199 research outputs found
Structure of neutron stars with unified equations of state
We present a set of three unified equations of states (EoSs) based on the
nuclear energy-density functional (EDF) theory.These EoSs are based on
generalized Skyrme forces fitted to essentially all experimental atomic mass
data and constrained to reproduce various properties of infinite nuclear matter
as obtained from many-body calculations using realistic two- and three-body
interactions. The structure of cold isolated neutron stars is discussed in
connection with some astrophysical observations.Comment: 4 pages, to appear in the proceedings of the ERPM conference, Zielona
Gora, Poland, April 201
Further explorations of Skyrme-Hartree-Fock-Bogoliubov mass formulas. XI: Stabilizing neutron stars against a ferromagnetic collapse
We construct a new Hartree-Fock-Bogoliubov (HFB) mass model, labeled HFB-18,
with a generalized Skyrme force. The additional terms that we have introduced
into the force are density-dependent generalizations of the usual and
terms, and are chosen in such a way as to avoid the high-density
ferromagnetic instability of neutron stars that is a general feature of
conventional Skyrme forces, and in particular of the Skyrme forces underlying
all the HFB mass models that we have developed in the past. The remaining
parameters of the model are then fitted to essentially all the available mass
data, an rms deviation of 0.585 MeV being obtained. The new model thus
gives almost as good a mass fit as our best-fit model HFB-17 ( = 0.581
MeV), and has the advantage of avoiding the ferromagnetic collapse of neutron
stars.Comment: accepted for publication in Physical Review
Neutron drip transition in accreting and nonaccreting neutron star crusts
The neutron-drip transition in the dense matter constituting the interior of
neutron stars generally refers to the appearance of unbound neutrons as the
matter density reaches some threshold density . This
transition has been mainly studied under the cold catalyzed matter hypothesis.
However, this assumption is unrealistic for accreting neutron stars. After
examining the physical processes that are thought to be allowed in both
accreting and nonaccreting neutron stars, suitable conditions for the onset of
neutron drip are derived and general analytical expressions for the neutron
drip density and pressure are obtained. Moreover, we show that the neutron-drip
transition occurs at lower density and pressure than those predicted within the
mean-nucleus approximation. This transition is studied numerically for various
initial composition of the ashes from X-ray bursts and superbursts using
microscopic nuclear mass models.Comment: 24 pages, accepted for publication in Physical Review
Symmetry energy: nuclear masses and neutron stars
We describe the main features of our most recent Hartree-Fock-Bogoliubov
nuclear mass models, based on 16-parameter generalized Skyrme forces. They have
been fitted to the data of the 2012 Atomic Mass Evaluation, and favour a value
of 30 MeV for the symmetry coefficient J, the corresponding root-mean square
deviation being 0.549 MeV. We find that this conclusion is compatible with
measurements of neutron-skin thickness. By constraining the underlying
interactions to fit various equations of state of neutron matter calculated
{\it ab initio} our models are well adapted to a realistic and unified
treatment of all regions of neutron stars. We use our models to calculate the
composition, the equation of state, the mass-radius relation and the maximum
mass. Comparison with observations of neutron stars again favours a value of J
= 30 MeV.Comment: 10 pages, 9 figures, to appear in EPJA special volume on symmetry
energ
Unified description of neutron superfluidity in the neutron-star crust with analogy to anisotropic multi-band BCS superconductors
The neutron superfluidity in the inner crust of a neutron star has been
traditionally studied considering either homogeneous neutron matter or only a
small number of nucleons confined inside the spherical Wigner-Seitz cell.
