Thu. May 26th, 2022

Astrophysicists explain the origin of unusually heavy neutron star binaries

In the late stages of binary neutron star formation, the giant star expands and engulfs the neutron star’s companion in a stage referred to as co-envelope evolution (a). Exhaust of the envelope leaves the neutron star in a tight orbit with a stripped envelope star. The development of the system depends on the mass ratio. Less massive stripped stars experience an additional mass transfer phase that further removes the star and reuses the pulsar companion, leading to systems such as the observed binary neutron stars in the Milky Way and GW170817 (b). More massive stripped stars do not expand as much, and therefore further stripping and concomitant recycling are avoided, leading to systems like GW190425 (c). Finally, even more massive stripped stars will lead to black-hole neutron star binaries, such as GW200115 (d). Credit: Vigna-Gomez et al.

A new study showing how the explosion of a stripped massive star in a supernova can lead to the formation of a heavy neutron star or a bright black hole solves one of the most challenging puzzles that arise from the discovery of neutron star fusions of 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 mostly met the expectations of astrophysicists. But the second discovery, in 2019, was a fusion of two neutron stars whose total mass was unexpectedly large.

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

Compact astrophysical objects such as neutron stars and black holes are challenging to study because when they are stable, they tend to be invisible and emit no detectable radiation. “It means we are biased in what we can observe,” Ramirez-Ruiz explained. “We have found neutron star binaries 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 a fusion between heavy neutron stars at a rate similar to the lighter binary system implies that heavy neutron star pairs must be relatively common. So why do they not show up in the pulsar population?

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

The study, published Oct. 8 in Astrophysical journal letters, was led by Alejandro Vigna-Gomez, an astrophysicist at the University of Copenhagen’s Niels Bohr Institute, where Ramirez-Ruiz holds a Niels Bohr professorship.

“We used detailed star models to follow the evolution of a stripped star until it explodes into a supernova,” Vigna-Gomez said. “When we reach the time of the supernova, we do a hydrodynamic study where we are interested in following the evolution of the exploding gas.”

The stripped star, in a binary system with a neutron star companion, starts ten times more massive than our sun, but so close is it smaller than the sun in diameter. The final step in its evolution is a nuclear collapse supernova that leaves either a neutron star or a black hole, depending on the final mass of the nucleus.

The team’s results showed that when the massive stripped star explodes, some of its outer layers are quickly pushed out of the binary system. However, some of the inner layers are not pushed out and eventually fall back onto the newly formed compact object.

“The amount of material added depends on the explosive energy — the higher the energy, the less mass you can retain,” Vigna-Gomez said. “For our star with ten solar masses removed, if the explosion energy is low, it will form a black hole. If the energy is large, it will retain less mass and form a neutron star.”

These results not only explain the formation of heavy neutron star binary systems, such as that revealed by the gravitational wave event GW190425, but also predict the formation of neutron stars and light black hollow binaries, such as the one that fused in the gravitational pull of the 2020 wave event GW200115.

Another important finding is that the mass of the helium nucleus of the stripped star is crucial in determining the nature of its interactions with its neutron star mate and the ultimate fate of the binary system. A sufficiently massive helium star can avoid transferring mass to the neutron star. However, with a less massive helium star, the mass transfer process can transform the neutron star into a rapidly rotating pulsar.

“When the helium nucleus is small, it expands, and then mass transfer spins the neutron star up to create a pulsar,” Ramirez-Ruiz explained. “However, massive helium nuclei are more gravity bound and do not expand, so there is no mass transfer. And if they do not spin up into a pulsar, we will not see them.”

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

“Transferring mass to a neutron star is an effective mechanism for creating rapidly rotating (millisecond) pulsars,” Vigna-Gomez said. “Avoidance of this mass transfer episode as we are suggesting that there is a radio-silent population of such systems in the Milky Way.”

Scientists puzzle about massive star system

More information:
Enrico Ramirez-Ruiz et al., Fallback supernova collection of heavy binary neutron stars and light black hollow neutron star pairs and the common star family of GW190425 and GW200115, Astrophysical journal letters (2021). DOI: 10.3847 / 2041-8213 / ac2903

Provided by the University of California – Santa Cruz

Citation: Astrophysicists Explain the Origin of Extraordinarily Heavy Neutron Star Binaries (2021, October 8) Retrieved October 9, 2021 from

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