even more energetic explosion. 

During their lifetimes, stars "burn" light elements such as hydrogen into heavier ones by nuclear fusion. This process produces energy until, at the end, an iron core is formed. Since iron has the largest binding energy of all nuclei, no heavier elements can be produced in fusion reactions and nuclear burning ceases. However, the iron core continues to grow by fusion processes at its surface. At this stage, gravity is balanced by the quantum mechanical pressure of the electrons. Similar to a white dwarf star, there is a critical mass above which the iron core can no longer resist the pull of gravity and collapses. Under appropriate conditions, this results in a powerful stellar explosion: a supernova. 

Already in the middle of the past century, first theories proposed the origin of the supernova energy: Because of the extreme gravitational force, the core collapses within fractions of a second to produce a neutron star. Gravitational binding energy is released and transported outwards by a shock front, but gets quickly absorbed by the outer layers of the iron core. To actually trigger the explosion, an additional effect is required: heating by neutrinos (see Highlight 2001). These elementary particles are generated in vast numbers in the new-born neutron star and propagate outwards relatively freely, once they are outside the neutron star's surface. Therefore they can extract energy from the so-called "cooling layer" and deposit this energy at greater distances from the neutron star where they are re-absorbed and thus heat the plasma in the so-called "heating layer" behind the shock wave. If the amount of deposited energy is large enough, the shock is pushed outwards, which eventually disrupts the star in a supernova. At least that is how the theory goes. 

The process to confirm this paradigm in detailed physical models, however, has been long: In the 1980s, the first star "exploded" in a computer, but only in spherically symmetric (i.e. one-dimensional) models and with some special assumptions to simplify the description of the physics involved. But the observation of supernova 1987A showed that multi-dimensional effects play an important role during the explosion. The shells surrounding the neutron star are mixed by convection, which further supports neutrino heating. After a few decades, scientists could confirm the basic functioning of the neutrino mechanism with two-dimensional models (see Press Release 2009). Still, the forced rotational symmetry about an arbitrary axis severely restricts motions of the stellar plasma. In addition, turbulent flows behave differently under these symmetry assumptions compared to three dimensions. It is therefore necessary to perform three-dimensional calculations to model all processes during the supernova correctly. 

So far, simulations have not yielded successful explosions in three dimensions (Press Release 2013 und Highlight 2014). But now, the scientists obtained their long desired result: the first successful, neutrino-driven explosion of a star with an initial mass of 9.6 solar masses in a three-dimensional, self-consistent simulation (see Fig. 1). The challenge was to describe the neutrinos as correctly as possible, so that the resulting complex calculation kept even supercomputers busy for a few months. The new method provides the currently most complete description of how neutrinos interact with matter in a supernova calculation. In particular, there is an open, controversial question whether three-dimensional turbulence in the neutrino heated plasma helps or hinders the explosion. 

In this case, the answer is definitely yes: three-dimensional turbulence leads to about 10% higher explosion energy. Turbulent effects in the heating layer change the flow of stellar material into the cooling layer, which means that the temperature in this region remains lower. As the cooling by neutrinos strongly depends on temperature, the energy loss by neutrino emission decreases at lower temperature and the explosion gets stronger. However, it is difficult to predict whether this phenomenon could play an equally important role for even more massive stars. To answer this question, the scientists need further simulations. They also plan to calculate the explosion with even higher resolution to better resolve turbulence and investigate it on smaller scales. Another important question is whether the star might have been asymmetric before collapse and how this would affect the explosion. So even with this significant milestone, the astrophysicists still have some way to go.

Tobias Melson, Hans-Thomas Janka
Public outreach: Hannelore Hämmerle

Publication:

T. Melson, H.-T. Janka, & A. Marek, Neutrino-driven supernova of a low-mass iron-core progenitor boosted by three-dimensional turbulent convection, Astrophysical Journal Letters, 801, L24 (2015)v

This project has been supported by the European Research Council through grant ERC-AdG No. 341157-COCO2CASA.

Furthermore, the authors acknowledge funding by the Deutsche Forschungsgemeinschaft through the Excellence Cluster EXC 153 "Origin and Structure of the Universe" and computing time provided by the European PRACE Initiative on SuperMUC (GCS@LRZ, Germany) and Curie (GENCI@CEA, France).

Source: Max Planck Institute for Astrophysics