Clusterized nuclear matter in the (proto-)neutron star crust and the symmetry energy

Though generally agreed that the symmetry energy plays a dramatic role in determining the structure of neutron stars and the evolution of core-collapsing supernovae, little is known in what concerns its value away from normal nuclear matter density and, even more important, the correct definition of this quantity in the case of unhomogeneous matter. Indeed, nuclear matter traditionally addressed by mean-field models is uniform while clusters are known to exist in the dilute baryonic matter which constitutes the main component of compact objects outer shells. In the present work we investigate the meaning of symmetry energy in the case of clusterized systems and the sensitivity of the proto-neutron star composition and equation of state to the effective interaction. To this aim an improved Nuclear Statistical Equilibrium (NSE) model is developed, where the same effective interaction is consistently used to determine the clusters and unbound particles energy functionals in the self-consistent mean-field approximation. In the same framework, in-medium modifications to the cluster energies due to the presence of the nuclear gas are evaluated. We show that the excluded volume effect does not exhaust the in-medium effects and an extra isospin and density dependent energy shift has to be considered to consistently determine the composition of subsaturation stellar matter. The symmetry energy of diluted matter is seen to depend on the isovector properties of the effective interaction, but its behavior with density and its quantitative value are strongly modified by clusterization.Comment: A contribution to the upcoming EPJA Special Volume on Nuclear Symmetry Energ

Statistical description of complex nuclear phases in supernovae and proto-neutron stars

We develop a phenomenological statistical model for dilute star matter at finite temperature, in which free nucleons are treated within a mean-field approximation and nuclei are considered to form a loosely interacting cluster gas. Its domain of applicability, that is baryonic densities ranging from about $\rho>10^8$ g $\cdot$ cm$^{-3}$ to normal nuclear density, temperatures between 1 and 20 MeV and proton fractions between 0.5 and 0, make it suitable for the description of baryonic matter produced in supernovae explosions and proto-neutron stars. The first finding is that, contrary to the common belief, the crust-core transition is not first order, and for all subsaturation densities matter can be viewed as a continuous fluid mixture between free nucleons and massive nuclei. As a consequence, the equations of state and the associated observables do not present any discontinuity over the whole thermodynamic range. We further investigate the nuclear matter composition over a wide range of densities and temperatures. At high density and temperature our model accounts for a much larger mass fraction bound in medium nuclei with respect to traditional approaches as Lattimer-Swesty, with sizeable consequences on the thermodynamic quantities. The equations of state agree well with the presently used EOS only at low temperatures and in the homogeneous matter phase, while important differences are present in the crust-core transition region. The correlation among the composition of baryonic matter and neutrino opacity is finally discussed, and we show that the two problems can be effectively decoupled.Comment: 40 pages, 25 figure

Multifragmentation and the symmetry term of the nuclear equation of state

We investigate the possibility to extract the symmetry energy from multifragmentation data. The applicability of the grandcanonical formula earlier proposed by Ono {\it et al.} [Phys. Rev. C {\bf 68}, 051601(R)] in the case of finite excited nuclei is tested within a microcanonical framework. Relatively good results are obtained except for large residual nuclei, especially when large sources are highly excited. Effects of secondary particle emission and the extent in which relevant information may be inferred from experimental observables are finally discussed.Comment: 13 pages, 4 figure

Unified treatment of sub-saturation stellar matter at zero and finite temperature

The standard variational derivation of stellar matter structure in the Wigner-Seitz approximation is generalized to the finite temperature situation where a wide distribution of different nuclear species can coexist in the same density and proton fraction condition, possibly out of $\beta$-equilibrium. The same theoretical formalism is shown to describe on one side the single-nucleus approximation (SNA), currently used in most core collapse supernova simulations, and on the other side the nuclear statistical equilibrium (NSE) approach, routinely employed in r- and p-process explosive nucleosynthesis problems. In particular we show that in-medium effects have to be accounted for in NSE to have a theoretical consistency between the zero and finite temperature modeling. The bulk part of these in-medium effects is analytically calculated and shown to be different from a van der Waals excluded volume term. This unified formalism allows controlling quantitatively the deviations from the SNA in the different thermodynamic conditions, as well as having a NSE model which is reliable at any arbitrarily low value of the temperature, with potential applications for neutron star cooling and accretion problems. We present different illustrative results with several mass models and effective interactions, showing the importance of accounting for the nuclear species distribution even at temperatures lower than 1 MeV.Comment: 26 pages, 17 figures; submitted to Phys. Rev.

