How are stars born, and how do they die? How do they produce the energy that keeps them burning for billions of years? How do they create the elements we observe today? Definitive answers to these questions continue to elude scientists in their quest to understand the processes that shape the chemical makeup of the universe.

Although the exact details of the reaction processes are unclear, understanding where and how elements are formed, and the processes of star formation, is essential for a comprehensive picture of the universe’s history, structure and evolution.

Recently, an international team, including researchers from the U.S. Department of Energy’s (DOE) Argonne National Laboratory, obtained new experimental data that clarifies how some of the heaviest elements in the universe are formed in stars. This discovery begins to answer fundamental questions about our origins.

The findings are published in the journal Physical Review Letters.

In particular, the team obtained the first experimental constraints for measuring the rate of the process in which neutrons collide and merge with a nucleus of the isotope barium-139 to form barium-140. Isotopes are members of a family of an element that all have the same number of protons but different numbers of neutrons. The reaction rate of barium-139 as it turns into barium-140 has been a dominant source of uncertainty in predictive models used to determine the presence of isotopes of heavy elements in stars.

Led by Artemis Spyrou, a professor in the Department of Physics and Astronomy at Michigan State University and the Facility for Rare Isotope Beams (FRIB), and Dennis Mücher, a professor at the Institute for Nuclear Physics at the University of Cologne, Germany, the team benefited from the use of CARIBU, a sophisticated source of radioactive ions located at the Argonne Tandem Linac Accelerator System (ATLAS), a DOE Office of Nuclear Physics user facility at Argonne.

“It is now clear that the synthesis of elements in stars is more complex than previously thought,” said Spyrou. “Only through this type of measurement will we be able to disentangle the contributions from different astrophysical processes.”

Scientists have known for a long time that the heavy elements in stars, such as barium, lanthanum and cesium, are created through rapid and slow nucleosynthesis processes. Nucleosynthesis is the formation of new atomic nuclei—the centers of atoms that are made up of protons and neutrons—or elements, by various processes in the universe.

The rapid or “r” process, which takes place in a matter of seconds, is thought to be responsible for nucleosynthesis in exploding stars, such as supernovae, and the small, dense stars that emerge after their collapse. Conversely, the slow or “s” process is thought to be responsible for nucleosynthesis primarily in brightly burning older stars, nearing the end of life.

Relatively new astronomical observations point to a nucleosynthesis pathway different from the rapid and slow processes. As some stars thought to be poor in metal have shown unusual abundance patterns of certain elements, scientists proposed an intermediate or “i” process to explain this phenomenon.

“What is most fascinating to me is that we find these different elements here on Earth, and often without knowing it, interact with them almost daily,” said Mücher. “However, we still don’t fully understand where they come from. Now, we have a better understanding that the i process is somehow related.”

Enabled by the CARIBU source at ATLAS, scientists have been able to study barium isotopes as they captured neutrons and eventually formed lanthanum—a byproduct of barium-139 decay—and a key indicator of the i process. However, determining this neutron-capture rate is especially challenging because the half-life of barium-139 is only 83 minutes.

With the aid of certain experimental techniques, researchers have found it is possible to indirectly determine this rate with a beam of the isotope cesium-140. This isotope undergoes radioactive decay into barium-140 and in doing so, emits a gamma ray, which researchers were able to detect and measure using FRIB’s Summing Nal (SuN) detector, a total absorption gamma-ray spectrometer. By more accurately capturing data for this process, researchers could indirectly calculate the reaction rate of barium-139 as it turns into barium-140, and the probability that this reaction will produce lanthanum.

“The technique that is being used requires radioactive beams of both fairly high intensity and very high purity,” said ATLAS director Guy Savard, an Argonne Distinguished Fellow. “CARIBU provides these conditions for a whole range of neutron-rich isotopes.”

Equipped with this newfound knowledge, researchers can apply what they’ve discovered in this study to other use cases at CARIBU and its near-future upgrade, nuCARIBU. There, they can further their understanding of how neutron capture works for neutron-rich isotopes in the i process. Eventually, they hope to find a more direct way to study the process.

“In the fall we’ll have a large experimental campaign enabled by nuCARIBU, making a number of measurements again, so we can expand the range over which this technique is applied, and look at many cases and try to understand the systematics of how this neutron capture on the neutron-rich isotopes works,” said Savard. “This is just the first step,” he added.

