Most 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 turn into a black gap. The decrease restrict additionally is smart given the mass of white dwarfs. While neutron stars defy gravitational collapse due to the strain between neutrons, white dwarfs defy gravity due to electron strain. 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 no less than that a lot mass. Otherwise, collapse would cease at a white dwarf. But that isn’t essentially true.
It is true that beneath easy hydrostatic collapse, something beneath 1.4 photo voltaic lots would stay a white dwarf. But bigger stars don’t merely run out of gas and collapse. They endure 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 might 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 fancy relativistic equation based mostly on sure assumed parameters. Using the very best knowledge we at the moment 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 tweak the parameters to probably the most excessive values allowed by commentary, 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. This is the main target of a brand new examine on the arXiv.
The examine appears to be like at knowledge from the third observing run of the Virgo and Advanced LIGO gravitational wave observatories. While 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 power of those smaller mergers is so near the noise stage of the gravitational wave detectors that you must have an concept of the kind of sign you’re in search of 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 better the deformation.
So the staff 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 seemed for these sorts of chirps within the knowledge of the third commentary run. While the staff discovered no proof for small-mass neutron stars, they had been capable of place an higher restrict on the hypothetical price of such mergers. Essentially, they discovered that there may be not more than 2,000 observable mergers involving a neutron star as much as 70% of the Sun’s mass. While that may not seem to be a lot of a restrict, it’s essential to do not forget that we’re nonetheless within the early phases of gravitational wave astronomy. In the approaching 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 Search for Low-Mass Neutron Stars within the Third Observing Run of Advanced LIGO and Virgo.” arXiv preprint arXiv:2412.05369 (2024).
