New Constraints on Radii and Tidal Deformabilities of Neutron Stars from GW170817

TLDR

The study parametrically explores a wide range of physically plausible equations of state for neutron stars, considering both purely hadronic matter and matter with a phase transition. By generating two million EOSs (one million each with and without phase transitions) and applying GW170817 constraints on maximum mass and tidal deformability, the authors compute over a billion equilibrium models to produce statistical distributions of stellar properties and determine the most probable radius for any mass. The analysis constrains the radius of a 1.4 M⊙ hadronic neutron star to 12.00–13.45 km (most likely 12.39 km) with Λ>375, while twin‑star EOSs allow radii as low as 8.53 km but are unlikely; a future binary with total mass 3.4 M⊙ and Λ>461 would exclude twin‑star solutions.

Abstract

We explore in a parameterized manner a very large range of physically plausible equations of state (EOSs) for compact stars for matter that is either purely hadronic or that exhibits a phase transition. In particular, we produce two classes of EOSs with and without phase transitions, each containing one million EOSs. We then impose constraints on the maximum mass (M<2.16 M_{⊙}) and on the dimensionless tidal deformability (Λ[over ˜]<800) deduced from GW170817, together with recent suggestions of lower limits on Λ[over ˜]. Exploiting more than 10^{9} equilibrium models for each class of EOSs, we produce distribution functions of all the stellar properties and determine, among other quantities, the radius that is statistically most probable for any value of the stellar mass. In this way, we deduce that the radius of a purely hadronic neutron star with a representative mass of 1.4 M_{⊙} is constrained to be 12.00<R_{1.4}/km<13.45 at a 2σ confidence level, with a most likely value of R[over ¯]_{1.4}=12.39 km; similarly, the smallest dimensionless tidal deformability is Λ[over ˜]_{1.4}>375, again at a 2σ level. On the other hand, because EOSs with a phase transition allow for very compact stars on the so-called "twin-star" branch, small radii are possible with such EOSs although not probable, i.e., 8.53<R_{1.4}/km<13.74 and R[over ¯]_{1.4}=13.06 km at a 2σ level, with Λ[over ˜]_{1.4}>35.5 at a 3σ level. Finally, since these EOSs exhibit upper limits on Λ[over ˜], the detection of a binary with a total mass of 3.4 M_{⊙} and Λ[over ˜]_{1.7}>461 can rule out twin-star solutions.