HomeNewsAstronomy and SpaceAstrophysicists clarify the origin of unusually heavy neutron star binaries

Astrophysicists clarify the origin of unusually heavy neutron star binaries

In the late levels of binary neutron star formation, the enormous star expands and engulfs the neutron star companion in a stage known as common-envelope evolution (a). Ejection of the envelope leaves the neutron star in a detailed orbit with a stripped-envelope star. The evolution of the system will depend on the mass ratio. Less-massive stripped stars expertise an extra mass switch phase that additional strips the star and recycles the pulsar companion, resulting in programs such because the noticed binary neutron stars within the Milky Way and GW170817 (b). More large stripped stars don’t develop as a lot, due to this fact avoiding additional stripping and companion recycling, resulting in programs corresponding to GW190425 (c). Finally, much more large stripped stars with will result in black hole-neutron star binaries corresponding to GW200115 (d). Credit:Vigna-Gomez et al.

A brand new research exhibiting how the explosion of a stripped large star in a supernova can result in the formation of a heavy neutron star or a lightweight black hole resolves some of the difficult puzzles to emerge from the detection of neutron star mergers 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 star merger that principally conformed to the expectations of astrophysicists. But the second detection, in 2019, was a merger of two whose mixed mass was unexpectedly giant.

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

Compact astrophysical objects like neutron stars and are difficult to check as a result of when they’re secure they are usually invisible, emitting no detectable radiation. “That means we are biased in what we can observe,” Ramirez-Ruiz defined. “We have detected neutron star binaries in our galaxy when one of them is a pulsar, and the masses of those pulsars are almost all identical—we don’t see any heavy neutron stars.”

LIGO’s detection of a heavy neutron star merger at a price much like the lighter binary system implies that heavy neutron star pairs must be comparatively widespread. So why do not they present up within the pulsar inhabitants?

In the brand new research, Ramirez-Ruiz and his colleagues centered on the supernovae of stripped stars in binary programs that may kind “double compact objects” consisting of both two neutron or a neutron star and a black hole. A stripped star, additionally known as a helium star, is a star that has had its hydrogen envelope eliminated by its interactions with a .

The research, printed October 8 in Astrophysical Journal Letters, was led by Alejandro Vigna-Gomez, an astrophysicist on the University of Copenhagen’s Niels Bohr Institute, the place Ramirez-Ruiz holds a Niels Bohr Professorship.

“We used detailed stellar models to follow the evolution of a stripped star until the moment it explodes in a supernova,” Vigna-Gomez mentioned. “Once we reach the time of the supernova, we do a hydrodynamical 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, begins out ten occasions extra large than our sun, however so dense it’s smaller than the sun in diameter. The remaining stage in its evolution is a core-collapse supernova, which leaves behind both a neutron star or a black hole, relying on the ultimate mass of the core.

The workforce’s outcomes confirmed that when the large stripped star explodes, a few of its outer layers are quickly ejected from the binary system. Some of the interior layers, nevertheless, should not ejected and finally fall again onto the newly shaped compact object.

“The amount of material accreted depends on the explosion energy—the higher the energy, the less mass you can keep,” Vigna-Gomez mentioned. “For our ten-solar-mass stripped star, 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 outcomes not solely clarify the formation of heavy neutron star binary programs, such because the one revealed by the gravitational wave occasion GW190425, but in addition predict the formation of neutron star and light-weight black hole binaries, such because the one which merged within the 2020 gravitational wave occasion GW200115.

Another essential discovering is that the mass of the helium core of the stripped star is important in figuring out the character of its interactions with its neutron star companion and the final word destiny of the binary system. A sufficiently large helium star can keep away from transferring mass onto the neutron star. With a much less large helium star, nevertheless, the mass switch course of can remodel the neutron star right into a quickly spinning pulsar.

“When the helium core is small, it expands, and then mass transfer spins up the neutron star to create a pulsar,” Ramirez-Ruiz defined. “Massive helium cores, however, are more gravitationally bound and don’t expand, so there is no mass transfer. And if they don’t spin up into a , we don’t see them.”

In different phrases, there might be a big undetected inhabitants of heavy neutron star binaries in our galaxy.

“Transferring mass onto a neutron star is an effective mechanism to create rapidly spinning (millisecond) pulsars,” Vigna-Gomez mentioned. “Avoiding this transfer episode as we suggest hints that there is a radio-quiet population of such systems in the Milky Way.”

Scientists puzzle over massive star system

More info:
Enrico Ramirez-Ruiz et al, Fallback supernova meeting of heavy binary neutron stars and light-weight black hole-neutron star pairs and the widespread stellar ancestry of GW190425 and GW200115, Astrophysical Journal Letters (2021). DOI: 10.3847/2041-8213/ac2903

Astrophysicists clarify the origin of unusually heavy neutron star binaries (2021, October 8)
retrieved 9 October 2021
from https://phys.org/news/2021-10-astrophysicists-unusually-heavy-neutron-star.html

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