Astronomers strike cosmic gold

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Artist’s conception of a binary neutron star merger, which creates a whirling cloud of radioactive debris and a short gamma-ray burst (jets). (Credit: National Science Foundation/LIGO/Sonoma State University/A. Simonnet)

See here for the original UC Berkeley press release article by Robert Sanders.

The first detection of gravitational waves from the cataclysmic merger of two neutron stars, and the observation of visible light in the aftermath of that merger, finally answer a long-standing question in astrophysics: Where do the heaviest elements, ranging from silver and other precious metals to uranium, come from?

Based on the brightness and color of the light emitted following the merger, which closely match predictions from theoretical simulations by N3AS physicists, astronomers can now say that the gold or platinum in your wedding ring was in all likelihood forged during the brief but violent merger of two orbiting neutron stars somewhere in the universe.

“We have been working for years to model what the elements produced in a neutron merger would look like,” said Daniel Kasen, a scientist at LBNL, associate professor of physics and astronomy at UC Berkeley, and N3AS member. “Now that theoretical speculation has suddenly come to life.”

The neutron star merger, dubbed GW170817, was detected on August 17 and immediately telegraphed to observers around the world, who turned their small and large telescopes on the region of the sky from which it came. The ripples in spacetime that LIGO/Virgo measured suggested a neutron star merger, since each star of the binary weighed between 1 and 2 times the mass of our sun.

Apart from black holes, neutron stars are the densest objects known in the universe. They are created when a massive star exhausts its fuel and collapses onto itself, compressing into what is essentially a giant atomic nucleus with a mass comparable the sun crammed into a sphere of radius about 10 kilometers.

Less than 11 hours after the gravitational wave detection, observers caught their first glimpse of visible light from the source, which was situated about 130 million light years from Earth in the direction of the constellation Hydra. On astronomical scales, that is surprisingly nearby, “practically in our own backyard”, according to Kasen.

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A team of UC Santa Cruz astronomers led by Ryan Foley was the first to detect the light from the neutron star merger 11 hours after the gravitational waves from the collision reached Earth. The left image shows that the glow (red arrow) was not there four months earlier. (Images courtesy of UCSC, Swopes Telescope and Hubble)

Genesis of the elements

While hydrogen and helium were formed in the Big Bang 13.8 billion years ago, heavier elements like carbon and oxygen were formed later in the cores of stars through nuclear fusion of hydrogen and helium. But this process can only build elements up to iron. Making the heaviest elements requires a special environment in which atoms are repeatedly bombarded by free neutrons. The neutrons are rapidly captured on atomic nuclei in a phenomenon called the ‘r-process’.

Where and how the r-process production occurs has been one of the longest standing questions in astrophysics. It is also one of the key focuses of a multi-institution collaboration called the “Network in Neutrinos, Nuclear Astrophysics, and Symmetries (N3AS)”, supported by the National Science Foundation and the Heising-Simons Foundation. The N3AS, which is led Berkeley physics professor Wick Haxton, and includes Kasen as a member, is dedicated to recruiting and training young postdoctoral scientists interested in nuclear astrophysics topics ranging from supernova and neutron star mergers, as well as the fundamental nature of neutrinos and dark matter.

Supercomputing simulations carried out by Kasen and collaborators show how during and immediately following the violent merger of neutron stars, a fraction of neutron star matter is thrown into space. This debris assembles into a strange mixture of heavy elements splashed across the periodic table, including precious metals like gold and platinum and radioactive isotopes like uranium.

 

Above: One day after the merger and explosion, there is a bright blue glow from lighter elements in the outer polar regions. Within three days after the merger, the blue glow is beginning to fade, giving way to the red glow from the heavier elements in the surrounding doughnut and spherical core. The red glow persisted for more than two weeks.

 

 

Above: Theoretical calculation of the evolution of the spectrum of light from a kilonova such as that associated with GW170817. The right panel shows an illustration of the expanding radioactive debris cloud ejected from a neutron star merger that gives rise to the light. The emission quickly evolves from blue to red.

But as the data trickled in, one night after the next, the images began to paint a satisfying and familiar picture. On the first night of observations, the event was relatively blue, with a brightness that matched the predictions of kilonova models with surface layers composed of lighter precious metals such as silver. After a few days, the light darkened into a deep red glow that persisted for over a week, exactly the signature that Kasen and Barnes had predicted if the debris interior was made of the heaviest elements, such as platinum, gold and uranium.

Kasen and his colleagues presented updated models and theoretical interpretations of the observed event in a paper released Oct. 16 in advance of publication in Nature. Their kilonova models are also being used to analyze a wide-ranging set of data presented in seven additional papers appearing in Nature, Science and the Astrophysical Journal.

Not only did the modeling provide direct confirmation that neutron star mergers make heavy elements, but it allowed Kasen and his colleagues to calculate the amount and make-up of the material produced. They determined a yield of about 6% of a solar mass, which is enough to imply that neutron star mergers likely account for all of the heavy r-process elements in the Universe. The amount of gold alone produced equaled the mass of roughly 200 earths.

“It’s a once in a lifetime discovery in astronomy.“ Kasen said “But it’s also a remarkable triumph for the field of theoretical astrophysics. Without the predictions from theory and simulations, I think everybody would have been looking up at the sky, mystified by exactly they we were seeing.”

Future advances in theory an computers simulations should provide even more insight as to how the heavy elements came to be, and the extreme physics of nuclear matter. “Most of the time in science you are working to gradually advance an established subject,” Kasen said. “It is rare to be around for the birth of an entirely new field of astrophysics. Very exciting times lie ahead.”

RELATED INFORMATION

CONTACTS

Daniel Kasen, kasen@berkeley.edu, 410-428-3565, 510-664-4838