Researchers Uncover New Insights into Neutron Star Matter

Paris, France (SPX) Jul 10, 2024 - Neutron stars, with their extreme densities and mysterious interiors, remain enigmatic to astrophysicists. Despite having a radius of around twelve kilometers, they can possess over twice the mass of the sun, with matter packed up to five times denser than an atomic nucleus. Alongside black holes, neutron stars are the densest objects in the universe. Under such intense conditions, matter can transform into exotic states, including a theorized formation known as "nuclear pasta," where protons and neutrons deform into plates and strings.

Scientists at the Department of Physics at TU Darmstadt and the Niels Bohr Institute in Copenhagen have adopted a novel theoretical approach to explore the state of nuclear matter within the inner crust of neutron stars. Their research has revealed that both neutrons and protons can "drip out" of atomic nuclei, stabilizing the nuclear pasta structure. This significant discovery was published in "Physical Review Letters."

Formation and Structure of Neutron Stars
Neutron stars originate from the explosive death of massive stars in supernova events. During this process, the outer layers of the star are expelled, while the core collapses under immense gravitational pressure. This crushing force fuses negatively charged electrons with positively charged protons, forming neutrons. The strong nuclear force then halts further collapse, resulting in a star composed predominantly of neutrons with a small fraction of protons.

Achim Schwenk and his team of theoretical nuclear physicists at Darmstadt have concentrated their research on the outer crust of neutron stars. Unlike the denser inner regions, the outer crust contains atomic nuclei and develops an excess of neutrons as density increases. These neutrons can "drip" out of the nuclei, creating a "neutron sauce" in which the atomic nuclei are immersed.

Discovery of Proton Drip
"We asked ourselves whether protons can drip out of the nuclei as well as," says Achim Schwenk. "The literature was not clear on this question," continues the physicist. Schwenk's team, including Jonas Keller and Kai Hebeler from TU Darmstadt and Christopher Pethick from the Niels Bohr Institute, calculated the state of nuclear matter in the neutron star crust, considering the energy as a function of proton fraction and interactions between nucleons.

Their method proved successful: the researchers demonstrated that protons also drip out of nuclei in the inner crust, confirming the existence of the "proton drip" phenomenon. "We were also able to show that this phase favors the phenomenon of nuclear pasta," says Schwenk. The inclusion of protons in the neutron-rich environment enhances the stability of nucleons in the form of spaghetti and lasagne shapes, refining our understanding of nuclear matter in neutron star crusts.

Implications for Astrophysical Observations
"The better we can describe neutron stars, the better we can compare with astrophysical observations," says Schwenk. Neutron stars present observational challenges; for instance, their radii are inferred indirectly through gravitational interactions with other neutron stars. Additional observable phenomena include pulsating radio emissions. The team's findings enhance theoretical models of neutron stars, facilitating new insights from astrophysical measurements.

Research Report:Neutron Star Matter as a Dilute Solution of Protons in Neutrons