In the late phases of binary neutron star development, the giant star expands and engulfs the neutron star buddy in a stage referred to as common-envelope evolution (a). Ejection of the envelope leaves the neutron star in a close orbit with a stripped-envelope star. Less-massive removed stars experience an extra mass transfer stage that even more strips the star and recycles the pulsar companion, leading to systems such as the observed binary neutron stars in the Milky Way and GW170817 (b). “We have identified neutron star binaries in our galaxy when one of them is a pulsar, and the masses of those pulsars are practically all similar– we do not see any heavy neutron stars.”
Another crucial finding is that the mass of the helium core of the stripped star is important in determining the nature of its interactions with its neutron star buddy and the supreme fate of the binary system.
In the late stages of binary neutron star development, the giant star expands and swallows up the neutron star buddy in a stage described as common-envelope evolution (a). Ejection of the envelope leaves the neutron star in a close orbit with a stripped-envelope star. The development of the system depends on the mass ratio. Less-massive removed stars experience an additional mass transfer phase that even more strips the star and recycles the pulsar buddy, causing systems such as the observed binary neutron stars in the Milky Way and GW170817 (b). More huge removed stars do not expand as much, for that reason avoiding further stripping and companion recycling, leading to systems such as GW190425 (c). Even more massive removed stars with will lead to black hole-neutron star binaries such as GW200115 (d). Credit: Vigna-Gomez et al., ApJL 2021
Astrophysicists Explain the Origin of Unusually Heavy Neutron Star Binaries
Simulations of supernova explosions of massive stars coupled with neutron stars can describe puzzling results from gravitational wave observatories.
A new research study showing how the surge of a removed enormous star in a supernova can lead to the development of a heavy neutron star or a light great void solves among the most tough 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 mostly complied with the expectations of astrophysicists. The second detection, in 2019, was a merger of 2 neutron stars whose combined mass was suddenly big.
” It was so stunning that we had to begin considering how to produce 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 challenging to study since when they are stable they tend to be unnoticeable, producing no detectable radiation. “That means we are prejudiced in what we can observe,” Ramirez-Ruiz described. “We have spotted neutron star binaries in our galaxy when among them is a pulsar, and the masses of those pulsars are practically all similar– we do not see any heavy neutron stars.”
LIGOs detection of a heavy neutron star merger at a rate comparable to the lighter double star suggests that heavy neutron star sets should be reasonably typical. Why do not they show up in the pulsar population?
In the brand-new study, Ramirez-Ruiz and his associates focused on the supernovae of stripped stars in binary systems that can form “double compact objects” including either 2 neutron stars or a neutron star and a great void. A stripped star, likewise called a helium star, is a star that has had its hydrogen envelope removed by its interactions with a buddy star.
The research study, published on October 8, 2021, in Astrophysical Journal Letters, was led by Alejandro Vigna-Gomez, an astrophysicist at the University of Copenhagens Niels Bohr Institute, where Ramirez-Ruiz holds a Niels Bohr Professorship.
” We used comprehensive excellent models to follow the advancement of a stripped star until the minute it blows up in a supernova,” Vigna-Gomez said. “Once we reach the time of the supernova, we do a hydrodynamical research study, where we have an interest in following the evolution of the taking off gas.”
The removed star, in a double star with a neutron star companion, starts ten times more enormous than our sun, however so dense it is smaller sized than the sun in diameter. The last in its development is a core-collapse supernova, which leaves behind either a neutron star or a black hole, depending upon the final mass of the core.
The teams outcomes showed that when the huge removed star explodes, a few of its outer layers are rapidly ejected from the double star. A few of the inner layers, however, are not ejected and ultimately fall back onto the recently formed compact things.
” The quantity of material accreted depends upon the explosion energy– the higher the energy, the less mass you can keep,” Vigna-Gomez stated. “For our ten-solar-mass removed star, if the surge energy is low, it will form a black hole; if the energy is big, it will keep less mass and form a neutron star.”
These results not only explain the formation of heavy neutron star double stars, such as the one revealed by the gravitational wave event GW190425, however likewise forecast the development of neutron star and light black hole binaries, such as the one that merged in the 2020 gravitational wave occasion GW200115.
Another crucial finding is that the mass of the helium core of the stripped star is necessary in determining the nature of its interactions with its neutron star buddy and the ultimate fate of the double star. An adequately huge helium star can prevent transferring mass onto the neutron star. With a less huge helium star, however, the mass transfer process can change the neutron star into a rapidly spinning pulsar.
” When the helium core is small, it expands, and after that mass transfer spins up the neutron star to create a pulsar,” Ramirez-Ruiz discussed. “Massive helium cores, nevertheless, are more gravitationally bound and do not broaden, so there is no mass transfer. And if they do not spin up into a pulsar, we do not see them.”
Simply put, there might well be a big unnoticed population of heavy neutron star binaries in our galaxy.
” Transferring mass onto a neutron star is an effective system to create rapidly spinning (millisecond) pulsars,” Vigna-Gomez stated. “Avoiding this mass transfer episode as we recommend tips that there is a radio-quiet population of such systems in the Milky Way.”
Referral: “Fallback Supernova Assembly of Heavy Binary Neutron Stars and Light Black Hole– Neutron Star Pairs and the Common Stellar Ancestry of GW190425 and GW200115” by Alejandro Vigna-Gómez, Sophie L. Schrøder, Enrico Ramirez-Ruiz, David R. Aguilera-Dena, Aldo Batta, Norbert Langer and Reinhold Willcox, 8 October 2021, Astrophysical Journal Letters.DOI: 10.3847/ 2041-8213/ ac2903.
In addition to Vigna-Gomez and Ramirez-Ruiz, the coauthors of the paper include Sophie Schroder at the Niels Bohr Institute; David Aguilera-Dena at the University of Crete; Aldo Batta at the National Institute of Astrophysics in Mexico; Norbert Langer at the University of Bonn, Germany; and Reinhold Willcox at Monash University, Australia. This work was supported by the Heising-Simons Foundation, the Danish National Research Foundation, and the U.S. National Science Foundation.
