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Astrophysicists clarify the origin of unusually heavy neutron star binaries

Astrophysicists explain 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 an in depth orbit with a stripped-envelope star. The evolution of the system relies on the mass ratio. Less-massive stripped stars expertise a further mass switch section that additional strips the star and recycles the pulsar companion, resulting in techniques such because the noticed binary neutron stars within the Milky Way and GW170817 (b). More large stripped stars don’t broaden as a lot, subsequently avoiding additional stripping and companion recycling, resulting in techniques equivalent to GW190425 (c). Finally, much more large stripped stars with will result in black hole-neutron star binaries equivalent to GW200115 (d). Credit:Vigna-Gomez et al.

A brand new examine 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 gap resolves one of the crucial 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 neutron star merger that largely conformed to the expectations of astrophysicists. But the second detection, in 2019, was a merger of two neutron stars whose mixed mass was unexpectedly massive.

“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 black holes are difficult to check as a result of when they’re steady they are typically 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 charge much like the lighter binary system implies that heavy neutron star pairs ought to be comparatively frequent. So why do not they present up within the pulsar inhabitants?

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

The examine, revealed 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 solar, however so dense it’s smaller than the solar in diameter. The last stage in its evolution is a core-collapse supernova, which leaves behind both a neutron star or a black gap, relying on the ultimate mass of the core.

The group’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 internal layers, nonetheless, are usually not ejected and ultimately fall again onto the newly fashioned 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 techniques, such because the one revealed by the gravitational wave occasion GW190425, but in addition predict the formation of neutron star and lightweight black gap 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 last 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, nonetheless, 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 pulsar, we don’t see them.”

In different phrases, there could 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 mass transfer episode as we suggest hints that there is a radio-quiet population of such systems in the Milky Way.”

Scientists puzzle over large star system

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

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University of California – Santa Cruz

Astrophysicists clarify the origin of unusually heavy neutron star binaries (2021, October 8)
retrieved 9 October 2021

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