Astrophysicists explain the origin of unusually heavy neutron star binaries

New study showing how the explosion of a massive naked star in a supernova can lead to the formation of a heavy neutron star or a light black hole solves one of the most difficult puzzles to emerge from detection mergers of neutron stars by the gravitational wave observatories LIGO and Virgo.

The first detection of gravitational waves by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) in 2017 was a fusion of neutron stars that was largely in line with the expectations of astrophysicists. But the second detection, in 2019, was a merger of two neutron stars with a surprisingly large combined mass.

“It was so shocking that we had to start thinking about how to create a heavy neutron star without making it a pulsar,” said Enrico Ramirez-Ruiz, professor of astronomy and astrophysics at UC Santa. Cruz.

Compact astrophysical objects like neutron stars and black holes are difficult to study because when stable they tend to be invisible, emitting no detectable radiation. “This means that we are biased in what we can observe,” explained Ramirez-Ruiz. “We have detected binaries of neutron stars in our galaxy when one of them is a pulsar, and the masses of these pulsars are almost all identical – we do not see any heavy neutron stars.”

LIGO’s detection of heavy neutron star fusion at a rate similar to that of the lighter binary system implies that heavy neutron star pairs should be relatively common. So why don’t they appear in the pulsar population?

In the new study, Ramirez-Ruiz and his colleagues focused on bare star supernovae in binary systems that can form “double compact objects” consisting of either two neutron stars or one neutron star and of a black hole. A bare star, also called a helium star, is a star whose hydrogen envelope has been removed by its interactions with a companion star.

The study, published on October 8 in Letters from astrophysical journals, was directed by Alejandro Vigna-Gomez, astrophysicist at the Niels Bohr Institute at the University of Copenhagen, where Ramirez-Ruiz holds a Niels Bohr chair.

“We used detailed stellar models to follow the evolution of a naked star until the moment it explodes in a supernova,” Vigna-Gomez said. “Once we hit the supernova time, we do a hydrodynamic study, where we are interested in following the evolution of the exploding gas.”

The bare star, in a binary system with a companion neutron star, is initially ten times as massive as our sun, but so dense that it is smaller than the sun in diameter. The final stage in its evolution is a core-collapsing supernova, which leaves behind either a neutron star or a black hole, depending on the final mass of the core.

The team’s results showed that when the massive bare star explodes, some of its outer layers are quickly ejected from the binary system. Some of the inner layers, however, are not ejected and eventually fall back onto the newly formed compact object.

“The amount of material accumulated depends on the energy of the explosion – the higher the energy, the less mass you can conserve,” Vigna-Gomez said. “For our star stripped of ten solar masses, if the explosion energy is low, it will form a black hole; if the energy is large, it will keep less mass and form a neutron star.”

These results not only explain the formation of binary heavy neutron star systems, like that revealed by the gravitational wave event GW190425, but also predict the formation of binary light neutron stars and black holes, like the one which merged into the gravitational cycle of 2020. wave event GW200115.

Another important finding is that the mass of the denuded star’s helium nucleus is critical in determining the nature of its interactions with its fellow neutron star and the ultimate fate of the binary system. A sufficiently massive helium star can avoid transferring mass to the neutron star. With a less massive helium star, however, the mass transfer process can transform the neutron star into a rapidly rotating pulsar.

“When the helium nucleus is small, it expands, and then the mass transfer causes the neutron star to spin to create a pulsar,” Ramirez-Ruiz explained. “However, massive helium nuclei are more gravitationally bound and don’t expand, so there’s no mass transfer. And if they don’t spin into a pulsar, we don’t see them.”

In other words, there could well be a large undetected population of heavy neutron star binaries in our galaxy.

“Mass transfer on a neutron star is an efficient mechanism for creating rapidly spinning (millisecond) pulsars,” said Vigna-Gomez. “Avoid this mass transfer episode as we suggest clues that there is a radio-silent population of such systems in the Milky Way.”