The ultimate fate of a massive star is intricately linked to its mass. Stars lighter than 8 solar masses end their life cycle as white dwarfs, supported by electron degeneracy pressure, with a well-known upper mass limit, the Chandrasekhar limit, near 1.4 solar masses. While for stars heavier than 8 but lighter than 25 solar masses, neutron stars will be produced, which instead, are mainly upheld by neutron degeneracy pressure. For non-rotating neutron stars, there is also a critical gravitational mass (i.e., M_TOV) that is known as the Oppenheimer limit, above which the neutron star will collapse into a black hole.

However, establishing a precise Oppenheimer limit is quite challenging. Only loose bounds can be set based on the first principle. Many specific evaluations are strongly model-dependent. The resulting M_TOV are diverse and the uncertainties are large.

FAN's team has refined the inference of M_TOV by incorporating robust multi-messenger observations and reliable nuclear physics data, circumventing the uncertainties present in earlier models. This includes leveraging recent advancements in mass/radius measurements from LIGO/Virgo gravitational-wave detectors and the Neutron star Interior Composition Explorer (NICER). In particular, they also incorporated the information of the maximum mass cutoff inferred from the neutron star mass distribution and significantly narrowed the parameter space, leading to an unprecedented precision in the inferred M_TOV. Three diverse equation of state (EoS) reconstruction models were employed to mitigate potential systematic errors, yielding almost identical results for M_TOV and the corresponding radius, which is 11.9 km with an uncertainty of 0.6 km in three independent EoS reconstruction approaches.

Almost identical posterior distributions of M_TOV and the corresponding radius found in three independent EoS reconstruction approaches (represented by the orange, purple and blue lines). The black lines shows the average results. (Image by FAN Yizhong)