Equation of state and neutron star properties constrained by nuclear physics and observation (1303.4662v2)

Abstract: Microscopic calculations of neutron matter based on nuclear interactions derived from chiral effective field theory, combined with the recent observation of a 1.97 +- 0.04 M_sun neutron star, constrain the equation of state of neutron-rich matter at sub- and supranuclear densities. We discuss in detail the allowed equations of state and the impact of our results on the structure of neutron stars, the crust-core transition density, and the nuclear symmetry energy. In particular, we show that the predicted range for neutron star radii is robust. For use in astrophysical simulations, we provide detailed numerical tables for a representative set of equations of state consistent with these constraints.

• The paper applies chiral effective field theory and microscopic calculations to constrain the equation of state for neutron-rich matter in neutron stars.
• The study integrates observational data, including a 1.97 solar mass neutron star, to rule out overly soft equations and limit the parameter space for EOS models.
• The paper extrapolates results to beta equilibrium to determine symmetry energy parameters and derive constraints on neutron star radii and internal structure.

Examination of Neutron Star Equation of State Constrained by Nuclear Physics and Observational Data

The paper "Equation of state and neutron star properties constrained by nuclear physics and observation" ventures into the challenging domain of ascertaining the equation of state (EOS) for neutron-rich matter, particularly within neutron stars. This area is pivotal primarily due to its implications for understanding matter at extreme densities. The authors leverage a combination of chiral effective field theory (EFT) for nuclear interactions alongside recent empirical data concerning massive neutron stars to impose constraints on the EOS.

Microscopic Calculations and Chiral EFT

The authors begin by discussing microscopic calculations of neutron matter utilizing nuclear interactions derived from chiral EFT, which has become a cornerstone in nuclear theory for systematically expanding nuclear forces at low momenta. Chiral EFT introduces a hierarchical structure of forces, typically dominated by two-nucleon (NN) interactions with notable contributions from three-nucleon (3N) forces, as highlighted in their computations. Within neutron stars, the matter ranges from electron-rich outer crusts to neutron-dominant outer cores, with potentially exotic states in the inner core at high densities.

Figure 1 in the paper illustrates the calculated energy per particle of neutron matter, which shows a close agreement across different computational methods. This convergence underscores the reliability of the chiral EFT framework in constraining low-density neutron-rich EOS.

Observational Constraints and Implications on the EOS

The incorporation of observational constraints from neutron star masses, including the discovery of a $1.97 \pm 0.04 \, M_{\odot}$ neutron star, plays a critical role in reinforcing EOS constraints at supra-saturation densities. Such high-mass observations disallow a multitude of EOS models that posit exotic phases typically leading to a softened EOS. One of the key arguments in the paper is that EOS models must remain consistent with causality and be able to support such massive neutron stars, limiting the parameter space for viable EOS models.

Asymmetric Matter and Beta Equilibrium

To extend the neutron matter results to beta equilibrium, critical for neutron star matter which contains both neutrons and protons, the authors utilize an interpolation between symmetric nuclear matter (SNM) and pure neutron matter (PNM). The discussion covers relevant parameters like the symmetry energy ($S_v$) and its density derivative ($L$), which are crucial for delineating the EOS of asymmetric nuclear matter. Based on these extrapolations, the paper provides stringent bounds on these parameters: $S_v$ within $29.7 - 33.5 \, \text{MeV}$ and $L$ ranging from $32.4 - 57.0 \, \text{MeV}$.

Structure of Neutron Stars

The EOS constraints naturally lead to corresponding constraints on the structural properties of neutron stars, such as their radii and core densities. The authors discuss the implications on the crust-core transition point and the resulting impact on properties like the moment of inertia and crustal mass fractions. The Tolman-Oppenheimer-Volkov equations are employed to investigate such structural aspects, against several proposed extensions of the EOS at high densities via piecewise polytropes.

Conclusion and Astrophysical Utilities

Ultimately, the findings culminate in providing representative EOSs that satisfy the combined theoretical and observational constraints. This work is indispensable for astrophysical simulations that require EOS models consistent with both nuclear physics and observed neutron star properties. Furthermore, future measurements regarding the radii of neutron stars and the discovery of more massive neutron stars could further refine these constraints.

In sum, the paper provides rigorous bounds on the neutron star EOS shaped by modern nuclear interactions and valuable astrophysical data. This comprehensive paper not only narrows down the viable EOS models but also sets a stage for future explorations that might integrate newer observational insights and advanced theoretical progress innovations in EFT.