Monthly Roundup: Where Elements Are Made

Roughly thirteen billion years ago, the periodic table was easy to memorize: hydrogen, helium, and lithium were the only chemical constituents of the infant universe. Today, things are a little more complicated. Through billions of years of stellar alchemy, the universe is now awash with an abundance of metals — what astronomers call elements heavier than helium. Some of these metals are forged in the cores of stars, while others require explosive events to form. In today’s post, we’ll take a look at three research articles that examine the creation of heavy elements in exotic environments across the universe.

Making Metals in Supernovae

Core-collapse supernovae are one of the sites of element formation, which is also called nucleosynthesis. Core-collapse supernovae are triggered by the collapse of a massive star as the star exhausts its ability to hold off the inward pressure of gravity with the outward pressure of radiation generated by nuclear reactions in its core. When the star’s core collapses, its outer layers recoil from the condensed core and explode into space.

The hot, dense ejecta of a supernova explosion may be a good place for elements to be created through r-process nucleosynthesis. In this process, multiple free-wheeling neutrons pack onto nearby atoms, creating heavier isotopes and elements fast enough that unstable isotopes don’t have the chance to decay. (The counterpart to the rapid r-process is the slow s-process, in which a trickle of neutrons builds elements and isotopes more gradually. The s-process tends to take place in stellar interiors, without the need for a cataclysmic explosion.) It’s not yet clear how much element creation via the r-process happens in core-collapse supernovae or how this quantity depends on the mass of the star or other factors.

Anirudh Patel (Columbia University) and collaborators used simulations to understand how r-process nucleosynthesis might leave its mark on the light curves of core-collapse supernovae. Patel’s team produced one set of models in which no r-process reactions take place and another in which r-process elements are produced deep within the explosion and then mixed throughout the ejected material. The team varied the amount of r-process material and how thoroughly it’s mixed with other material.

Patel and coauthors found that if the amount of r-process material is small — less than a hundredth of the mass of the Sun — the light curve looks scarcely different than if there is no r-process material at all. For larger amounts of r-process material and greater degrees of mixing, the “plateau” phase of the light curve shortens and dims, and the supernova appears redder than it otherwise would. The team’s research suggests that r-process-enriched supernovae should be distinguishable from regular supernovae but may be confused with certain types of rare supernovae.

The Influence of Magnetic Fields

Many core-collapse supernovae leave behind a neutron star: an extraordinarily dense sphere of neutrons about 10 kilometers in radius and about the mass of the Sun. When two neutron stars collide, the merger creates ideal conditions for element formation through the r-process. In a recent article, Kelsey Lund (North Carolina State University and Los Alamos National Laboratory) and collaborators examined element creation in the case of a neutron star merger than produces a black hole surrounded by a hot, dense accretion disk.

The merger of a binary pair of neutron stars can create a bright electromagnetic signal called a kilonova. The kilonova emission is powered by the radioactive decay of r-process elements that are produced when the stars collide. One of the many uncertainties that surround the creation of a kilonova is the impact of the magnetic field. The magnetic field is thought to control how quickly material flows from the accretion disk onto the black hole as well as how rapidly the material flows out from the disk — two factors that influence the production of r-process material.

Lund’s team used general relativistic magnetohydrodynamics simulations to trace the evolution of the disk that forms after the stars merge. The three simulations capture the nucleosynthesis that occurs within about 127 milliseconds of the collision. When the magnetic field is stronger, more mass is ejected by the merger and larger amounts of elements in the lanthanide and actinide groups — the two rows of heavy elements separated from the rest of the periodic table — are produced. The amounts of lanthanide-group and actinide-group elements both increase with increasing magnetic field strength, but the increase is larger for the actinide-group elements.

This last finding could explain a curious feature of some old stars: while many old stars contain about the same amount of lanthanide-group elements, there is a broad range of actinide-group element abundances. This may reflect the different magnetic field conditions in the neutron star mergers where the elements formed, long before the stars themselves were born.

Collapsar Jets and Nucleosynthesis

The final article of today’s post examines a variety of element-making methods in the aftermath of a collapsar: a rapidly spinning, massive star that collapses into a black hole, slinging jets of material into space in the process. Zhenyu He (Beihang University) and collaborators used an extensive network of nuclear reactions to model the fusion, fission, and decay taking place in the collapsar’s jets.

He’s team found that in the immediate aftermath of a collapsar, the r-process is in full swing, packing neutrons onto outflowing atoms and forming elements like gold and platinum. After about five seconds, the outflow has cooled enough that the r-process stalls. This doesn’t mean that nucleosynthesis stops, though: the team found evidence that the slower s-process and the intermediate-speed i-process take over and continue to churn out heavier atomic species for several hours. The later, slower nucleosynthesis is mainly powered by neutrons from the fission of fermium and rutherfordium.

Previous modeling of other r-process sites like supernovae has not revealed this later stage of nucleosynthesis, but this work suggests that the s-process and i-process are important for shaping the chemical makeup of collapsar outflows. In particular, the pattern of elements with even numbers of protons being more abundant than elements with odd numbers of protons might be enhanced by these processes. To learn more, He’s team proposed, researchers will need to study old, metal-poor stars in the distant halo of the Milky Way, which may have formed from gas enriched by collapsars rather than neutron star mergers.

Citation

“The Effects of r-Process Enrichment in Hydrogen-Rich Supernovae,” Anirudh Patel et al 2024 ApJ 966 212. doi:10.3847/1538-4357/ad37fe

“Magnetic Field Strength Effects on Nucleosynthesis from Neutron Star Merger Outflows,” Kelsey A. Lund et al 2024 ApJ 964 111. doi:10.3847/1538-4357/ad25ef

“Possibility of Secondary i– and s-Processes Following r-Process in the Collapsar Jet,” Zhenyu He et al 2024 ApJL 966 L37. doi:10.3847/2041-8213/ad444c