The Equation of State for the Nucleonic and Hyperonic Core of Neutron Stars

Abstract

We reexamine the equation of state for the nucleonic and hyperonic inner core of neutron stars that satisfies the 2M observations as well as the recent determinations of stellar radii below 13 km, while fulfilling the saturation properties of nuclear matter and finite nuclei together with the constraints on the high-density nuclear pressure coming from heavy-ion collisions. The recent nucleonic FSU2R and hyperonic FSU2H models are updated in order to improve the behavior of pure neutron matter at subsaturation densities. The corresponding nuclear matter properties at saturation, the symmetry energy and its slope turn out to be compatible with recent experimental and theoretical determinations. We obtain the mass, radius and composition of neutron stars for the two updated models and study the impact on these properties of the uncertainties in the hyperon-nucleon couplings estimated from hypernuclear data. We find that the onset of appearance of each hyperon strongly depends on the hyperon-nuclear uncertainties, whereas the maximum masses for neutron stars differ by at most 0.1 M , although a larger deviation should be expected tied to the lack of knowledge of the hyperon potentials at the high densities present in the center of 2M stars. For easier use, we provide tables with the results from the FSU2R and FSU2H models for the equation of state and the neutron star mass-radius relation.

Interplay between Delta Particles and Hyperons in Neutron Stars

“…One must also consider the particular value of the coupling strength g ρN . In general, models that have a larger L value (implying a stiffer nuclear symmetry energy), also have a more moderate g ρN value, see Tolos et al (2017a), so that they may end producing ∆ − particles at relatively low densities, as is the case in Li et al (2018). Models with lower L values (softer nuclear symmetry energy), which usually come with larger g ρN strengths, give rise to larger ∆ − onset densities, as found here and in Drago et al (2014b).…”

“…Mmax of M = 1.4M , with respect to the EoS without ∆ isobars. Increasing the x σ∆ and x ω∆ parameters so as to fulfill the maximum 2M condition, gives rise to more compact stars, especially if a small value for x ρ∆ is also considered, hence producing canonical NSs with realistic radii (Tolos et al 2017a).…”

Strangeness in nuclei and neutron stars

“…Since then, the study of the properties of hyperons inside NSs have been studied by many authors using phenomenological schemes (see for example Refs. [372][373][374][375][376][377][378]), or microscopical models (see [292,307,[309][310][311][312][313][314][315][316]379] as examples). These approaches agree that hyperons may appear in the inner core of neutron stars at densities of ≈ 2-3ρ 0 .…”

Hyperonic Stars and the Nuclear Symmetry Energy

“…The rearrangement term is only present in the densitydependent models and ensures thermodynamic consistency. Besides the two models with density-dependent parameters, DD2 and DDME2, we will also consider the following set of RMF models with constant couplings (see Table I for their properties): FSU2 [33], FSU2H and FSU2R [34,39], NL3 [40], NL3 σ ρ and NL3 ωρ [2,41], TM1 [42], TM1ωρ and TM1σ ρ [41,43], TM1-2 and TM1-2 ωρ [8]. TABLE I.…”

“…Following the approach described in [22], we have obtained calibrated couplings for the FSU2 [33], and the FSU2R and FSU2H RMF parametrizations recently proposed in [34]. The last two parametrizations have been fitted to both properties of nuclear matter and finite nuclei and NS properties.…”