Low‐Mass Neutron Stars and the Equation of State of Dense Matter

Abstract

Neutron-star radii provide useful information on the equation of state of neutron rich matter. Particularly interesting is the density dependence of the equation of state (EOS). For example, the softening of the EOS at high density, where the pressure rises slower than anticipated, could signal a transition to an exotic phase. However, extracting the density dependence of the EOS requires measuring the radii of neutron stars for a broad range of masses. A "normal" 1.4M⊙ (M⊙=solar mass) neutron star has a central density of a few times nuclear-matter saturation density (ρ0). In contrast, low mass (≃ 0.5M⊙) neutron stars have central densities near ρ0 so its radius provides information on the EOS at low density. Unfortunately, low-mass stars are rare because they may be hard to form. Instead, a precision measurement of nuclear radii on atomic nuclei may contain similar information. Indeed, we find a strong correlation between the neutron radius of 208 Pb and the radius of a 0.5M⊙ neutron star. Thus, the radius of a 0.5M⊙ neutron star can be inferred from a measurement of the the neutron radius of 208 Pb. Comparing this value to the measured radius of a ≃ 1.4M⊙ neutron star should provide the strongest constraint to date on the density dependence of the equation of state.

Vlasov formalism for extended relativistic mean field models: The crust-core transition and the stellar matter equation of state

“…Our results agree well with those of Refs. [46,47]. The neutron radius and, therefore, the neutron skin thickness, decrease with decreasing L for both families.…”

“…The models NL3ωρ, Z271ωρ, and Z271σρ were taken from Ref. [47]. The others; namely, NL3σρ, TM1ωρ, and TM1σρ, are obtained by varying either the ω or σ coupling constants and by calculating the new g ρ constants, so that the symmetry energy at ρ = 0.1 fm −3 has the same value as the reference model of the family.…”

“…We analyze the effect of the σρ and ωρ couplings on the crust-core transition density and pressure at zero temperature, starting from three different relativistic mean field (RMF) models, TM1 [48], NL3 [42], and Z271 [46,47]. These three models will be designated as the heads of the families of the models that we are going to construct by adding the terms σρ or ωρ with different coupling strengths.…”

“…We consider terms of the form f 2 i ρ 2 , with f i = ω or σ , as in Refs. [46,47], which will affect the neutron skin thickness of nuclei but do not change some well-established properties of nuclei, such as the binding energy and the charge radius. In Ref.…”

Soft nuclear equation-of-state from heavy-ion data and implications for compact stars

“…With their immense interest in nuclear physics [22], neutron star radii are a focus of major attention [23][24][25][26][27][28][29][30]. Limits on radii are obtained from X-ray bursters and lowmass X-ray binaries (see [24,25,[30][31][32][33][34][35][36] and references therein) whereas such studies could be substantially refined in the future with the Large Observatory For X-ray Timing (LOFT) [37].…”