On the Maximum Mass of Neutron Stars (1307.3995v3)

Abstract: One of the most intringuing questions about neutron stars concerns their maximum mass. The answer is intimately related to the properties of matter at densities far beyond that found in heavy atomic nuclei. The current view on the internal constitution of neutron stars and on their maximum mass, both from theoretical and observational studies, are briefly reviewed.

• The paper reviews theoretical models and observational data that constrain neutron star maximum masses.
• It employs advanced quantum many-body and relativistic mean-field models to predict mass limits between 1.8 and 2.5 solar masses.
• Observations of pulsars, including PSR J1614-2230, reinforce the push toward a 3 solar mass upper limit.

Evaluating the Maximum Mass of Neutron Stars

The paper of neutron stars (NSs) and their maximum mass is a critical sector of astrophysics as it provides insights into the behavior of ultra-dense matter and the fundamental properties of matter under such extreme conditions. In the paper titled "On the Maximum Mass of Neutron Stars" by Chamel et al., the authors undertake a comprehensive review of theoretical and observational perspectives regarding the maximum mass limits of neutron stars, addressing both historical developments and modern understandings.

Core Components and Theoretical Foundations

Neutron stars are the remnants of supernova explosions and represent the densest stellar objects observed, with densities surpassing that of atomic nuclei up to tenfold. The maximum mass of a neutron star, designated as $M_{\rm max}$, is fundamentally constrained by general relativity and the equation of state (EoS), which describes the relation between density and pressure in the stellar interior.

The authors trace the derivation of mass limits from early Newtonian approximations, such as the Chandrasekhar limit for white dwarfs, to more comprehensive treatments incorporating the principles of general relativity. Oppenheimer and Volkoff's introduction of a relativistic framework to describe neutron stars indicated a substantially lower mass limit, setting the stage for evolving models factoring in nuclear interactions, hyperonic matter, mesons, and a potential transition to quark states.

EoS and Beyond Neutron Matter

At densities considerably exceeding that found in atomic nuclei, neutron star matter is expected to include a variety of constituents beyond pure neutrons. Different compositions, such as nucleons mixed with hyperons, mesonic condensates, or even deconfined quark matter, lead to different EoS characterizations, directly affecting the predicted $M_{\rm max}$. The uncertainty in the EoS at high densities remains one of the significant challenges in determining the true mass limits of neutron stars.

Modern calculations employ advanced many-body quantum theories, simulations with relativistic mean-field models, and effective field theories to understand these dense states. The results portray a range in maximum masses due to EoS variances, generally between 1.8 to 2.5 $M_\odot$, where $M_\odot$ represents solar mass.

Observational Constraints

Observational data serves as critical constraints on these theoretical predictions. The precise measurements of neutron star masses in binary systems, especially those with relativistic effects, such as pulsars, have provided tangible mass estimates. The measurements of systems such as PSR J1614-2230 indicate neutron star masses as high as 2 $M_\odot$, necessitating sufficiently stiff EoS models.

Such observational data not only refine theoretical models but also challenge some EoS constructions, particularly those involving significant softening due to hyperons or other new degrees of freedom unless compensated by factors like additional repulsive interactions or phase transitions.

Implications and Future Directions

The paper implies that while the theoretical upper limits on NS mass established by considering extreme stiffness constraints (approximately 3 $M_\odot$) are helpful, the precise determination of $M_{\rm max}$ is better informed through a synergy of accurate observations and theoretical advancements.

Future observational campaigns, including gravitational wave astronomy and continued monitoring of pulsars, combined with explorations into high-energy astrophysics and nuclear physics, will undoubtedly refine these mass estimates further. The evolution in constraints of $M_{\rm max}$ will enhance the understanding of matter interactions under such extreme conditions, testing the limits of dissident theories including modifications to gravity or exotic forms of matter.

In conclusion, Chamel et al.'s review of our current understanding of neutron star maximum mass findings emphasizes a complex interplay between observations and advanced modeling, suggesting a window between 2 and approaching the hard theoretical cap of 3 $M_\odot$, gravitating increasingly towards stronger constraints with each observational and theoretical advance in the field.