In addition to Spyrou, Mücher and Savard, authors include P.A. Denissenkov, F. Herwig, E.C. Good, G. Balk, H.C. Berg, D.L. Bleuel, J.A. Clark, C. Dembski, P.A. DeYoung, B. Greaves, M. Guttormsen, C. Harris, A.C. Larsen, S.N. Liddick, S. Lyons, M. Markova, M.J. Mogannam, S. Nikas, J. Owens-Fryar, A. Palmisano-Kyle, G. Perdikakis, F. Pogliano, M. Quintieri, A.L. Richard, D. Santiago-Gonzalez, M.K. Smith, A. Sweet, A. Tsantiri and M. Wiedeking.

A newly discovered mechanism for the flow and freezing of ice sheet meltwater could improve estimates of sea level rise around the globe.

Researchers from The University of Texas at Austin, in collaboration with NASA’s Jet Propulsion Laboratory (JPL) and the Geological Survey of Denmark and Greenland (GEUS), have found a new mechanism that explains the process of how impermeable horizontal ice layers are formed below the surface, a process critical for determining the contribution of ice sheet meltwater to sea level rise.

The work by Mohammad Afzal Shadab, a graduate student at UT’s Oden Institute for Computational Engineering and Sciences, was published in Geophysical Research Letters. Shadab was supervised by study co-authors Marc Hesse and Cyril Grima at UT’s Jackson School of Geosciences.

The world’s two largest freshwater reservoirs, the Greenland and Antarctica ice sheets, are covered in old snow, known as firn, that’s not yet compacted into solid ice. Because the firn is porous, melted snow can drain down into the firn and freeze again rather than running into the sea. This process is thought to decrease meltwater runoff by about half.

However, it’s also possible to form impermeable ice layers that can serve as barriers for meltwater—and divert meltwater to the sea, said Shadab.

“So, there are cases where these ice layers in firn accelerate the rate of meltwater running into the oceans,” he said.

The potential for glacial meltwater to freeze in firn or flow off existing ice barriers makes understanding freezing dynamics within the firn layer an important part of estimating sea level rise, according to the researchers.

Previous work on firn in mountains, which also contain ice layers, found that these ice layers are created when rainwater accumulates, or ponds, on older layers within the firn and then refreezes. But according to Hesse, it didn’t seem to work that way for ice sheets.

“When we looked at the data from Greenland, the actual amount of melt that’s being produced, even in an extreme melt event, is not enough to produce ponds,” said Hesse. “And that’s really where this study has come up with a new mechanism for ice layer formation.”

This new research presents ice layer formation as a competition between two processes: warmer meltwater flowing down through the porous firn (advection) and the cold ice freezing the water in place by heat conduction. The depth where heat conduction begins to dominate over heat advection determines the location where a new ice layer forms.

“Now that we know the physics of the formation of those ice layers, we will be able to better predict the meltwater retention capability of firn,” said study co-author Surendra Adhikari, a geophysicist at JPL.

Anja Rutishauser, a former UT postdoctoral researcher who is now a now at GEUS, also co-authored the study.

To ground truth this new mechanism, the researchers compared their models to a dataset collected in 2016 in which scientists dug a hole in Greenland’s firn and heavily equipped it with thermometers and radar that could measure the movement of meltwater. While previous hydrological models deviated from the measurements, the new mechanism successfully mirrored observations.

An unexpected finding of the new work was that the location of the ice layers may act as a record of the thermal conditions under which they formed.

“In the warming scenario, we found that the ice layers form deeper and deeper into the firn chronologically in a top-down fashion,” said Shadab. “And in a colder condition, ice layers form closer to the surface in a bottom-up scenario.”

Today, the amount of water running into the sea from Greenland currently outpaces Antarctica’s, about 270 billion tons per year compared to Antarctica’s 140 billion tons. Together, that’s more than two and a half Lake Tahoes’ worth each year. But future predictions of how much the two ice sheets will contribute to sea level rise are highly variable, fluctuating from 5 to 55 centimeters by 2100. And it’s clear that ice layers play a key—and until now, poorly understood—role.

“Things are much more complex in reality than what has been captured by existing models,” said Adhikari. “If we really want to improve our predictions, this is where we’re really advancing the state of the art.”