Cosmic chocolate pralines? General neutron star structure revealed

Cosmic chocolate pralines: general structure of neutron stars revealed

The study of the speed of sound has revealed that heavy neutron stars have a stiff mantle and a soft core, while light neutron stars have a soft mantle and a stiff core, much like various chocolate pralines. Credits: Peter Kiefer & Luciano Rezzolla

Until now, little is known about the interior of neutron stars, those extremely compact objects that can form after the death of a star. The mass of our sun or even more is compressed into a sphere the diameter of a large city. Since their discovery more than 60 years ago, scientists have been trying to decipher their structure.

The biggest challenge is to imitate the extreme conditions in neutron stars, because they can hardly be imitated in the laboratory on Earth. There are therefore many models in which different properties – of density and temperature – are described using so-called equations of state. These equations attempt to describe the structure of neutron stars from the surface of the star to the inner core.

Now physicists at Frankfurt’s Goethe University have managed to add even more crucial pieces to the puzzle. The working group, led by Prof. Luciano Rezzolla from the Institute of Theoretical Physics, has developed more than a million different equations of state that meet the constraints posed by data from theoretical nuclear physics on the one hand and astronomical observations on the other. other. Their work has been published in The Astrophysical Journal Letters.

In evaluating the equations of state, the working group made a surprising discovery: “light” neutron stars (with masses less than about 1.7 solar masses) appear to have a soft mantle and a rigid core, while “heavy” neutron stars (with masses greater than 1. .7 solar masses) instead have a rigid mantle and a soft core.

“This result is very interesting because it gives us a direct measure of how compressible the center of neutron stars can be,” says Prof. Luciano Rezzolla, “Neutron stars apparently behave a bit like chocolate pralines: light stars resemble chocolates with a hazelnut in the center surrounded by soft chocolate, while heavy stars can be considered more like those chocolates where a hard layer contains a soft filling.”

Crucial to this insight was the speed of sound, a study focus of undergraduate student Sinan Altiarmak. This measure of quantity describes how fast sound waves propagate within an object and depends on how stiff or soft the matter is. Here on Earth, the speed of sound is used to explore the planet’s interior and discover oil deposits.

Modeling the equations of state also allowed the physicists to uncover other previously unexplained properties of neutron stars. For example, regardless of their mass, they most likely have a radius of only 12 km. They are therefore as large in diameter as Frankfurt, the birthplace of Goethe University.

Study author Dr. Christian Ecker explains: “Our extensive numerical study not only allows us to make predictions for the radii and maximum masses of neutron stars, but also to set new limits on their deformability in binary systems, that is, how strongly they distort each other through their gravitational fields. These insights will become extremely important to localize the unknown equation of state with future astronomical observations and detections of gravitational waves from merging stars.”

More information:
On the speed of sound in neutron stars, The Astrophysical Journal Letters (2022). DOI: 10.3847/2041-8213/ac9b2a

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