Neutron Stars’ Surprising Secrets Revealed by Gravitational Waves

Binary Neutron Star Formation

In the late stages of binary neutron star formation, the giant star expands and engulfs the neutron star companion 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. The evolution of the system depends on the mass ratio. Less-massive stripped stars experience an additional mass transfer phase that further 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). More massive stripped stars do not expand as much, therefore avoiding further stripping and companion recycling, leading to systems such as GW190425 (c). Finally, even more massive stripped stars with will lead to black hole-neutron star binaries such as GW200115 (d). Credit: Vigna-Gomez et al., ApJL 2021

The confirmation of gravitational waves back in 2017 continues to unlock whole new worlds of physics but also continues to elicit further questions.  The detection of each gravitational wave brings a new challenge – how to find out what caused the event.  Sometimes that is harder than it sounds.  Now a team led by Alejandro Vigna-Gomez of the University of Copenhagen thinks they found a model of star death that helps to explain some previously inexplicable findings – and points to a galaxy with many more massive neutron stars than previously thought.

In science, it is common to collect data that doesn’t seem to fit the current scientific theory. That sort of unexpected data came from the Laser Interferometer Gravitational-Wave Observatory’s (LIGO) second-ever gravitational wave finding.  Usually, LIGO would record gravitational waves resulting from the collision of two massively dense objects, such as a black hole and a neutron star.  In the case of its second positive recording, initially recorded in 2019 and now known as GW190425, the data pointed to the source as being two merging neutron stars, but they were surprisingly big.  

Average neutron stars are tough to “see” in the traditional sense. Like their closely related cousin, the black hole, they usually form only after a supermassive star has imploded. However, occasionally they form pulsars, creating a form of star that is one of the most visible in the universe. Typically, the only way to see a binary neutron star system, such as the one that created the GW190425 gravitational wave signal, is if one of the two stars in the system is a pulsar and then interacts with its regular neutron star neighbor. But none of the known binary neutron star systems had heavy enough stars to match the signal seen by LIGO.

They lacked such stars partially due to larger stars turning into black holes rather than neutron stars when they die. However, the gravitational signals were coming from merging giant neutron stars, not merging black holes. So what is causing the formation of these large neutron stars, and why aren’t they showing up in binary pairs with pulsars?

According to Dr. Vigna-Gomez, the answer might lie in a type of star called a “stripped star.”  Also called a helium star, these stellar objects only form in binary systems and have their hydrogen outer shell forced away by the other star in the system, leaving a core of pure helium.  The team modeled these types of stars to understand what happens to them after a supernova.  It depends on two factors: the weight of the core that’s left and the forcefulness of its supernova explosion.

Using stellar evolution models, the team showed that for helium stars, some of the outer layers of helium can be blown off in the explosion, lowering the weight of the star to the point where it is no longer able to become a black hole.  That could potentially explain where the heavy neutron stars come from, but why aren’t they more noticeable in binary systems with pulsars?

The answer comes from a standard process in binary systems – mass transfer.  Often, one star in a binary system loses some of its material to the other, more massive, star in a process known as mass transfer.  In neutron star systems, this mass transfer can sometimes spin up a neutron star into a pulsar.  However, the larger the star’s helium core, the less likely that mass transfer process is.  So in systems that form massive neutron stars, it is less likely they would end up in a binary system with a pulsar.  They are more able to hold on to their mass rather than transferring it to their binary companion, letting it light up as a pulsar.

Other data from LIGO back up this theory.  It appears that heavy neutron star mergers are just as common in the universe as mergers of slightly less heavy neutron stars with pulsars.  An entire population of large neutron star binary systems might exist, invisible to our usual detection methods. But now, with LIGO, we should at least be able to see when they merge, and that is another step towards truly understanding them.

Originally published on Universe Today.

For more on this research, see Astrophysicists Explain Puzzling Results From Gravitational Wave Observatories.

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.
DOI: 10.3847/2041-8213/ac2903

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