A New Phase in Neutron Stars that Promotes ‘Nuclear Pasta’ is Predicted by Scientists

Astrophysicists are unable to observe inside neutron stars, which are extreme and enigmatic objects. They have the potential to have more than twice the mass of the sun inside a radius of about 12 kilometers. Together with black holes, they are the densest objects in the universe because of the matter that is packed in them up to five times more densely than in an atomic nucleus.

Extreme circumstances can cause matter to take on unusual states. One theory is that protons and neutrons, the fundamental building blocks of atomic nuclei, deform into strings and plates like lasagna or spaghetti. For this reason, specialists refer to this phenomenon as “nuclear pasta.”

A new theoretical technique has been taken by researchers at the Niels Bohr Institute in Copenhagen and the Department of Physics at TU Darmstadt to study the condition of nuclear materials in the inner crust of neutron stars. It was demonstrated that protons and neutrons may both “drip out” of atomic nuclei and stabilize “nuclear pasta.” Physical Review Letters report on their findings.

When enormous stars explode in a supernova, their interior collapses and their outer shells are launched into space, giving rise to neutron stars. The strong gravitational pull practically crushes the atoms. In the atomic nucleus, the negatively charged electrons are forced so near to the positively charged protons that they are converted into neutrons despite their initial repulsion.

Then, additional collapse is prevented by the potent nuclear force. Approximately 95% of the resulting object is made up of neutrons, and the remaining 5% is composed of protons, creating a “neutron star.”

One area of expertise for the Darmstadt researchers under Achim Schwenk’s direction is theoretical nuclear physics, and one of their areas of interest is neutron stars. They are currently concentrating on these extreme objects’ crusts. Atomic nuclei are still present in the outer crust, but it is less dense than the inside.

An overabundance of neutrons forms in the atomic nuclei as the density rises. Under some conditions, neutrons may “drip” out of the nuclei; this is referred to as “neutron drip.” For this reason, atomic nuclei “swim” in a kind of neutron sauce.

According to Achim Schwenk, “We asked ourselves whether protons can drip out of the nuclei as well.” “The physicist goes on, “The literature was not clear on this question,” continues the physicist. The team with Jonas Keller and Kai Hebeler of TU Darmstadt and Christopher Pethick from the Niels Bohr Institute in Copenhagen has calculated the state of nuclear matter under the conditions in the neutron star crust.”

They determined its energy directly as a function of the proton percentage, unlike previously. They also accounted for the interactions between three nucleons and pairwise interactions between particles in their computations.

The strategy worked, as the scientists were able to show that protons in the inner crust likewise leak out of nuclei. Thus, “proton drip” is a real phenomenon. Protons make up this phase that coexists with neutrons.

“We were also able to show that this phase favors the phenomenon of nuclear pasta,” Schwenk says. The protons strewn throughout the “sauce” enable the nucleons to coexist more comfortably in the shapes of lasagna and spaghetti. As a result, the team was able to improve their understanding of nuclear materials within neutron star crust.

“The better we can describe neutron stars, the better we can compare with astrophysical observations,” Schwenk states. Astrophysics finds it hard to understand neutron stars. For instance, our sole indirect knowledge of their radius comes from the gravitational pull of another neutron star. In addition, other phenomena can be seen, like the radio radiation from neutron stars that pulses.

The team’s discovery advances our theoretical knowledge of neutron stars and helps us decipher these cosmic riddles through new astrophysical measurement data.

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