CASTRO Simulations reveal an unusual death for ancient stars

This image is a slice through the interior of a supermassive star of 55,500 solar masses along the axis of symmetry. 

It shows the inner helium core in which nuclear burning is converting helium to oxygen, powering various fluid instabilities (swirling lines). 

This "snapshot" from a CASTRO simulation shows one moment a day after the onset of the explosion, when the radius of the outer circle would be slightly larger than that of the orbit of the Earth around the sun. 

Visualizations were done in VisIT. 

Credit: Ken Chen, University of California at Santa Cruz

Certain primordial stars, those 55,000 and 56,000 times the mass of our Sun, or solar masses, may have died unusually.

In death, these objects, among the Universe's first-generation of stars, would have exploded as supernovae and burned completely, leaving no remnant black hole behind.

Astrophysicists at the University of California, Santa Cruz (UCSC) and the University of Minnesota came to this conclusion after running a number of supercomputer simulations at the US Department of Energy's (DOE's) National Energy Research Scientific Computing Center (NERSC) and Minnesota Supercomputing Institute at the University of Minnesota.

They relied extensively on CASTRO, a compressible astrophysics code developed at DOE's Lawrence Berkeley National Laboratory's (Berkeley Lab's) Computational Research Division (CRD).

Their findings were recently published in Astrophysical Journal (ApJ).

First-generation stars are especially interesting because they produced the first heavy elements, or chemical elements other than hydrogen and helium.

In death, they sent their chemical creations into outer space, paving the way for subsequent generations of stars, solar systems and galaxies.

With a greater understanding of how these first stars died, scientists hope to glean some insights about how the Universe, as we know it today, came to be.

"We found that there is a narrow window where supermassive stars could explode completely instead of becoming a supermassive black hole, no one has ever found this mechanism before," says Ke-Jung Chen, a postdoctoral researcher at UCSC and lead author of the ApJ paper.

"Without NERSC resources, it would have taken us a lot longer to reach this result."

"From a user perspective, the facility is run very efficiently and it is an extremely convenient place to do science."

More information: Astrophysical Journal, iopscience.iop.org/0004-637X/790/2/162

Simulations re-create X-rays emerging from the neighbourhood of black holes

Black holes may be dark, but the areas around them definitely are not. These dense, spinning behemoths twist up gas and matter just outside their event horizon, and generate heat and energy that gets radiated, in part, as light and when black holes merge, they produce a bright intergalactic burst that may act as a beacon for their collision.

Karl Schwarzschild

Astrophysicists became deeply interested in black holes in the 1960s, but the idea of their event horizon was first intimated in a paper by Karl Schwarzschild published after Einstein introduced general relativity in 1915.

Knowledge about black holes—these still-unseen objects—has grown tremendously in recent years.

Part of this growth comes from researchers' ability to use detailed numerical models and powerful supercomputers to simulate the complex dynamics near a black hole.

This is no trivial matter. Warped spacetime, gas pressure, ionizing radiation, magnetized plasma—the list of phenomena that must be included in an accurate simulation goes on and on.

Scott Noble

"It's not something that you want to do with a paper and pencil," said Scott Noble, an astrophysicist at the Rochester Institute of Technology (RIT).

Working with Jeremy Schnittman of Goddard Space Flight Center and Julian Krolik of Johns Hopkins University, Noble and his colleagues created a new tool that predicts the light that an accreting black hole would produce.

They did so by modeling how photons hit gas particles in the disk around the black hole (also known as an accretion disk), generating light—specifically light in the X-ray spectrum—and producing signals detected with today's most powerful telescopes.

In their June 2013 paper in the Astrophysical Journal, the researchers presented the results of a new global radiation transport code coupled to a relativistic simulation of an accreting, non-rotating black hole.

For the first time, they were able to re-create and explain nearly all the components seen in the X-ray spectra of stellar-mass black holes.

The ability to generate realistic light signals from a black hole simulation is a first and brings with it the possibility of explaining a whole host of observations taken with multiple X-ray satellites during the past 40 years.

"We felt excited and also incredibly lucky, like we'd turned up ten heads in a row," Noble said. "The simulations are very challenging and if you don't get it just right, it won't give you an accurate answer."

"This was the first time that people have put all of the pieces together from first principles in such a thorough way."

More information: "X-Ray Spectra from Magnetohydrodynamic Simulations of Accreting Black Holes." Jeremy D. Schnittman, Julian H. Krolik, Scott C. Noble. 2013 ApJ 769 156. DOI: 10.1088/0004-637X/769/2/156