New simulations reveal hot neutrinos trapped in neutron star collisions

When stars collapse, they can leave behind incredibly dense but relatively small and cold remnants called neutron stars. If two stars collapse nearby, the remaining 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 (tiny, essentially massless particles that rarely interact with other matter) created during the collision can be briefly trapped at these interfaces and remain out of equilibrium with the cold nuclei of the molten stars for 2 to 4 hours. 3 milliseconds. Meanwhile, simulations show that neutrinos can interact weakly with star matter, helping to bring particles back toward equilibrium and opening new insights into the physics of these powerful events.

A paper describing the simulations, written by a research team led by Penn State physicists, appeared in the journal Physical exam letters.

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

“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 a merger of binary neutron stars goes through simulations based on mathematics from Einstein’s theory of general relativity.

Neutron stars get their name because they are thought to be composed almost entirely of neutrons, 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 across, but has about one and a half times the mass of our sun, which is about 1.4 million kilometers across. A teaspoon of neutron star material can weigh as much as a mountain, or tens or hundreds of millions of tons.

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

“When they collide they can become very hot, the interface of colliding stars can be heated to temperatures of several billion degrees Kelvin. However, they are so dense that photons cannot instead, we think they are so dense that photons cannot escape to dissipate heat by emitting neutrinos.

According to the researchers, neutrinos are created during the collision when neutrons from stars collide and are destroyed into protons, electrons and neutrinos. What happens next in the first moments after a collision is an open question in astrophysics.

To try to answer this question, the research team created simulations requiring enormous amounts of computing power that model the merger of binary neutron stars and all the associated physics. The simulations showed for the first time that, even briefly, even neutrinos can be trapped by the heat and density of fusion. Hot neutrinos are out of balance with the still-cold cores of stars and can interact with star matter.

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

“The period that merging stars are out of equilibrium is only 2 to 3 milliseconds, but like temperature, time here is relative, the orbital period of the two stars before the merger can be as short as 1 millisecond. -The equilibrium phase is where 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 impact the types of signals that can be observed on Earth when binary stars merge.

“How neutrinos interact with star matter and are ultimately emitted can impact the oscillations of the merged remnants of the two stars, which in turn can impact the appearance of electromagnetic signals and gravitational waves fusion when they reach us here on Earth,” Espino said.

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

In addition to Espino and Radice, the research team includes postdoctoral researchers Peter Hammond and Rossella Gamba of Penn State; Sebastiano Bernuzzi, Francesco Zappa and Luís Felipe Longo Micchi at the Friedrich-Schiller-Universität Jena in Germany; and Albino Perego at the University of Trento in Italy.