Neutron stars: two variants of the interior – Depending on their mass, neutron stars have a hard or soft core

Surprising differences: Neutron stars may look the same on the outside, but on the inside they are not; there are apparently two fundamentally different types, as one study suggests. According to this, the interior of the lightest neutron stars with less than 1.7 solar masses is soft on the outside and hard on the inside, while the opposite is true for heavier specimens. In the future, these differences could even be detected by gravitational wave detectors.

A neutron star forms when a massive star explodes in a supernova at the end of its life cycle. What remains is a stellar core in which the mass of one or two suns is compressed to a diameter of just 20 to 30 kilometers. Their enormous gravity makes neutron stars the most perfect spheres in the cosmos. Inside, the density and pressure are so high that even atoms disintegrate, leaving only neutrons. As a result, an exotic, superfluid state of matter is formed in the nucleus – this is the assumption.

Sound waves as tracks
However, the nature of the interior of neutron stars can only be reconstructed theoretically so far. Because the extreme conditions inside these stellar remnants cannot be reproduced in terrestrial laboratories. Therefore, there are currently many different physical models that try to describe the structure of neutron stars, from the surface to the inner core, with the help of so-called equations of state.

An important quantity in these equations is the speed of sound in dense matter. Because it provides information about how stiff and therefore hard the medium is. In the case of neutron stars, this can be used to determine how well the dense, compressed matter can withstand the enormous pressure of gravity. The longer, and therefore stiffer, the exotic mix of neutrons at the core of the stellar remnant, the faster sound will propagate in it.

For their study, physicists led by Luciano Rezzolla of the Goethe University of Frankfurt have constructed more than a million equations of state for the interiors of neutron stars and have compared them with astronomical observations and nuclear physics parameters.

The maximum speed is the same, the position of the maximum is different
The result: Unsurprisingly, sound waves don’t travel at maximum speed near the surface of neutron stars because the matter there is less dense. As the calculations showed, its behavior there is also largely independent of the mass of the neutron star. However, this changes as depth increases. The speed of the sound waves increases towards the interior until reaching a maximum value. Above a certain mass, this is the same for all neutron stars, as the team reports.

What is surprising, however, is that the position of this maximum is highly dependent on the mass of the neutron star. In light stellar remnants up to 1.7 solar masses, sound waves are fastest in the core; therefore, it is the hardest and most rigid part of the neutron star. But as the mass of the neutron star increases, the maximum of the wave wave approaches the surface. For heavy specimens, it’s even in the outer layers, the researchers determined. These stellar remnants are therefore harder on the outside than on the inside.

According to this, neutron stars have a very different structure depending on their mass. Rezzola likens this to the consistency of different types of chocolate: “Light neutron stars resemble pralines with a hard nut covered in smooth chocolate. The heavy stars, on the other hand, are more like bonbons with a hard chocolate shell and a creamy, smooth filling,” explains the researcher. “This is an extremely interesting result because it sheds light on how compressible a neutron star’s core can be.”

Detectable via gravitational waves?
Also exciting: So far, these findings are based only on theoretical models, but there might be a chance to verify them through astronomical observations, as Rezzola and his team report. Because, as they have determined, the rigidity of a neutron star could be revealed in collisions of such stellar remnants. “This property is related to a value that can be directly measured in gravitational waves,” explain the astrophysicists.

In this way, gravitational wave detectors such as LIGO, Virgo and KAGRA could help to find out more about the internal structure of neutron stars and to specify common theoretical models: “These findings will play a particularly important role in the elaboration of the equation currently State unknown more precise with future gravitational wave measurements of neutron star collisions to determine,” says co-author Christian Ecker of Trinity College Dublin. (The Astrophysical Journal Letters, 2022; doi: 10.3847/2041-8213/ac9b2a; doi: 10.3847/2041-8213/ac8674)

Source: Goethe University of Frankfurt am Main