Neutron Stars With Much less Mass Than A White Dwarf Would possibly Exist, and LIGO and Virgo May Discover Them

Many of the neutron stars we all know of have a mass between 1.4 and a couple of.0 Suns. The higher restrict is smart, since, past about two photo voltaic lots, a neutron star would collapse to develop into a black gap. The decrease restrict additionally is smart given the mass of white dwarfs. Whereas neutron stars defy gravitational collapse due to the stress between neutrons, white dwarfs defy gravity due to electron stress. As first found by Subrahmanyan Chandrasekhar in 1930, white dwarfs can solely assist themselves up to what’s now generally known as the Chandrasekhar Limit, or 1.4 photo voltaic lots. So it’s simple to imagine {that a} neutron star should have at the least 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 lots would stay a white dwarf. However bigger stars don’t merely run out of gasoline and collapse. They endure 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 lots. The query is whether or not it may very well be secure as a small neutron star. That will depend on 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 posh relativistic equation primarily based on sure assumed parameters. Utilizing the very best knowledge we at present have, the TOV equation of state places an higher mass restrict for a neutron star at 2.17 photo voltaic lots and a decrease mass restrict round 1.1 photo voltaic lots. In case you tweak the parameters to probably the most excessive values allowed by statement, the decrease restrict can drop to 0.4 photo voltaic lots. 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 examine on the arXiv.

The examine seems at knowledge from the third observing run of the Virgo and Superior LIGO gravitational wave observatories. Whereas many of the noticed occasions are the mergers of stellar-mass black holes, the observatories may seize mergers between two neutron stars or a neutron star and a black gap companion. The sign energy of those smaller mergers is so near the noise degree of the gravitational wave detectors that it’s good to have an thought of the kind of sign you’re searching for to seek out it. For neutron star mergers, that is sophisticated 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 larger the deformation.

So the group simulated how sub-white-dwarf mass neutron stars would tidally deform as they merge, then calculated how that will have an effect on the noticed gravitational chirp. They then regarded for these sorts of chirps within the knowledge of the third statement run. Whereas the group discovered no proof for small-mass neutron stars, they have been in a position to place an higher restrict on the hypothetical price of such mergers. Basically, they discovered that there will 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 appear to be a lot of a restrict, it’s necessary to keep in mind that we’re nonetheless within the early levels of gravitational wave astronomy. Within the coming a long time, we can 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 Search for Low-Mass Neutron Stars in the Third Observing Run of Advanced LIGO and Virgo.” arXiv preprint arXiv:2412.05369 (2024).