Scientists Discover Explosive Process That Creates Massive Neutron Stars

New research has revealed how a massive neutron star can be created by the supernova explosion of a star that has lost its outer layers of hydrogen. The same explosive process could also lead to the creation of a small black hole.

The investigation was inspired by the detection of gravitational waves by the Laser-Interferometer Gravitational-Wave Observatory (LIGO) from the event GW190425.

In April 2019, operators of Laser-Interferometer Gravitational-Wave Observatory (LIGO) detected gravitational waves from an event they labeled GW190425.

Investigating this event, they determined that it was the merger between two massive neutron stars.

This suggested to astronomers that mergers between such large neutron stars are about as common as collisions between their smaller counterparts. This means massive neutron stars should be fairly common in the Milky Way.

The problem is astronomers do not find examples of such massive neutron stars when they search for pulsars—rotating neutron stars that blast out pulses of radiation at regular intervals that make these objects visible.

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

Enrico Ramirez-Ruiz is one of the authors of a paper documenting the finding published in The Astrophysical Journal Letters.

Both black holes and neutron stars form when massive stars run out of fuel and can no longer support themselves against gravitational collapse. This collapse triggers a massive supernova that can often outshine every other star in the galaxy in which it originates.

Yet, despite this bright and very detectable birthing event, once formed, black holes and neutron stars are difficult to spot because, if they are stable and not gobbling matter, they emit no detectable radiation.

"That means we are biased in what we can observe," Ramirez-Ruiz explained. "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."

To solve the problem of how massive neutron star binaries or light black hole pairings form, the team of researchers led by astrophysicist at the University of Copenhagen's Niels Bohr Institute, Alejandro Vigna-Gómez, investigated what happens when a "stripped star" explodes.

A stripped star, or a helium star, designates a star that has lost its outer hydrogen envelope as a result of an interaction with its partner star in a binary pair.

A binary pair of stars designates stellar bodies that are formed at the same time and go on to orbit around each other. Sometimes, one of these stars steals material from the other and becomes a compact stellar remnant—a black hole or a neutron star.

In some systems, this can lead to binary pairs of neutron stars, black holes, or even a mixed pairing of a black hole and a neutron star. When these objects spiral together it leads to mergers.

"We used detailed stellar models to follow the evolution of a stripped star until the moment it explodes in a supernova," Vigna-Gómez said. "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 team considered a stripped star with a mass around 10 times that of the Sun but denser and therefore smaller than our star, in a binary system with a neutron star.

The team's research showed that when the stripped star goes supernova, its outer layers and some of its inner layers are shed. While the outer layers are blown out of the binary system, the inner layers remain close and eventually fall back to the exploded star, now a compact stellar remnant. This remnant is either a neutron star or a black hole, depending on its starting mass.

"The amount of material accreted depends on the explosion energy—the higher the energy, the less mass you can keep," Vigna-Gómez said. "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."

The team also found that if the stripped star's helium core is large enough it can prevent its material from being transferred to its neutron star partner. In a system with a more diminutive stripped star, however, material is transferred to the neutron star carrying with it angular momentum creating a pulsar.

"When the helium core is small, it expands, and then mass transfer spins up the neutron star to create a pulsar," Ramirez-Ruiz explained. "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."

Incidents in which this mass transfer is avoided lead to so-called "radio-quiet" massive neutron stars, implying that there could be a large population of such objects lurking in the Milky Way waiting to be discovered.

Colliding Neutron Stars
An artist's rendition of the merger of two massive neutron stars. Researchers have found a way to simulate the explosion that gives rise to such massive neutron stars. University of Warwick/Mark Garlick/ESO