Dynamics of fragment formation in neutron-rich matter

P.ALCAIN; C. O. DORSO

Physical Review C

American Physical Society

Lugar: New York; Año: 2018 vol. 97 p. 1 - 7

2469-9985

Background:Neutron stars are astronomical systems with nucleons subjected to extreme conditions. Due to thelonger range Coulomb repulsion between protons, the system has structural inhomogeneities. Several interactionstailored to reproduce nuclear matter plus a screened Coulomb term reproduce these inhomogeneities known asnuclear pasta. These structural inhomogeneities, located in the crusts of neutron stars, can also arise in expandingsystems depending on the thermodynamic conditions (temperature, proton fraction, etc.) and the expansionvelocity.Purpose:We aim to find the dynamics of the fragment formation for expanding systems simulated according tothe little big bang model. This expansion resembles the evolution of merging neutron stars.Method:We study the dynamics of the nucleons with semiclassical molecular dynamics models. Startingwith an equilibrium configuration, we expand the system homogeneously until we arrive at an asymptoticconfiguration (i.e., very low final densities). We study, with four different cluster recognition algorithms, thefragment distribution throughout this expansion and the dynamics of the cluster formation.Results:Studying the topology of the equilibrium states, before the expansion, we reproduced the known pastaphases plus a novel phase we calledpregnocchi, consisting of proton aggregates embedded in aneutron sea.Wehave identified different fragmentation regimes, depending on the initial temperature and fragment velocity. Inparticular, for the already mentioned pregnocchi, a neutron cloud surrounds the clusters during the early stagesof the expansion, resulting in systems that give rise to configurations compatible with the emergence of therprocess.Conclusions:We showed that a proper identification of the cluster distribution is highly dependent on the clusterrecognition algorithm chosen, and found that the early cluster recognition algorithm (ECRA) was the most stableone. This approach allowed us to identify the dynamics of the fragment formation. These calculations pave theway to a comparison between Earth experiments and neutron star studies.