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Red Dwarf Physics to be Modeled
Red dwarfs represent the majority of stars in our galaxy, yet astrophysicists lack full understanding of the interior physics for this type of star. Heat from hydrogen fusion emanates from a red dwarf’s core to its surface; but whether heat transport occurs through radiation (heat transfer through electromagnetic waves), convection (heat transfer by way of fluid currents), or both remains uncertain. Because a star’s energy output is primarily determined by its mass, low-mass red dwarfs are relatively cool and dim. The smallest red dwarfs can burn for trillions of years, and energy transport near their surface is dominated by convection. At larger masses, the possibility of a radiative core increases, and energy is dispelled as electromagnetic waves.
In research published August 12, 2022, in Physics of Plasmas, Livermore scientists and a group of international researchers proposed an experiment to elucidate the nature of red dwarf interiors. “Understanding radiative properties of complex plasmas within a host star is crucial when judging the possibility of an exoplanet to host life—especially for red dwarfs, where the habitable zone is thought to be relatively close to the star itself due to low surface temperature,” says Livermore co-author Tilo Doeppner.
The proposed experiment, to be conducted at the National Ignition Facility, would measure the opacity of dense hydrogen plasma by reducing implosion velocity, creating colder plasmas than necessary for sustained fusion reactions. Findings will help determine which interior regions of the stars are dominated by convective or radiative transport in addition to enhancing models of dense plasma behavior.
Contact: Tilo Doeppner (925) 422-2147 (doeppner1@llnl.gov).
Smaller, Safer Samples for Testing
Studying uncommon, radioactive compounds deepens fundamental understanding of chemical elements and their behavior. With state-of-the-art characterization methods, scientists can perform physicochemical analyses with only milligrams of a substance. However, for the most scarce, unstable, and toxic ingredients, one milligram may outweigh annual production worldwide, posing logistical challenges of high cost and low availability of samples. Livermore scientists collaborated with researchers at Oregon State University to detail a new process for isolating rare, often hazardous, elements for further study. Their work was published on September 1, 2022, in Nature Chemistry and was selected as the journal’s December cover story.
Led by Livermore’s Gauthier Deblonde, the research team synthesized coordination complexes containing rare isotopes to enable detailed characterization of radioactive compounds while using as little as one thousandth the mass previously required. The study demonstrated that polyoxometalates (POMs) can be used to form crystallized metal–ligand complexes with multiple f-block elements, including transplutonium elements. Further, researchers found previously unnoticeable differences between their solution- and solid-state chemistries. The team produced and analyzed three new complexes of curium (a third of all described curium–ligand complexes since the element’s discovery) and achieved several new POM complexes with lanthanide elements.
“The simplicity, efficacy, and modularity of the newly proposed method are astonishing,” says Deblonde. “The method significantly decreases the radiation exposure to workers, preserves the nation’s isotope resources, and drastically cuts costs.” The team’s findings could help facilitate investigation of compounds using even rarer materials such as actinium, short-lived metal isotopes, and transcalifornium elements to uncover additional isotopic and bonding trends in a most inaccessible region of the periodic table.
Contact: Gauthier Deblonde (925) 423-2068 (deblonde1@llnl.gov).
Extreme Pressure Yields Experimental First
As the world’s most energetic laser, the National Ignition Facility (NIF) provides unique opportunities to investigate matter under extreme conditions. In a study published September 19, 2022, in Nature Physics, a research team led by Martin Gorman used NIF to observe how solid-state matter reacts to enormous pressures comparable to those of giant planetary cores. Computer simulations had predicted that materials such as magnesium will form novel phases of matter at immense pressures due to quantum mechanical forces beginning to dictate atomic and subatomic interactions. While the predictions shattered traditional understanding of bonding and crystallization, they had not been experimentally verified.
Employing NIF’s shaped laser pulses, the team crushed a small sample of magnesium foil with pressure reaching 1.3 terapascals (TPa) to initiate a structural phase transition—an experimental first in the TPa compression regime. “Our observations confirm theoretical predictions for magnesium and demonstrate how TPa pressures—10 million times atmospheric pressure—force materials to adopt fundamentally new chemical and structural behaviors,” says Gorman.
X-ray diffraction analysis revealed that valence electrons of magnesium atoms, which normally travel freely throughout the material, became localized to interatomic cavities under these conditions, forming crystalline, ionic salts in which electrons negatively charged ions. Direct observation of the phenomenon provides valuable insight into the way that valence–core and core–core electron interactions can influence material properties at high pressure. The success of this effort presents opportunities for further high-pressure research at the limits of experimental feasibility.
Contact: Martin Gorman (925) 422-7741 (gorman11@llnl.gov).
