Using more than 10 million randomly generated equations of state that satisfy nuclear theory and astronomical observations, astrophysicists from the Institut für Theoretische Physik at Goethe Universität and elsewhere constructed a novel description of the sound speed in neutron stars. Their results suggest that ‘heavy’ neutron stars have a stiff mantle and a soft core, while ‘light’ neutron stars have a soft mantle and a stiff core.
Ecker et al. suggest that light (heavy) stars have stiff (soft) cores and soft (stiff) outer layers. Image credit: Peter Kiefer / Luciano Rezzolla.
“We 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,” explained Goethe Universität’s Professor Luciano Rezzolla and colleagues.
“When evaluating the equations of state, we made a surprising discovery: light neutron stars (with masses smaller than about 1.7 solar masses) seem to have a soft mantle and a stiff core, whereas heavy 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 center of neutron stars can be.”
“Neutron stars apparently behave a bit like chocolate pralines: light stars resemble those chocolates that have a hazelnut in their center 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,” they added.
“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 modeling the equations of state, the physicists were also able to uncover previously unexplained internal properties of neutron stars.
For example, regardless of their mass, these compact objects very probably have a radius of only 12 km.
“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,” said Goethe Universität’s Dr. Christian Ecker.
“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.”
The findings appear in two papers in the Astrophysical Journal Letters.
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Christian Ecker & Luciano Rezzolla. 2022. A General, Scale-independent Description of the Sound Speed in Neutron Stars. ApJL 939, L35; doi: 10.3847/2041-8213/ac8674
Sinan Altiparmak et al. 2022. On the Sound Speed in Neutron Stars. ApJL 939, L34; doi: 10.3847/2041-8213/ac9b2a
