The Nuclear Equation of State and Neutron Star Masses (1305.3510v1)

Abstract: Neutron stars are valuable laboratories for the study of dense matter. Recent observations have uncovered both massive and low-mass neutron stars and have also set constraints on neutron star radii. The largest mass measurements are powerfully influencing the high-density equation of state because of the existence of the neutron star maximum mass. The smallest mass measurements, and the distributions of masses, have implications for the progenitors and formation mechanisms of neutron stars. The ensemble of mass and radius observations can realistically restrict the properties of dense matter, and, in particular, the behavior of the nuclear symmetry energy near the nuclear saturation density. Simultaneously, various nuclear experiments are progressively restricting the ranges of parameters describing the symmetry properties of the nuclear equation of state. In addition, new theoretical studies of pure neutron matter are providing insights. These observational, experimental and theoretical constraints of dense matter, when combined, are now revealing a remarkable convergence.

• The paper demonstrates that precise neutron star mass measurements, such as the 1.97M☉ for PSR J1614-2230, critically constrain the nuclear equation of state.
• It employs mass-radius analyses and symmetry energy parameters to bridge experimental results with theoretical models of dense matter.
• The study highlights that future gravitational wave observations are poised to further refine EOS models and narrow key parameter ranges.

The Nuclear Equation of State and Neutron Star Masses

The intricate relationship between the nuclear equation of state (EOS) and neutron star masses forms the core of Lattimer's work, as published in the Annual Review of Nuclear and Particle Science. For experts across nuclear physics and astrophysics, this paper delivers a detailed examination of how observations of neutron star properties translate into constraints on the EOS, which in turn informs our understanding of dense matter.

Key Observations and Their Implications

Neutron stars, born from the gravitational collapse of massive stellar cores, provide a unique opportunity to explore the behavior of matter under extreme densities, exceeding that found in atomic nuclei. Observations highlight that both massive and low-mass neutron stars exist, with precise mass determinations from binary pulsar systems acting as a crucial observational tool. For instance, the well-measured mass of PSR J1614-2230 at 1.97±0.04M☉ challenges and refines theoretical models of the EOS by confirming the existence of extremely massive neutron stars.

Moreover, these observations imply significant constraints on the high-density EOS, affecting theoretical models of the nuclear symmetry energy near saturation density, $n_s$, which are vital for comprehending both the pressure and energy densities within neutron stars. The diverse outcomes of mass and radius observations, alongside theoretical studies, are converging towards a compressed range of parameter values, illustrating a soft symmetry energy at lower densities and stiffening at higher densities to support such massive neutron configurations.

Theoretical and Experimental Convergence

The behavior of the nuclear symmetry energy is pivotal in determining the mass-radius relationship of neutron stars as well as their maximum mass limit. Utilizing mass-radius analyses, Steiner et al. proposed likely ranges for the nuclear symmetry parameters that align with observational data, such as the relatively narrow radii of ~11.5 km for 1.4 $M_\odot$ neutron stars. Intriguingly, these astrophysical findings are in agreement with results derived from nuclear experiments and neutron matter calculations. For instance, constraints derived from studies of giant dipole resonances and neutron skin thicknesses increasingly align with the symmetry parameters essential for understanding the EOS.

Nonetheless, the implications for the nuclear symmetry energy parameters $S_v$ and $L$ extend beyond theoretical models and demand precise laboratory results. Various experimental techniques, ranging from nuclear mass fits to neutron skin thickness and isospin diffusion measurements, continue to refine these parameters. The present consistency between experimental and observational findings is compelling, while sustaining the necessity for ongoing research to resolve discrepancies and improve the precision of these measures.

Future Directions and Implications

This paper foresees significant advancements in our understanding stemming from future observations, such as gravitational wave detections from neutron star mergers. These events have the potential to provide precise mass and radius data that could further restrict the permissible models of the nuclear EOS. Such progress, complemented by laboratory constraints and theoretical innovations, will illuminate the mysterious nature of ultra-dense matter in neutron stars.

Conclusively, Lattimer's meticulous review underscores a period of remarkable convergence in neutron star physics. By synthesizing astrophysical, experimental, and theoretical approaches, this work delineates a pathway for unraveling the complex dynamics of these extraordinary celestial objects and the nuclear forces at their core. Further developments in observational technology and theoretical modeling will undoubtedly refine and expand our comprehension of the nuclear EOS and its profound astrophysical implications.