Astrophysicists Explain the Origin of Unusually Heavy Neutron Star Binaries
Simulations of supernova explosions of massive stars paired with neutron stars can explain puzzling results from gravitational wave observatories.
A new study showing how the explosion of a stripped massive star in a supernova can lead to the formation of a heavy neutron starA neutron star is the collapsed core of a large (between 10 and 29 solar masses) star. Neutron stars are the smallest and densest stars known to exist. Though neutron stars typically have a radius on the order of just 10 – 20 kilometers (6 – 12 miles), they can have masses of about 1.3 – 2.5 that of the Sun.”>neutron star or a light black holeA black hole is a place in space where the pull of gravity is so strong not even light can escape it. Astronomers classify black holes into three categories by size: miniature, stellar, and supermassive black holes. Miniature black holes could have a mass smaller than our Sun and supermassive black holes could have a mass equivalent to billions of our Sun.”>black hole resolves one of the most challenging puzzles to emerge from the detection of neutron star mergers by the gravitational wave observatories LIGOThe Laser Interferometer Gravitational-Wave Observatory (LIGO) is a large-scale physics experiment and observatory supported by the National Science Foundation and operated by Caltech and MIT. It’s designed to detect cosmic gravitational waves and to develop gravitational-wave observations as an astronomical tool. It’s multi-kilometer-scale gravitational wave detectors use laser interferometry to measure the minute ripples in space-time caused by passing gravitational waves. It consists of two widely separated interferometers within the United States—one in Hanford, Washington and the other in Livingston, Louisiana.”>LIGO and Virgo.
The first detection of gravitational wavesGravitational waves are distortions or ripples in the fabric of space and time. They were first detected in 2015 by the Advanced LIGO detectors and are produced by catastrophic events such as colliding black holes, supernovae, or merging neutron stars.”>gravitational waves by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) in 2017 was a neutron star merger that mostly conformed to the expectations of astrophysicists. But the second detection, in 2019, was a merger of two neutron stars whose combined mass was unexpectedly large.
“It was so shocking that we had to start thinking about how to create a heavy neutron star without making it a pulsarFirst observed at radio frequencies, a pulsar is a rotating neutron star that emits regular pulses of radiation. Astronomers developed three categories for pulsars: accretion-powered pulsars, rotation-powered pulsars, and nuclear-powered pulsars; and have since observed them at X-ray, optical, and gamma-ray energies.”>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 because when they are stable they tend to be invisible, emitting 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.”
LIGO’s detection of a heavy neutron star merger at a rate similar to the lighter binary system implies that heavy neutron star pairs should be relatively common. So why don’t they show up in the pulsar population?
In the new study, Ramirez-Ruiz and his colleagues focused on the supernovae of stripped stars in binary systems that can form “double compact objects” consisting of either two neutron stars or a neutron star and a black hole. A stripped star, also called a helium star, is a star that has had its hydrogen envelope removed by its interactions with a companion star.
The study, published on October 8, 2021, in Astrophysical Journal Letters, was led by Alejandro Vigna-Gomez, an astrophysicist at the University of Copenhagen’s Niels Bohr Institute, where 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 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 stripped star, in a binary system with a neutron star companion, starts out ten times more massive than our sun, but so dense it is smaller than the sun in diameter. The final stage in its evolution is a core-collapse supernova, which leaves behind either a neutron star or a black hole, depending on the final mass of the core.
The team’s results showed that when the massive stripped star explodes, some of its outer layers are rapidly ejected from the binary system. Some of the inner layers, however, are not ejected and eventually fall back onto the newly formed compact object.
“The amount of material accreted depends on the explosion energy—the higher the energy, the less mass you can keep,” Vigna-Gomez 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.”
These results not only explain the formation of heavy neutron star binary systems, such as the one revealed by the gravitational wave event GW190425, but also predict the formation of neutron star and light black hole binaries, such as the one that merged in the 2020 gravitational wave event GW200115.
Another important finding is that the mass of the helium core of the stripped star is essential in determining the nature of its interactions with its neutron star companion and the ultimate fate of the binary system. A sufficiently massive helium star can avoid transferring mass onto the neutron star. With a less massive helium star, however, the mass transfer process can transform the neutron star into a rapidly 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 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.”
In other words, there may well be a large undetected population 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 said. “Avoiding this mass transfer episode as we suggest hints that there is a radio-quiet population of such systems in the Milky WayThe Milky Way is the galaxy that contains the Earth, and is named for its appearance from Earth. It is a barred spiral galaxy that contains an estimated 100-400 billion stars and has a diameter between 150,000 and 200,000 light-years.”>Milky Way.”
Reference: “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.
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.