Thermodynamics of baryonic matter with strangeness within non-relativistic energy density functional model

We study the thermodynamical properties of compressed baryonic matter with strangeness within non-relativistic energy density functional models with a particular emphasis on possible phase transitions found earlier for a simple $n,p,e,\Lambda$-mixture. The aim of the paper is twofold: I) examining the phase structure of the complete system, including the full baryonic octet and II) testing the sensitivity of the results to the model parameters. We find that, associated to the onset of the different hyperonic families, up to three separate strangeness-driven phase transitions may occur. Consequently, a large fraction of the baryonic density domain is covered by phase coexistence with potential relevance for (proto)-neutron star evolution. It is shown that the presence of a phase transition is compatible both with the observational constraint on the maximal neutron star mass, and with the present experimental information on hypernuclei. In particular we show that two solar mass neutron stars are compatible with important hyperon content. Still, the parameter space is too large to give a definitive conclusion of the possible occurrence of a strangeness driven phase transition, and further constraints from multiple-hyperon nuclei and/or hyperon diffusion data are needed.Comment: 11 pages, 7 figure

Modification of magicity towards the dripline and its impact on electron-capture rates for stellar core-collapse

The importance of microphysical inputs from laboratory nuclear experiments and theoretical nuclear structure calculations in the understanding of the core collapse dynamics, and the subsequent supernova explosion, is largely recognized in the recent literature. In this work, we analyze the impact of the masses of very neutron rich nuclei on the matter composition during collapse, and the corresponding electron capture rate. To this aim, we introduce an empirical modification of the popular Duflo-Zuker mass model to account for possible shell quenching far from stability, and study the effect of the quenching on the average electron capture rate. We show that the preeminence of the $N=50$ and $N=82$ closed shells in the collapse dynamics is considerably decreased if the shell gaps are reduced in the region of $^{78}$Ni and beyond. As a consequence, local modifications of the overall electron capture rate up to 30\% can be expected, with integrated values strongly dependent on the stiffness of magicity quenching and progenitor mass and potential important consequences on the entropy generation, the neutrino emissivity, and the mass of the core at bounce. Our work underlines the importance of new experimental measurements in this region of the nuclear chart, the most crucial information being the nuclear mass and the Gamow-Teller strength. Reliable microscopic calculations of the associated elementary rate, in a wide range of temperatures and electron densities, optimized on these new empirical information, will be additionally needed to get quantitative predictions of the collapse dynamics.Comment: 12 pages, 10 figure

Levels of abstraction in human supervisory control teams

This paper aims to report a study into the levels of abstraction hierarchy (LOAH) in two energy distribution teams. The original proposition for the LOAH was that it depicted five levels of system representation, working from functional purpose through to physical form to determine causes of a malfunction, or from physical form to functional purpose to determine the purpose of system function. The LOAH has been widely used throughout human supervisory control research to explain individual behaviour. The research seeks to focus on the application the LOAH to human supervisory control teams in semi-automated “intelligent” systems

A Mechanism for Hadron Molecule Production in p pbar(p) Collisions

We propose a mechanism allowing the formation of loosely bound molecules of charmed mesons in high energy proton-(anti)proton collisions.Comment: 4 pages, 3 figure

Testing Hollnagel's contextual control mod

Please contact publisher for further reprinting or re-useThis article sets out to test the hypothetical COtext and COntrol Model (COCOM) developed by Hollnagel (1993). Essentially, Hollnagel develops the argument that team behavior should be analyzed at a macro, rather than micro, level. He proposes 4 principal models of team activity: strategic, tactical, opportunistic, and scrambled. These modes of team behavior vary in terms of the degree of forward planning (highest in the strategic mode) and reactivity to the environment (highest in the scrambled mode). He further hypothesizes a linear progression through the modes from strategic to tactical to opportunistic to scrambled, depending on context, and vice versa. To test the COCOM model, we placed teams of people in a simulated energy distribution system. Our results confirm Hollnagel's hypothesized model in 2 main ways. First, we show that the team behavior could be categorized reliably into the 4 control modes and this provided a useful way of distinguishing between experimental conditions. Second, the progression between control modes conformed to the linear progression as predicted. This research provided the first independent test of the COCOM model and lends empirical support to the hypotheses