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Maximum Mass of a Non-Rotating Neutron Star Has Been Precisely Inferred to be 2.25 Solar Masses

A recent study spearheaded by Professor FAN Yizhong from the Purple Mountain Observatory, Chinese Academy of Sciences, has achieved a landmark precision in determining the upper mass limit for non-rotating neutron stars, a pivotal aspect in the study of nuclear physics and astrophysics. According to their investigation, the maximum gravitational mass of a non-rotating neutron star is approximately 2.25 solar masses, with an uncertainty of just 0.07 solar mass. This study was published in Physical Review D.

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., MTOV) 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 MTOV are diverse and the uncertainties are large.

FAN's team has refined the inference of MTOV 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 MTOV. Three diverse equation of state (EoS) reconstruction models were employed to mitigate potential systematic errors, yielding almost identical results for MTOV 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 MTOV 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)

The precisely evaluated MTOV carries profound implications for both nuclear physics and astrophysics. It indicates a moderately stiff EoS for neutron star matter and suggests that the compact objects with masses in the range of approximately 2.5 to 3.0 solar masses, detected by LIGO/Virgo, are more likely to be the lightest black holes. Furthermore, the merger remnants of binary neutron star systems exceeding a total mass of roughly 2.76 solar masses would collapse into black holes, while lighter systems would result in the formation of (supramassive) neutron stars.