Astrophysicists explain the origin of unusually heavy neutron star binaries

The final stages of binary neutron stars formation see the giant star expanding and engulfing the neutron star companion. This stage is known as common-envelope development (a). The neutron star is left in close orbit by the stripped-envelope star after the envelope is ejected. The mass ratio is a key factor in the evolution of the system. The mass ratio of less-massive stripped star undergoes an additional mass transfer phase, which further strips the star and recycles its pulsar companion. This results in systems like the binary neutron stars observed in the Milky Way (b) and GW170817(c). Larger stripped stars don't expand as much and are therefore less likely to be stripped or recycled. This is why systems like GW190425 can be created (c). Black hole-neutron star binaries will be possible with even larger stripped stars, such as GW200115 (d). Credit:Vigna-Gomez et al.
This new study shows how supernova explosions can result in the formation of a neutron star or a black hole. It is one of the most difficult puzzles that has been solved by the gravitational wave observatories LIGO/Virgo.

In 2017, the Advanced Laser Interferometer Gravitational Wave Observatory (LIGO), detected gravitational waves for the first time. It was a neutron-star merger, which mostly met the expectations of astrophysicists. The second detection was made in 2019, however, by a merger between two neutron stars with a combined mass that was surprisingly high.

Enrico Ramirez Ruiz, professor at UC Santa Cruz of astronomy-astrophysics, said that the shock was so overwhelming that they had to think about ways to make a neutron star out of heavy neutrons.

Because they emit no radiation and are invisible when they are stable, compact astrophysical objects such as black holes or neutron stars are difficult to study. Ramirez Ruiz stated that this means that we are limited in what we can see. "We have found neutron star binaries within our galaxy when one is a Pulsar. The masses of those pulsars almost all match, so we don't see any neutron stars."

LIGO's detection at a rate comparable to that of the lighter binary system of a heavy-neutron star merger suggests that heavy neutron pair should be fairly common. Why don't they appear in the pulsar population, then?

Ramirez Ruiz and his collaborators focused their new study on supernovae that form stripped stars in binary systems. These can either be two neutron stars, or one neutron star and one black hole. A stripped star is also known as a helium-star, or a star whose hydrogen envelope has been removed by interactions with another star.

Alejandro Vigna Gomez, an astrophysicist from the University of Copenhagen's Niels Bohr Institute where Ramirez Ruiz holds a Niels Bohr Chair, led the study. It was published in Astrophysical Journal letters on October 8.

Vigna-Gomez stated that detailed stellar models were used to track the evolution of a star stripped until it explodes in supernovae. "Once we reach supernova time, we do hydrodynamical studies, where we are interested to follow the evolution of the exploding gases."

In a binary system, the stripped star is ten times larger than our sun but so dense that it is less than our sun's diameter. Its final stage of evolution is a supernova core-collapse, which, depending on its final mass, leaves behind either a nucleon star or black hole.

According to the team's findings, when the massive star stripped explodes, some of the outer layers are quickly ejected by the binary system. However, some of the inner layers are not ejected, and instead fall back onto the compact object.

Vigna-Gomez stated that the amount of material that is accreted will depend on the explosion energy. The higher the energy, it will keep less mass." For our ten-solar mass stripped star, if it has low explosion energy, it will form black holes. If it has high energy, it will retain more mass and become a neutron star.

These results are not only able to explain the formation heavy neutron star binary system, such the one revealed by gravitational waves event GW190425, they also predict the formation neutron star binaries and light black holes binaries, such the one that merged during the 2020 gravitational waves event GW200115.

Another important discovery is that the helium core mass of the stripped star is crucial in determining its interactions with its neutron companion star and the fate of the binary system. A sufficient helium star of sufficient mass can avoid the transfer of mass onto the neutron stars. However, a smaller helium star can prevent the mass transfer process from transforming the neutron star to a rapidly spinning Pulsar.

Ramirez Ruiz explained that helium cores shrink when they are small. Then mass transfer spins up neutron stars to create a pulsestar. Mass transfer is not possible for massive helium cores. They are gravitationally bound, don't expand, and therefore, they are less likely to expand. We don't see them if they don’t spin up into an pulsar.

This means that there could be an undetected large population of heavy neutron star binary stars in our galaxy.

Vigna-Gomez stated that "transferring mass onto neutron stars is an effective mechanism for creating rapidly spinning (milliseconds) pulsars." We suggest that this mass transfer episode should be avoided as it suggests that there may be a radio-quiet number of such systems in our Milky Way.

Continue reading Scientists solve mystery of massive star system

Enrico Ramirez Ruiz, Fallback supernova assembly heavy binary neutron star and light black hole–neutron star pair and the common stellar ancestry GW190425 & GW200115. Astrophysical Journal Letters (2021). Information from Astrophysical Journal Letters Enrico Ramirez Ruiz, Fallback supernova assembly heavy binary neutron star and light black hole–neutron star pair and the common stellar ancestry GW190425 & GW200115 (2021). DOI: 10.3847/2041-8213/ac2903