Scientists predict new phase of neutron stars that favors ‘nuclear pasta’

Neutron stars are extreme and mysterious objects that astrophysicists cannot see inside. With a radius of around 12 kilometers, they can have more than twice the mass of the Sun. The matter in them is packed up to five times as densely as in the atomic nucleus; with black holes they are the densest objects in the universe.

Under extreme conditions, matter can acquire exotic states. One hypothesis is that the building blocks of atomic nuclei – protons and neutrons – deform into plates and strings, much like lasagna or spaghetti, which is why experts call this “nuclear pasta”.

Scientists from TU Darmstadt’s Department of Physics and the Niels Bohr Institute in Copenhagen have now taken a new theoretical approach to investigating the state of nuclear matter in the inner crust of neutron stars. They showed that both neutrons and protons can ‘drip’ from atomic nuclei and stabilize ‘nuclear pasta’. Their findings are presented in Physical inspection letters.

Neutron stars are formed when massive stars explode in a supernova: while the star’s outer envelopes are ejected into space, its interior collapses. Atoms are literally crushed by the massive force of gravity. Despite their repulsion, the negatively charged electrons are pushed so close to the positively charged protons in the atomic nucleus that they are converted into neutrons.

The strong nuclear force will then prevent further collapse. The result is an object that consists of approximately 95% neutrons and 5% protons – a “neutron star”.

The Darmstadt scientists, led by Achim Schwenk, are experts in theoretical nuclear physics, with neutron stars as one of their research interests. In their current work, they focus on the bark of these extreme objects. Matter in the outer crust is not as dense as in the interior, and atomic nuclei still exist.

As the density increases, an excess of neutrons is created in atomic nuclei. Neutrons can then ‘drip’ from the nuclei, a phenomenon known as ‘neutron dripping’. Atomic nuclei therefore “float” in a kind of neutron sauce.

“We asked ourselves whether protons can also drip from nuclei,” says Achim Schwenk. “The literature has not been clear on this issue,” the physicist continues. A team with Jonas Keller and Kai Hebeler from TU Darmstadt and Christopher Pethick from the Niels Bohr Institute in Copenhagen calculated the state of nuclear matter under conditions in the crust of a neutron star.

Unlike before, they directly calculated its energy as a function of proton fraction. In addition, they included pairwise interactions between particles as well as interactions between three nucleons in their calculations.

The method was successful: The researchers were able to demonstrate that protons in the inner crust also drip from the cores. So the “proton drop” really exists. This phase consisting of protons coexists with neutrons.

“We were also able to demonstrate that this phase supports the nuclear pasta phenomenon,” says Schwenk. Thanks to protons sprinkled in the “sauce”, nucleons can better exist in the shape of spaghetti and lasagna. This allowed the team to refine the image of nuclear matter in the crust of neutron stars.

“The better we can describe neutron stars, the better we can compare with astrophysical observations,” says Schwenk. Neutron stars are difficult to grasp astrophysically. For example, we only know their radius indirectly from the gravitational effects on another neutron star. In addition, other phenomena such as pulsating radio emission from neutron stars can be observed.

The team’s result improves the theoretical understanding of neutron stars and contributes to new insights into these mysteries of the universe from astrophysical measurements.