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Compressibility measurements reach white dwarf pressures

20 August 2020

With spherically converging shock waves, researchers can probe material properties under extreme conditions.

Much of the baryonic matter in the universe—the interiors of planets, stars, and especially dense stellar remnants such as white dwarfs—exists in forms unlike anything we encounter on Earth’s surface. At pressures above 1 Mbar (a million times the pressure of Earth’s atmosphere, but less than a third of the pressure at the planet’s core), chemical bonds start to be ripped apart. Above 100 Mbar, even the core electrons in elements such as carbon are torn away from their nuclei.

Diagram of the NIF lasers
Credit: Courtesy of Lawrence Livermore National Laboratory

Those changes naturally influence material properties, including how much a material compresses in response to additional pressure. And those properties, in turn, manifest in observable behavior, such as the brightness variations in pulsating white dwarfs. To accurately model the pulsations and other aspects of stellar evolution, researchers need to understand how matter behaves under extreme pressure.

The best chance of accessing those pressures in terrestrial experiments is through shock waves, fleeting pressure disturbances propagating through a material. (See the article by Paul Drake, Physics Today, June 2010, page 28.) But reaching the megabar regime requires concentrating an enormous amount of energy—from lasers, magnets, or other sources—into a few cubic millimeters of matter. Previous laboratory measurements of compressibility have probed only up to about 60 Mbar.

Now researchers working at Lawrence Livermore National Laboratory’s National Ignition Facility (NIF) have advanced that bound by nearly an order of magnitude. Designed for researching nuclear fusion, NIF uses lasers (whose beams are shown in blue in the figure) to deliver hundreds of kilojoules of energy to a centimeter-long cavity (dull gold) containing a sample (white). Key to the high pressures in the new work was launching the shock wave in a spherically convergent geometry. Although the shock takes just 9 ns to traverse the sample’s 1 mm radius, the researchers track its progress using streaked x-ray radiography, and they probe a continuum of pressures as the shock wave closes in on the sphere’s center. From their measurements, they deduce the carbonaceous sample’s compressibility curve from 100 Mbar up to 450 Mbar.

Even at the high end of that range, the experiment is far from simulating the interior of a white dwarf. But the pressures are typical of those found in a white dwarf’s convective outer envelope, the region most responsible for the pulsation modes. With new experimental constraints on the envelope’s compressibility, astronomers will be able to better model how pulsating white dwarfs form and behave. (A. L. Kritcher et al., Nature 584, 51, 2020.)

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