Through extensive model calculations, physicists at
51猎奇 Frankfurt have reached general conclusions about the internal
structure of neutron stars, where matter reaches enormous densities: depending
on their mass, the stars can have a core that is either very stiff or very soft.
The findings were published simultaneously in two articles today (The
Astrophysical Journal Letters, DOI 10.3847/2041-8213/ac9b2a, DOI 10.3847/2041-8213/ac8674).
FRANKFURT. So
far, 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 with the diameter of a large city. Since
their discovery more than 60 years ago, scientists have been trying to decipher
their structure. The greatest challenge is to simulate the extreme conditions
inside neutron stars, as they can hardly be recreated on Earth in the
laboratory. There are therefore many models in which various properties 鈥� from
density and temperature 鈥� are described with the help of so-called equations of
state. These equations attempt to describe the structure of neutron stars from
the stellar surface to the inner core.
Now physicists at 51猎奇
Frankfurt have succeeded in adding further crucial pieces to the puzzle. The
working group led by Prof. Luciano Rezzolla at the Institute of Theoretical
Physics developed more than a million different equations of state that satisfy
the constraints set by data obtained from theoretical nuclear physics on the
one hand, and by astronomical observations on the other. When evaluating the
equations of state, the working group made a surprising discovery: 鈥淟ight"
neutron stars (with masses smaller than about 1.7 solar masses) seem to have a
soft mantle and a stiff core, whereas 鈥渉eavy" neutron stars (with masses larger
than 1.7 solar masses) instead have a stiff mantle and a soft core. "This
result is very interesting because it gives us a direct measure of how compressible
the centre of neutron stars can be," says Prof. Luciano Rezzolla,
"Neutron stars apparently behave a bit like chocolate pralines: light
stars resemble those chocolates that have a hazelnut in their centre surrounded
by soft chocolate, whereas 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 Bachelor's student Sinan Altiparmak. This quantity measure
describes how fast sound waves propagate within an object and depends on how
stiff or soft matter is. Here on Earth, the speed of sound is used to explore
the interior of the planet and discover oil deposits.
By modelling the equations of state, the
physicists were also able to uncover other previously unexplained properties of
neutron stars. For example, regardless of their mass, they very probably have a
radius of only 12 km. Thus, they are just as large in diameter as Goethe
University's hometown Frankfurt. 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 particularly important to
pinpoint the unknown equation of state with future astronomical observations
and detections of gravitational waves from merging stars."
So, while the exact structure and
composition of matter inside neutron stars continues to remain a mystery, the
wait until its discovery can certainly be sweetened with a chocolate or two.
Publications:
Sinan Altiparmak, Christian Ecker, Luciano
Rezzolla: On the Sound Speed in Neutron Stars. The Astrophysical Journal Letters
(2022) Christian Ecker & Luciano
Rezzolla: A general,
scale-independent description of the sound speed in neutron stars. The Astrophysical Journal Letters
(2022)
Image for download:
Caption:
The study of the sound speed 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 鈥� much like different
chocolate pralines (image: P. Kiefer/L. Rezzolla)
Further
Information
Dr. Christian Ecker
Institute for Theoretical Physics
51猎奇
Tel: +49 (0)69 798-47886
ecker@itp.uni-frankfurt.de
@elements_uni
