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 be more than twice the mass of the sun. The matter in them is packed up to five times more densely than in an atomic nucleus; with black holes, they are the most dense 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 plates and strings, similar to lasagna or spaghetti, which is why experts call this “nuclear pasta”.

Researchers at 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” from atomic nuclei and stabilize “nuclear noodles”. Their findings are reported in Physical review papers.

Neutron stars form when massive stars explode in a supernova: as the star’s outer shells are thrown into space, its interior collapses. Atoms are literally crushed by the massive gravitational force. Despite their repulsion, the negatively charged electrons are pressed so close to the positively charged protons in the atomic nucleus that they become neutrons.

The strong nuclear force then prevents further collapse. The result is an object that consists of about 95% neutrons and 5% 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. The matter in the outer crust is not as dense as in the interior and still has atomic nuclei.

As the density increases, an excess of neutrons develops in atomic nuclei. Neutrons can then “drip” from the nuclei, a phenomenon known as “neutron dripping”. Therefore, atomic nuclei “float” in a kind of neutron sauce.

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

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

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

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

“The better we describe neutron stars, the better we can compare with astrophysical observations,” says Schwenk. Neutron stars are difficult to understand astrophysically. For example, we know their radius only indirectly from gravitational effects on another neutron star. Additionally, 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 obtaining new insights into these mysteries of the universe from astrophysical measurements.