33,471 research outputs found
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
Light clusters in nuclear matter: Excluded volume versus quantum many-body approaches
The formation of clusters in nuclear matter is investigated, which occurs
e.g. in low energy heavy ion collisions or core-collapse supernovae. In
astrophysical applications, the excluded volume concept is commonly used for
the description of light clusters. Here we compare a phenomenological excluded
volume approach to two quantum many-body models, the quantum statistical model
and the generalized relativistic mean field model. All three models contain
bound states of nuclei with mass number A <= 4. It is explored to which extent
the complex medium effects can be mimicked by the simpler excluded volume
model, regarding the chemical composition and thermodynamic variables.
Furthermore, the role of heavy nuclei and excited states is investigated by use
of the excluded volume model. At temperatures of a few MeV the excluded volume
model gives a poor description of the medium effects on the light clusters, but
there the composition is actually dominated by heavy nuclei. At larger
temperatures there is a rather good agreement, whereas some smaller differences
and model dependencies remain.Comment: 12 pages, 6 figures, published version, minor change
Statistical Model for a Complete Supernova Equation of State
A statistical model for the equation of state (EOS) and the composition of
supernova matter is presented with focus on the liquid-gas phase transition of
nuclear matter. It consists of an ensemble of nuclei and interacting nucleons
in nuclear statistical equilibrium. A relativistic mean field model is applied
for the nucleons. The masses of the nuclei are taken from nuclear structure
calculations which are based on the same nuclear Lagrangian. For known nuclei
experimental data is used directly. Excluded volume effects are implemented in
a thermodynamic consistent way so that the transition to uniform nuclear matter
at large densities can be described. Thus the model can be applied at all
densities relevant for supernova simulations, i.e. rho=10^5 - 10^15 g/cm^3, and
it is possible to calculate a complete supernova EOS table. The model allows to
investigate the role of shell effects, which lead to narrow-peaked
distributions around the neutron magic numbers for low temperatures. At larger
temperatures the distributions become broad. The significance of the
statistical treatment and the nuclear distributions for the composition is
shown. We find that the contribution of light clusters is very important and is
only poorly represented by alpha-particles alone. The results for the EOS are
systematically compared to two commonly used models for supernova matter which
are based on the single nucleus approximation. Apart from the composition, in
general only small differences of the different EOSs are found. The differences
are most pronounced around the (low-density) liquid-gas phase transition line
where the distribution of light and intermediate clusters has an important
effect. Possible extensions and improvements of the model are discussed.Comment: 33 pages, 22 page
A statistical study of the mass and density structure of Infrared Dark Clouds
How and when the mass distribution of stars in the Galaxy is set is one of
the main issues of modern astronomy. Here we present a statistical study of
mass and density distributions of infrared dark clouds (IRDCs) and fragments
within them. These regions are pristine molecular gas structures and
progenitors of stars and so provide insights into the initial conditions of
star formation. This study makes use of a IRDC catalogue (Peretto & Fuller
2009), the largest sample of IRDC column density maps to date, containing a
total of ~11,000 IRDCs with column densities exceeding N_{H2} = 1 X10^{22}
cm^{-2} and over 50,000 single peaked IRDC fragments. The large number of
objects constitutes an important strength of this study, allowing detailed
analysis of the completeness of the sample and so statistically robust
conclusions. Using a statistical approach to assigning distances to clouds, the
mass and density distributions of the clouds and the fragments within them are
constructed. The mass distributions show a steepening of the slope when
switching from IRDCs to fragments, in agreement with previous results of
similar structures. IRDCs and fragments are divided into unbound/bound objects
by assuming Larson's relation and calculating their virial parameter. IRDCs are
mostly gravitationally bound, while a significant fraction of the fragments are
not. The density distribution of gravitationally unbound fragments shows a
steep characteristic slope. (see paper for full Abstract).Comment: 15 pages, accepted for publication in Ap
Superfluid Properties of the Inner Crust of Neutron Stars
Superfluid properties of the inner crust matter of neutron stars, formed by
nuclear clusters immersed in a dilute neutron gas, are analysed in a self-
consistent HFB approach. The calculations are performed with two pairing
forces, fixed so as to obtain in infinite nuclear matter the pairing gaps
provided by the Gogny force or by induced interactions. It is shown that the
nuclear clusters can either suppress or enhance the pairing correlations inside
the inner crust matter, depending on the density of the surrounding neutrons.
The profile of the pairing field in the inner crust is rather similar for both
pairing forces, but the values of the pairing gaps are drastically reduced for
the force which simulates the polarisation effects in infinite neutron matter.Comment: 13 pages, 6 figures. Corrected typos and new format. To appear in
Phys. Rev.
Strange hadron matter and SU(3) symmetry
We calculate saturation curves for strange hadron matter using recently
constructed baryon-baryon potentials which are constrained by SU(3) symmetry.
All possible interaction channels within the baryon octet (consisting of ,
, , and ) are considered. It is found that a small
fraction in nuclear matter slightly increases binding, but that
larger fractions () rapidly cause a decrease. Charge-neutral
systems, with equal densities for nucleons and cascades, are
only very weakly bound. The dependence of the binding energies on the
strangeness per baryon, , is predicted for various and
systems. The implications of our results in
relativistic heavy-ion collisions and the core of a dense star are discussed.
We also discuss the differences between our results and previous hadron matter
calculations.Comment: 14 pages RevTeX, 7 postscript figure
- …