Experiments replicate high densities in 'white dwarf' stars
Findings could shed light on creating new materials on Earth

Date:
August 17, 2020
Source:
University of Rochester
Summary:
Engineers have simulated the crushing pressure created as white
dwarf stars cease to produce their own fuel, leaving only an
extremely dense core. The results add to growing evidence about
the evolution of astrophysical bodies -- and possible approaches
to creating novel materials in laboratories on Earth.



FULL STORY ==========================================================================
For the first time, researchers have found a way to describe conditions
deep in the convection zone of "white dwarf" stars, which are home to
some of the densest collections of matter in the Universe.


==========================================================================
In a project conducted at the National Ignition Facility at Lawrence
Livermore National Laboratory, the research team, including University
of Rochester engineering professor Gilbert (Rip) Collins, simulated
the crushing pressure created as stars cease to produce their own fuel,
leaving only an extremely dense core.

"This is the first time we have been able to lock down an equation of
state, describing the behavior of matter that is intrinsic to white
dwarf stars, in particular the regime in a part of white dwarfs where oscillations occur that have been particularly difficult to model,"
says Collins, who was a coauthor on the team's paper published in Nature.

Collins is the director of science, technology, and academics at the
Laboratory for Laser Energetics and is the Tracy Hyde Harris Professor
of Mechanical Engineering and is a professor in the Department of Physics
and Astronomy.

The results are important because they add to the growing body of evidence being collected by high-energy-density researchers about the formation
and evolution of planets, stars, and other astrophysical bodies, which
in turn can suggest possible approaches to creating novel materials in laboratories on Earth.

"Decades ago, underground nuclear tests made a couple of measurements
in a similar regime, but now we're able to do this with a much higher
level of accuracy and precision," says Collins.

Inwardly converging shock waves White dwarf stars, sometimes called
"star corpses" in popular literature, are what stars like our sun become
after they have exhausted their nuclear fuel and expelled most their
outer material. The process leaves behind a hot core that cools down
over the next billion years or so, according to information from NASA's
Goddard Space Flight Center. A white dwarf star the size of the Earth
is 200,000 times as dense.

The density is achieved when the star is no longer able to create
internal, outwardly directed pressure, because fusion has ceased. As
that happens, gravity compacts the star's matter inward until even the electrons that compose the dwarf star's atoms are smashed together. One
recent analysis has suggested that white dwarf stars are an important
source of carbon found in galaxies.

To study the process, researchers fired nanometer laser light into a
hohlraum - - a tiny gold cylinder -- bathing a spherical 1 mm sample
of a carbon-based compound known as CH (methylidyne) in x-ray radiation
heated to nearly 3.5 million degrees, at pressures ranging from 100 to
450 million atmospheres.

The experiments described in the paper simulate what happens in
hot DQ white dwarf stars, first discovered in 2007, which contain a
carbon and oxygen core surrounded by an envelope, or atmosphere, of
mostly carbon. The researchers focused specifically on replicating the
high pressure regimes that occur in an area of oscillating pulsations
where previous attempts to model the behavior of matter have produced inconsistent results.

The paper describes how the x-ray radiation bath in the hohlraum is
absorbed by an outer region (ablator) of the spherical fuel sample, which
heats and expands, launching inwardly converging shock waves toward the
center of sphere.

The shocks coalesce into a single strong shock, traveling at a speed
of 150 to 220 kilometers per second and traversing the sample in about
9 nanoseconds.


========================================================================== Story Source: Materials provided by University_of_Rochester. Original
written by Bob Marcotte. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Andrea L. Kritcher, Damian C. Swift, Tilo Do"ppner, Benjamin
Bachmann,
Lorin X. Benedict, Gilbert W. Collins, Jonathan L. DuBois, Fred
Elsner, Gilles Fontaine, Jim A. Gaffney, Sebastien Hamel, Amy
Lazicki, Walter R.

Johnson, Natalie Kostinski, Dominik Kraus, Michael J. MacDonald,
Brian Maddox, Madison E. Martin, Paul Neumayer, Abbas Nikroo, Joseph
Nilsen, Bruce A. Remington, Didier Saumon, Phillip A. Sterne, Wendi
Sweet, Alfredo A. Correa, Heather D. Whitley, Roger W. Falcone,
Siegfried H.

Glenzer. A measurement of the equation of state of carbon
envelopes of white dwarfs. Nature, 2020; 584 (7819): 51 DOI:
10.1038/s41586-020-2535-y ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200817123039.htm

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