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Abstract
The appearance of delta isobars in beta-stable matter is regulated by the be- haviour of the symmetry energy at densities larger than saturation density. We show that by taking into account recent constraints on the density derivative of the symmetry energy and the theoretical and experimental results on the excitations of delta isobars in nuclei, delta isobars are necessary ingredients for the equations of state used for studying neutron stars. We analyze the effect of the appearance of deltas on the structure of neutron stars: as in the case of hyperons, matter containing delta is too soft for allowing the existence of 2Mneutron stars. Quark stars on the other hand, could reach very massive configu- rations and they could form from a process of conversion of hadronic stars in which an initial seed of strangeness appears through hyperons.
Highlights
Understanding the properties of strongly interacting matter at densities of a few times saturation density, which are likely to be reached in the core of neutron stars, is one of the most interesting and challenging problem in nuclear and hadron physics
We have considered the problem of formation of baryons heavier than the nucleon in neutron star matter
While a huge literature is available for what concerns hyperons, little has be done regarding the formation of delta isobars
Summary
Understanding the properties of strongly interacting matter at densities of a few times saturation density, which are likely to be reached in the core of neutron stars, is one of the most interesting and challenging problem in nuclear and hadron physics. Considering hyperons for instance, while in relativistic mean field models the free parameters can be tuned to fulfill the observational limit (see for instance [8]), in more realistic calculations based in microscopic nucleon-nucleon interactions the appearance of hyperons is accompanied by a strong softening of the equation of state which leads to maximum masses much below 2M [9,10,11]: the so called “hyperon puzzle” stems from the difficulties in fulfilling the astrophysical limits and, at the same time, to account for the formation of hyperons which, being only slightly heavier than the nucleons, should form at large densities. We will review in the following the main results concerning the formation of delta isobars in neutron stars and we will discuss how to reconcile the precise measurements of high masses and the hints of small radii within a scenario of two coexisting families of compact stars
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