2,246 research outputs found
The neutron star in Cassiopeia A: equation of state, superfluidity, and Joule heating
The thermomagnetic evolution of the young neutron star in Cassiopea A is
studied by considering fast neutrino emission processes. In particular, we
consider neutron star models obtained from the equation of state computed in
the framework of the Brueckner-Bethe-Goldstone many-body theory and variational
methods, and models obtained with the Akmal-Pandharipande-Ravenhall equation of
state. It is shown that it is possible to explain a fast cooling regime as the
one observed in the neutron star in Cassiopea A if the Joule heating produced
by dissipation of the small-scale magnetic field in the crust is taken into
account. We thus argue that it is difficult to put severe constraints on the
superfluid gap if the Joule heating is considered.Comment: 4 pages, 2 figures, to appear on A&A Letter
Chaoticity and Dissipation of Nuclear Collective Motion in a Classical Model
We analyze the behavior of a gas of classical particles moving in a
two-dimensional "nuclear" billiard whose multipole-deformed walls undergo
periodic shape oscillations. We demonstrate that a single particle Hamiltonian
containing coupling terms between the particles' motion and the collective
coordinate induces a chaotic dynamics for any multipolarity, independently on
the geometry of the billiard. The absence of coupling terms allows us to
recover qualitatively the "wall formula" predictions. We also discuss the
dissipative behavior of the wall motion and its relation with the
order-to-chaos transition in the dynamics of the microscopic degrees of
freedom.Comment: LateX, 11 pages, 7 figures available on request, to appear in the
Proceedings of XXXIV Winter Meeting on Nuclear Physics, Bormio 22-27 January,
199
Chaos vs. Linear Instability in the Vlasov Equation: A Fractal Analysis Characterization
In this work we discuss the most recent results concerning the Vlasov
dynamics inside the spinodal region. The chaotic behaviour which follows an
initial regular evolution is characterized through the calculation of the
fractal dimension of the distribution of the final modes excited. The ambiguous
role of the largest Lyapunov exponent for unstable systems is also critically
reviewed.Comment: 10 pages, RevTeX, 4 figures not included but available upon reques
Relativistic Approach to Superfluidity in Nuclear Matter
Pairing correlations in symmetric nuclear matter are studied within a
relativistic mean-field approximation based on a field theory of nucleons
coupled to neutral ( and ) and to charged () mesons.
The Hartree-Fock and the pairing fields are calculated in a self-consistent
way. The energy gap is the result of a strong cancellation between the scalar
and vector components of the pairing field. We find that the pair amplitude
vanishes beyond a certain value of momentum of the paired nucleons. This fact
determines an effective cutoff in the gap equation. The value of this cutoff
gives an energy gap in agreement with the estimates of non relativistic
calculations.Comment: 21 pages, REVTEX, 8 ps-figures, to appear in Phys.Rev.C. e-mail:
[email protected]
Neutron-antineutron Oscillations in the Trapping Box
We have reexamined the problem of oscillations for ultra-cold
neutrons (UCN) confined within a trap. We have shown that the growth of the
component with time is to a decent accuracy given by where is the mixing parameter,
sec in the neutron propagation time between subsequent collisions
with the trap walls. Possible corrections to this law and open questions are
discussed.Comment: 11 pages, LaTeX2
Elementary excitations in homogeneous superfluid neutron star matter: Role of the proton component
The thermal evolution of neuron stars depends on the elementary excitations
affecting the stellar matter. In particular, the low-energy excitations, whose
energy is proportional to the transfered momentum, can play a major role in the
emission and propagation of neutrinos. In this paper, we focus on the density
modes associated with the proton component in the homogeneous matter of the
outer core of neutron stars (at density between one and three times the nuclear
saturation density, where the baryonic constituants are expected to be neutrons
and protons). In this region, it is predicted that the protons are
superconductor. We study the respective roles of the proton pairing and Coulomb
interaction in determining the properties of the modes associated with the
proton component. This study is performed in the framework of the Random Phase
Approximation, generalized in order to describe the response of a superfluid
system.The formalism we use ensures that the Generalized Ward's Identities are
satisfied. An important conclusion of this work is the presence of a
pseudo-Goldstone mode associated with the proton superconductor in neutron-star
matter. Indeed, the Goldstone mode, which characterizes a pure superfluid, is
suppressed in usual superconductors due to the long-range Coulomb interaction,
which only allows a plasmon mode. However, for the proton component of stellar
matter, the Coulomb field is screened by the electrons and a pseudo-Goldstone
mode occurs, with a velocity increased by the Coulomb interaction.Comment: Submitted for publicatio
Screening Effects in Superfluid Nuclear and Neutron Matter within Brueckner Theory
Effects of medium polarization are studied for pairing in neutron and
nuclear matter. The screening potential is calculated in the RPA limit,
suitably renormalized to cure the low density mechanical instability of nuclear
matter. The selfenergy corrections are consistently included resulting in a
strong depletion of the Fermi surface. All medium effects are calculated based
on the Brueckner theory. The gap is determined from the generalized gap
equation. The selfenergy corrections always lead to a quenching of the gap,
which is enhanced by the screening effect of the pairing potential in neutron
matter, whereas it is almost completely compensated by the antiscreening effect
in nuclear matter.Comment: 8 pages, 6 Postscript figure
Critical Enhancement of the In-medium Nucleon-Nucleon Cross Section at low Temperatures
The in-medium nucleon-nucleon cross section is calculated starting from the
thermodynamic T-matrix at finite temperatures. The corresponding
Bethe-Salpeter-equation is solved using a separable representation of the Paris
nucleon-nucleon-potential. The energy-dependent in-medium N-N cross section at
a given density shows a strong temperature dependence. Especially at low
temperatures and low total momenta, the in-medium cross section is strongly
modified by in-medium effects. In particular, with decreasing temperature an
enhancement near the Fermi energy is observed. This enhancement can be
discussed as a precursor of the superfluid phase transition in nuclear matter.Comment: 10 pages with 4 figures (available on request from the authors),
MPG-VT-UR 34/94 accepted for publication in Phys. Rev.
- …