Researchers predict new phase in neutron stars that favors ‘nuclear pasta’

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

Under extreme conditions, matter can take on exotic states. One hypothesis is that the building blocks of atomic nuclei—protons and neutrons—deform into sheets and strings, similar to lasagna or spaghetti, which is why experts call it “nuclear pasta.”

Researchers from the Department of Physics at TU Darmstadt and the Niels Bohr Institute in Copenhagen have now adopted a new theoretical approach to investigate the state of nuclear matter in the inner crust of neutron stars. They showed that both neutrons and protons can “drip” out of atomic nuclei and stabilize the “nuclear paste.” Their findings are reported in Physical assessment letters.

Neutron stars are formed when massive stars explode in a supernova: while the outer shells of the star are hurled into space, the interior collapses. The atoms are literally crushed by the enormous force of gravity. Despite their repulsion, the negatively charged electrons are pressed so close to the positively charged protons in the atomic nucleus that they are transformed into neutrons.

The strong nuclear force then prevents further collapse. The result is an object that is about 95 percent neutrons and 5 percent protons—a “neutron star.”

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

As density increases, an excess of neutrons develops in the nuclei. Neutrons can then “drip” out of the nuclei, a phenomenon known as “neutron drip.” Atomic nuclei thus “swim” in a kind of neutron sauce.

“We wondered whether protons can drip out of the cores just as well as,” says Achim Schwenk. “The literature was unclear on this question,” the physicist continues. The 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 the conditions in the crust of neutron stars.

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

The method was successful: the researchers were able to show that protons in the inner crust also drip from the cores. “Proton drip” therefore does exist. This phase consisting of protons coexists with the neutrons.

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

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

The team’s results improve the theoretical understanding of neutron stars and contribute to gaining new insights into the mysteries of the universe through astrophysical measurements.