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 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 often called the Chandrasekhar Restrict, or 1.4 photo voltaic lots. So it’s straightforward to imagine {that a} neutron star will need to have at the very 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 underneath easy hydrostatic collapse, something underneath 1.4 photo voltaic lots 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 lots. 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 posh relativistic equation based mostly on sure assumed parameters. Utilizing one of the best information 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. If you happen to tweak the parameters to probably the most excessive values allowed by remark, the decrease restrict can drop to 0.4 photo voltaic lots. If we are able to observe low-mass neutron stars, it might additional constrain the TOV parameters and enhance our understanding of neutron stars. That is the main target of a brand new examine on the arXiv.
The examine seems to be at information 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 also can 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 searching for to search 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 higher the deformation.
So the workforce simulated how sub-white-dwarf mass neutron stars would tidally deform as they merge, then calculated how that may have an effect on the noticed gravitational chirp. They then seemed 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 had been in a position to place an higher restrict on the hypothetical charge of such mergers. Primarily, they discovered that there could 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 look like a lot of a restrict, it’s essential to do not forget that we’re nonetheless within the early levels 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’ll’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).
