Supernova simulations show that low-mass star in binary system J0453+1559 could be a neutron star

In 2015, astrophysicists discovered a system consisting of two compact stars orbiting each other: a pulsar (i.e., a highly magnetized rapidly rotating, light-emitting neutron star) and a so-called companion star. The companion star in this system has a mass that is 1.174 solar masses (M_⊙), which is significantly lower than that of other known neutron stars with accurately measured masses.

The nature of this light mass star in the J0453+1559 binary pulsar system has not yet been fully elucidated. Over the past few years, some physicists have been trying to shed more light on its nature, using both theory and computational tools.

While some researchers have suggested that it could be a neutron star, others have questioned this hypothesis, instead suggesting that it might be a white dwarf. Notably, these two types of stars have different origins: a neutron star is formed when a massive star undergoes a supernova explosion, while a white dwarf emerges when a star with a low to medium mass exhausts its nuclear fuel and sheds its outer layers.

Researchers at Monash University and Swinburne University of Technology recently ran 3D simulations of supernova explosions, to explore the possibility that the low-mass star in the J0453+1559 system is a neutron star. Their paper, published in Physical Review Letters, suggests that this star could in fact be a neutron star and not a white dwarf, as some earlier studies suggested.

"We had been working with collaborators on another paper that has now been published in Nature Astronomy," Bernhard Müller, first author of the paper, told Phys.org. "This paper had assembled neutron star observations to reconstruct the neutron star mass distribution. The origin of a very low-mass neutron star in system J0453+1559 had been debated for a while, but the observations also showed a few other neutron stars with quite low mass."

Initially, Müller and his colleagues Alexander Heger and Jade Powell started looking for possible progenitor stars for the low-mass star within the large database of stellar evolution models available to them. They then performed supernova simulations to determine how current theories of stellar evolution could explain a neutron star with such a low mass.

"We looked at 25 stellar evolution models of stars close to the minimum mass required for supernova explosions," explained Müller. "We zeroed in on five of these where we expected the neutron star mass to be particularly low because of the core structure of the progenitor, specifically the mass of the iron and silicon core.

"However, the amount of mass that ends up in the neutron star is subject to complex explosion dynamics (e.g., matter can still fall onto the neutron star while the explosion is already developing)."

The researchers ran 3D supernova simulations for the five progenitor stars they identified, which spanned from their collapse to their explosion. These simulations allowed them to predict the masses of the neutron stars that would emerge from the explosions with good accuracy.

"The study makes it plausible that the mysterious low-mass object of 1.17 solar masses in system J0453+1559 is indeed a neutron star and not a white dwarf, as some authors have recently proposed," said Müller. "On a bigger scale, the study demonstrates the power of big 3D simulations to explain features of the neutron star and black hole mass distribution and elucidate the underlying stellar evolution and explosion physics."

Overall, the simulations performed by Müller, Heger and Powell suggest that the low-mass star in the binary system J0453 + 1559 could in fact be a neutron star. In their next studies, the researchers plan to run more 3D simulations to further improve the current understanding of how different celestial objects are born and their unique properties, particularly neutron stars and black holes.

"With more computer power, we hope that big simulations can explain the birth properties of neutron stars and black holes in detail, e.g., their masses, their birth velocities, and how fast they spin," added Müller.

"A particularly challenging problem, which we are currently working on, is the origin of neutron star magnetic fields. It remains to be explained why some neutron stars, called magnetars, are born with very high surface field strengths of order 10¹⁵ Gauss. 3D simulations will hopefully contribute to explaining their origin."

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