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When stars collapse, they can leave behind incredibly dense but relatively small and cold remnants called neutron stars. If two stars collapse in close proximity, the remnant binary neutron stars spiral and eventually collide, and the interface where the two stars begin to merge becomes incredibly hot.

New simulations of these events show that hot neutrinos – small, essentially massless particles that rarely interact with other matter – that are created during the collision can be briefly trapped at these interfaces and remain out of equilibrium with the cold nuclei of merging stars in 2 to 3 milliseconds. During this time, simulations show that neutrinos can weakly interact with the stars’ matter, helping to bring the particles back into equilibrium and providing new insight into the physics of these powerful events.

A paper describing the simulations, by a research team led by Penn State physicists, appeared in the journal Letters for physical examinations.

“For the first time in 2017, we observed here on Earth signals of various kinds, including gravitational waves, from a binary neutron star merger,” said Pedro Luis Espino, a postdoctoral researcher at Penn State and UC Berkeley who led the study.

“This has led to a huge surge of interest in the astrophysics of binary neutron stars. There is no way to reproduce these events in a laboratory to study them experimentally, so the best window we have to understand what happens during the merger of binary neutron stars, is through simulations based on mathematics derived from Einstein’s theory of general relativity.”

Neutron stars get their name because they are thought to be composed almost entirely of neutrons, the uncharged particles that, along with positively charged protons and negatively charged electrons, make up atoms. Their incredible density — only black holes are smaller and denser — is thought to squeeze protons and electrons together, fusing them into neutrons.

A typical neutron star is only tens of kilometers in diameter, but has about one and a half times the mass of our sun, which is about 1.4 million kilometers in diameter. A teaspoon of material from a neutron star can weigh as much as a mountain, tens or hundreds of millions of tons.

“Pre-merger neutron stars are essentially cold, although they may be billions of degrees Kelvin, their incredible density means that this heat contributes very little to the energy of the system,” said David Radiss, assistant professor of physics and astronomy and astrophysics at Eberly College of Science at Penn State and leader of the research team.

“As they collide, they can get really hot, the interface of the colliding stars can heat up to temperatures in the trillions of degrees Kelvin. However, they are so dense that photons cannot escape to dissipate the heat; instead, we think they cool by emitting neutrinos.”

According to the researchers, neutrinos are created during the collision as neutrons in the stars smash into each other and decay into protons, electrons and neutrinos. What happens in those first moments after a collision is an open question in astrophysics.

To try to answer this question, the research team created simulations requiring massive amounts of computing power that modeled the merger of binary neutron stars and all the physics involved. The simulations showed for the first time that, albeit briefly, even neutrinos can be trapped by the heat and density of the merger. Hot neutrinos are out of equilibrium with the still cool cores of the stars and can interact with the stellar matter.

“These extreme events push the boundaries of our understanding of physics, and studying them allows us to learn new things,” Radis said.

“The period that the merging stars are out of equilibrium is only 2 to 3 milliseconds, but like temperature, time is relative here, the orbital period of the two stars before the merger can be as little as 1 millisecond. This short out-of-equilibrium phase is when the most interesting physics occurs. Once the system returns to equilibrium, the physics is better understood.”

The researchers explained that the precise physical interactions that occur during the merger can affect the types of signals that can be observed on Earth from merging binary stars.

“The way neutrinos interact with the stars’ matter and are ultimately emitted can affect the oscillations of the merging remnants of the two stars, which in turn can affect what the electromagnetic and gravitational wave signals from the merger look like , when they reach us here on Earth,” Espino said.

“Next-generation gravitational wave detectors can be designed to look for these kinds of signal differences. In this way, these simulations play a crucial role in allowing us to gain insight into these extreme events while informing future experiments and observations in a kind of feedback loop.” cycle.”

In addition to Espino and Radis, the research team includes postdoctoral fellows Peter Hammond and Rosella Gamba of Penn State; Sebastiano Bernuzzi, Francesco Zappa and Luis Felipe Longo Miki at the Friedrich-Schiller-University of Jena in Germany; and Albino Perego at the Università di Trento in Italy.