Neutron Stars With Less Mass Than A White Dwarf Might Exist, and LIGO and Virgo Could Find 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 sensible, since, past about two photo voltaic plenty, a neutron star would collapse to develop into a black gap. The decrease restrict additionally is sensible given the mass of white dwarfs. Whereas neutron stars defy gravitational collapse because of the stress between neutrons, white dwarfs defy gravity because of electron stress. As first found by Subrahmanyan Chandrasekhar in 1930, white dwarfs can solely help themselves up to what’s now referred to as the Chandrasekhar Limit, or 1.4 photo voltaic plenty. So it’s straightforward to imagine {that a} neutron star will need to 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 below easy hydrostatic collapse, something below 1.4 photo voltaic plenty would stay a white dwarf. However bigger stars don’t merely run out of gas and collapse. They bear cataclysmic explosions as a supernova. If such an explosion had been to squeeze the central core quickly, you might need a core of neutron matter with lower than 1.4 photo voltaic plenty. The query is whether or not it could possibly be secure as a small neutron star. That is dependent upon 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 based mostly on sure assumed parameters. Utilizing the most effective knowledge 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. In case you tweak the parameters to essentially the most excessive values allowed by statement, the decrease restrict can drop to 0.4 photo voltaic plenty. If we will observe low-mass neutron stars, it will 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 seems to be at knowledge 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 also can 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 stage of the gravitational wave detectors that you could have an concept of the kind of sign you’re on the lookout for to search 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 larger the deformation.

So the crew 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 knowledge of the third statement run. Whereas the crew discovered no proof for small-mass neutron stars, they had been in a position to place an higher restrict on the hypothetical charge of such mergers. Basically, 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 which may not seem to be a lot of a restrict, it’s essential to keep in mind that we’re nonetheless within the early levels of gravitational wave astronomy. Within the coming a long time, we may have extra delicate gravitational telescopes, which can 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).