Introduction

A startling hypothesis is taking shape in the realm of astrophysics: neutron stars with masses lighter than even the humble white dwarf could be lurking in the cosmos. Typically, neutron stars are observed to have masses between 1.4 and 2.0 times that of our Sun. This upper limit is well understood; any neutron star exceeding about two solar masses is destined to collapse into a black hole. On the flip side, the lower limit aligns with the characteristics of white dwarfs, which rely on electron pressure to resist gravitational collapse as discovered by Subrahmanyan Chandrasekhar in 1930, known famously as the Chandrasekhar Limit of 1.4 solar masses.

Challenging Assumptions

However, a prevailing assumption that a neutron star must always exceed this mass level is being challenged. While conventional models suggest that stellar entities beneath 1.4 solar masses should remain confined to white dwarf status, the scenario changes dramatically during a supernova. When massive stars reach their life’s end, they undergo spectacular supernova explosions that could rapidly compress the central core, potentially creating a neutron core of mass under 1.4 solar masses. This leads to an intriguing inquiry: can such neutron 'stars' truly exist in stability, or would they succumb to gravitational collapse?

The Tolman-Oppenheimer-Volkoff Equation

The stability of neutron matter is governed by the complex Tolman-Oppenheimer-Volkoff (TOV) equation, which predicts the mass limits for neutron stars. Current parameters provided by the TOV equation suggest an upper limit of approximately 2.17 solar masses while proposing a lower threshold around 1.1 solar masses. Intriguingly, when parameters are pushed to their extremes based on observational data, this lower limit may even dip as low as 0.4 solar masses.

Research Insights

A research study recently published on arXiv focuses on this ambiguity, examining gravitational wave data from the third observational run of the LIGO and Virgo observatories. Astrophysicists have mostly detected mergers between black holes; however, the gravitational wave detectors also capture signals from the merging of two neutron stars or between a neutron star and a black hole. The challenge lies in the fact that these lighter neutron stars would exhibit significant tidal deformations, altering the characteristic 'chirp' of the gravitational wave signals produced during mergers. The smaller the neutron star, the more pronounced the deformation, complicating detection efforts.

Simulation and Findings

To explore this, the research team simulated potential merger scenarios involving neutron stars with masses below that of white dwarfs, calculating how these deformations would shift the observable chirp in the gravitational wave data. Although their investigation did not reveal any small-mass neutron stars, they managed to establish an upper limit on the plausible frequency of such mergers. Their findings conclude that there can be a maximum of 2,000 detectable mergers involving neutron stars weighing up to 70% of the Sun’s mass.

Conclusion

While this may seem like a modest figure, it is imperative to consider that we are still on the frontier of gravitational wave astronomy. With advancements in technology over the next few decades, more sensitive gravitational wave detectors will emerge, making it possible to either uncover these elusive small neutron stars or ultimately prove their nonexistence. This provocative discovery raises tantalizing possibilities about the diversity of stellar remnants in our universe and the mysteries that still await revelation in the cosmic symphony of gravitational waves. Keep your eyes on the skies, as the next breakthroughs in our understanding of neutron stars might just be around the corner!