Many of the neutron stars we all know of have a mass between 1.4 and a pair of.0 Suns. The higher restrict is smart, since, past about two photo voltaic plenty, a neutron star would collapse to turn out to be a black gap. The decrease restrict additionally is smart given the mass of white dwarfs. Whereas neutron stars defy gravitational collapse because of the strain between neutrons, white dwarfs defy gravity because of electron strain. As first found by Subrahmanyan Chandrasekhar in 1930, white dwarfs can solely assist themselves up to what’s now referred to as the Chandrasekhar Restrict, or 1.4 photo voltaic plenty. So it’s simple to imagine {that a} neutron star should have a minimum of that a lot mass. In any other case, collapse would cease at a white dwarf. However that isn’t essentially true.

It’s true that beneath easy hydrostatic collapse, something beneath 1.4 photo voltaic plenty would stay a white dwarf. However bigger stars don’t merely run out of gasoline and collapse. They bear cataclysmic explosions as a supernova. If such an explosion have been to squeeze the central core quickly, you may need a core of neutron matter with lower than 1.4 photo voltaic plenty. The query is whether or not it may very well be secure as a small neutron star. That is determined by how neutron matter holds collectively, which is described by its equation of state.

Neutron star matter is ruled by the Tolman–Oppenheimer–Volkoff, which is a fancy relativistic equation primarily based on sure assumed parameters. Utilizing one of the best information we presently have, the TOV equation of state places an higher mass restrict for a neutron star at 2.17 photo voltaic plenty and a decrease mass restrict round 1.1 photo voltaic plenty. When you tweak the parameters to essentially the most excessive values allowed by remark, the decrease restrict can drop to 0.4 photo voltaic plenty. If we are able to observe low-mass neutron stars, it could additional constrain the TOV parameters and enhance our understanding of neutron stars. That is the main focus of a brand new research on the arXiv.

The research appears at information from the third observing run of the Virgo and Superior LIGO gravitational wave observatories. Whereas a lot of the noticed occasions are the mergers of stellar-mass black holes, the observatories can even seize mergers between two neutron stars or a neutron star and a black gap companion. The sign power of those smaller mergers is so near the noise degree of the gravitational wave detectors that you could have an concept of the kind of sign you’re on the lookout for to seek out it. For neutron star mergers, that is difficult by the truth that neutron stars are delicate to tidal deformations. These deformations would shift the “chirp” of the merger sign, and the smaller the neutron star, the higher the deformation.

So the workforce simulated how sub-white-dwarf mass neutron stars would tidally deform as they merge, then calculated how that might have an effect on the noticed gravitational chirp. They then regarded for these sorts of chirps within the information of the third remark run. Whereas the workforce discovered no proof for small-mass neutron stars, they have been capable of place an higher restrict on the hypothetical fee of such mergers. Primarily, they discovered that there might be not more than 2,000 observable mergers involving a neutron star as much as 70% of the Solar’s mass. Whereas that may not look like a lot of a restrict, it’s necessary to keep in mind that we’re nonetheless within the early phases of gravitational wave astronomy. Within the coming many years, we could have extra delicate gravitational telescopes, which is able to both uncover small neutron stars or show that they will’t exist.

Reference: Kacanja, Keisi, and Alexander H. Nitz. “A Seek for Low-Mass Neutron Stars within the Third Observing Run of Superior LIGO and Virgo.” arXiv preprint arXiv:2412.05369 (2024).