Drawing analogies with the recently discovered multi-band superconductors, we
have solved the anisotropic multi-band BCS gap equations with Bloch boundary
conditions, thus providing a unified description taking consistently into
account both the free neutrons and the nuclear clusters. Calculations have been
carried out using the effective interaction underlying our recent
Hartree-Fock-Bogoliubov nuclear mass model HFB-16. We have found that even
though the presence of inhomogeneities lowers the neutron pairing gaps, the
reduction is much less than that predicted by previous calculations using the
Wigner-Seitz approximation. We have studied the disappearance of superfluidity
with increasing temperature. As an application we have calculated the neutron
specific heat, which is an important ingredient for modeling the thermal
evolution of newly-born neutron stars. This work provides a new scheme for
realistic calculations of superfluidity in neutron-star crusts.Comment: 15 pages, 31 figures, accepted for publication in Physical Review
Giant Pulsar Glitches and the Inertia of Neutron-Star Crusts
Giant pulsar frequency glitches as detected in the emblematic Vela pulsar
have long been thought to be the manifestation of a neutron superfluid
permeating the inner crust of a neutron star. However, this superfluid has been
recently found to be entrained by the crust, and as a consequence it does not
carry enough angular momentum to explain giant glitches. The extent to which
pulsar-timing observations can be reconciled with the standard vortex-mediated
glitch theory is studied considering the current uncertainties on dense-matter
properties. To this end, the crustal moment of inertia of glitching pulsars is
calculated employing a series of different unified dense-matter equations of
state.Comment: 11 pages, 6 figures, submitted to PR
Finite-size effects and collective vibrations in the inner crust of neutron stars
We study the linear response of the inner crust of neutron stars within the
Random Phase Approximation, employing a Skyrme-type interaction as effective
interaction. We adopt the Wigner-Seitz approximation, and consider a single
unit cell of the Coulomb lattice which constitutes the inner crust, with a
nucleus at its center, surrounded by a sea of free neutrons. With the use of an
appropriate operator, it is possible to analyze in detail the properties of the
vibrations of the surface of the nucleus and their interaction with the modes
of the sea of free neutrons, and to investigate the role of shell effects and
of resonant states
Entrainment coefficient and effective mass for conduction neutrons in neutron star crust: II Macroscopic treatment
Phenomena such as pulsar frequency glitches are believed to be attributable
to differential rotation of a current of ``free'' superfluid neutrons at
densities above the ``drip'' threshold in the ionic crust of a neutron star.
Such relative flow is shown to be locally describable by adaption of a
canonical two fluid treatment that emphasizes the role of the momentum
covectors constructed by differentiation of action with respect to the
currents, with allowance for stratification whereby the ionic number current
may be conserved even when the ionic charge number Z is altered by beta
processes. It is demonstrated that the gauge freedom to make different choices
of the chemical basis determining which neutrons are counted as ``free'' does
not affect their ``superfluid'' momentum covector, which must locally have the
form of a gradient (though it does affect the ``normal'' momentum covector
characterising the protons and those neutrons that are considered to be
``confined'' in the nuclei). It is shown how the effect of ``entrainment''
(whereby the momentum directions deviate from those of the currents) is
controlled by the (gauge independent) mobility coefficient K, estimated in
recent microscopical quantum mechanical investigations, which suggest that the
corresponding (gauge dependent) ``effective mass'' m* of the free neutrons can
become very large in some layers. The relation between this treatment of the
crust layers and related work (using different definitions of ``effective
mass'') intended for the deeper core layers is discussed.Comment: 21 pages Latex. Part II of article whose Part I (Simple microscopic
models) is given by nucl-th/0402057. New version extended to include figure
Masses of neutron stars and nuclei
We calculate the maximum mass of neutron stars for three different equations
of state (EOS) based on generalized Skyrme functionals that are simultaneously
fitted to essentially all the 2003 nuclear mass data (the rms deviation is 0.58
MeV in all three cases) and to one or other of three different equations of
state of pure neutron matter, each determined by a different many-body
calculation using realistic two- and three-body interactions but leading to
significantly different degrees of stiffness at the high densities prevailing
in neutron-star interiors. The observation of a neutron star with mass 1.97
0.04 eliminates the softest of our models (BSk19),
but does not discriminate between BSk20 and BSk21. However, nuclear-mass
measurements that have been made since our models were constructed strongly
favor BSk21, our stiffest functional.Comment: 11 pages, 4 figures; Physical Review C in pres
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