88 research outputs found
Echos of the liquid-gas phase transition in multifragmentation
A general discussion is made concerning the ways in which one can get
signatures about a possible liquid-gas phase transition in nuclear matter.
Microcanonical temperature, heat capacity and second order derivative of the
entropy versus energy formulas have been deduced in a general case. These
formulas are {\em exact}, simply applicable and do not depend on any model
assumption. Therefore, they are suitable to be applied on experimental data.
The formulas are tested in various situations. It is evidenced that when the
freeze-out constraint is of fluctuating volume type the deduced (heat capacity
and second order derivative of the entropy versus energy) formulas will prompt
the spinodal region through specific signals. Finally, the same microcanonical
formulas are deduced for the case when an incomplete number of fragments per
event are available. These formulas could overcome the freeze-out backtracking
deficiencies.Comment: accepted to Nuclear Physics
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
g cm 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 -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.
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 and closed shells in the collapse dynamics is considerably
decreased if the shell gaps are reduced in the region of 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
Break-up fragment topology in statistical multifragmentation models
Break-up fragmentation patterns together with kinetic and configurational
energy fluctuations are investigated in the framework of a microcanonical model
with fragment degrees of freedom over a broad excitation energy range. As far
as fragment partitioning is approximately preserved, energy fluctuations are
found to be rather insensitive to both the way in which the freeze-out volume
is constrained and the trajectory followed by the system in the excitation
energy - freeze-out volume space. Due to hard-core repulsion, the freeze-out
volume is found to be populated un-uniformly, its highly depleted core giving
the source a bubble-like structure. The most probable localization of the
largest fragments in the freeze-out volume may be inferred experimentally from
their kinematic properties, largely dictated by Coulomb repulsion
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
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
-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
Densities and energies of nuclei in dilute matter
We explore the ground-state properties of nuclear clusters embedded in a gas
of nucleons with the help of Skyrme-Hartree-Fock microscopic calculations. Two
alternative representations of clusters are introduced, namely coordinate-space
and energy-space clusters. We parameterize their density profiles in spherical
symmetry in terms of basic properties of the energy density functionals used
and propose an analytical, Woods-Saxon density profile whose parameters depend,
not only on the composition of the cluster, but also of the nucleon gas. We
study the clusters' energies with the help of the local-density approximation,
validated through our microscopic results. We find that the volume energies of
coordinate-space clusters are determined by the saturation properties of
matter, while the surface energies are strongly affected by the presence of the
gas. We conclude that both the density profiles and the cluster energies are
strongly affected by the gas and discuss implications for the nuclear EoS and
related perspectives. Our study provides a simple, but microscopically
motivated modeling of the energetics of clusterized matter at subsaturation
densities, for direct use in consequential applications of astrophysical
interest.Comment: 20 pages, incl. 12 figure
Fragment isospin distributions and the phase diagram of excited nuclear systems
Fragment average isospin distributions are investigated within a
microcanonical multifragmentation model in different regions of the phase
diagram. The results indicate that in the liquid phase versus is
monotonically increasing, in the phase coexistence region it has a rise and
fall shape and in the gas phase it is constant. Deviations from this behavior
may manifest at low fragment multiplicity as a consequence of mass/charge
conservation. Characterization of the "free" and "bound" phases function of
fragment charge reconfirms the neutron enrichment of the "free" phase with
respect to the "bound" one irrespectively the localization of the
multifragmentation event in the phase diagram.Comment: 23 pages, 12 figure
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