1,151 research outputs found
The Effects of Competition on Variation in the Quality and Cost of Medical Care
We estimate the effects of hospital competition on the level of and the variation in quality of care and hospital expenditures for elderly Medicare beneficiaries with heart attack. We compare competition's effects on more-severely ill patients, whom we assume value quality more highly, to the effects on less-severely ill, low-valuation patients. We find that low-valuation patients in less-competitive markets receive more intensive treatment than in more-competitive markets, but have statistically similar health outcomes. In contrast, high-valuation patients in less-competitive markets receive less intensive treatment than in more-competitive markets, and have significantly worse health outcomes. Since this competition-induced increase in variation in expenditures is, on net, expenditure-decreasing and outcome-beneficial, we conclude that it is welfare-enhancing. These findings are inconsistent with conventional models of vertical differentiation, although they can be accommodated by more recent models.
Hall drift of axisymmetric magnetic fields in solid neutron-star matter
Hall drift, i. e., transport of magnetic flux by the moving electrons giving
rise to the electrical current, may be the dominant effect causing the
evolution of the magnetic field in the solid crust of neutron stars. It is a
nonlinear process that, despite a number of efforts, is still not fully
understood. We use the Hall induction equation in axial symmetry to obtain some
general properties of nonevolving fields, as well as analyzing the evolution of
purely toroidal fields, their poloidal perturbations, and current-free, purely
poloidal fields. We also analyze energy conservation in Hall instabilities and
write down a variational principle for Hall equilibria. We show that the
evolution of any toroidal magnetic field can be described by Burgers' equation,
as previously found in plane-parallel geometry. It leads to sharp current
sheets that dissipate on the Hall time scale, yielding a stationary field
configuration that depends on a single, suitably defined coordinate. This
field, however, is unstable to poloidal perturbations, which grow as their
field lines are stretched by the background electron flow, as in instabilities
earlier found numerically. On the other hand, current-free poloidal
configurations are stable and could represent a long-lived crustal field
supported by currents in the fluid stellar core.Comment: 8 pages, 5 figure panels; new version with very small correction;
accepted by Astronomy & Astrophysic
Evidence for Heating of Neutron Stars by Magnetic Field Decay
We show the existence of a strong trend between neutron star surface
temperature and the dipolar component of the magnetic field extending through
three orders of field magnitude, a range that includes magnetars, radio-quiet
isolated neutron stars, and many ordinary radio pulsars. We suggest that this
trend can be explained by the decay of currents in the crust over a time scale
of few Myr. We estimate the minimum temperature that a NS with a given magnetic
field can reach in this interpretation.Comment: 4 pages, 1 figures, version accepted for publication in Phys. Rev.
Let
Turning Points in the Evolution of Isolated Neutron Stars' Magnetic Fields
During the life of isolated neutron stars (NSs) their magnetic field passes
through a variety of evolutionary phases. Depending on its strength and
structure and on the physical state of the NS (e.g. cooling, rotation), the
field looks qualitatively and quantitatively different after each of these
phases. Three of them, the phase of MHD instabilities immediately after NS's
birth, the phase of fallback which may take place hours to months after NS's
birth, and the phase when strong temperature gradients may drive thermoelectric
instabilities, are concentrated in a period lasting from the end of the
proto--NS phase until 100, perhaps 1000 years, when the NS has become almost
isothermal. The further evolution of the magnetic field proceeds in general
inconspicuous since the star is in isolation. However, as soon as the product
of Larmor frequency and electron relaxation time, the so-called magnetization
parameter, locally and/or temporally considerably exceeds unity, phases, also
unstable ones, of dramatic changes of the field structure and magnitude can
appear. An overview is given about that field evolution phases, the outcome of
which makes a qualitative decision regarding the further evolution of the
magnetic field and its host NS.Comment: References updated, typos correcte
Thermal conductivity of ions in a neutron star envelope
We analyze the thermal conductivity of ions (equivalent to the conductivity
of phonons in crystalline matter) in a neutron star envelope.
We calculate the ion/phonon thermal conductivity in a crystal of atomic
nuclei using variational formalism and performing momentum-space integration by
Monte Carlo method. We take into account phonon-phonon and phonon-electron
scattering mechanisms and show that phonon-electron scattering dominates at not
too low densities. We extract the ion thermal conductivity in ion liquid or gas
from literature.
Numerical values of the ion/phonon conductivity are approximated by
analytical expressions, valid for T>10^5 K and 10^5 g cm^-3 < \rho < 10^14 g
cm^-3. Typical magnetic fields B~10^12 G in neutron star envelopes do not
affect this conductivity although they strongly reduce the electron thermal
conductivity across the magnetic field. The ion thermal conductivity remains
much smaller than the electron conductivity along the magnetic field. However,
in the outer neutron star envelope it can be larger than the electron
conductivity across the field, that is important for heat transport across
magnetic field lines in cooling neutron stars. The ion conductivity can greatly
reduce the anisotropy of heat conduction in outer envelopes of magnetized
neutron stars.Comment: 12 pages, 5 figures; to appear in MNRA
Flux Expulsion - Field Evolution in Neutron Stars
Models for the evolution of magnetic fields of neutron stars are constructed,
assuming the field is embedded in the proton superconducting core of the star.
The rate of expulsion of the magnetic flux out of the core, or equivalently the
velocity of outward motion of flux-carrying proton-vortices is determined from
a solution of the Magnus equation of motion for these vortices. A force due to
the pinning interaction between the proton-vortices and the neutron-superfluid
vortices is also taken into account in addition to the other more conventional
forces acting on the proton-vortices. Alternative models for the field
evolution are considered based on the different possibilities discussed for the
effective values of the various forces. The coupled spin and magnetic evolution
of single pulsars as well as those processed in low-mass binary systems are
computed, for each of the models. The predicted lifetimes of active pulsars,
field strengths of the very old neutron stars, and distribution of the magnetic
fields versus orbital periods in low-mass binary pulsars are used to test the
adopted field decay models. Contrary to the earlier claims, the buoyancy is
argued to be the dominant driving cause of the flux expulsion, for the single
as well as the binary neutron stars. However, the pinning is also found to play
a crucial role which is necessary to account for the observed low field binary
and millisecond pulsars.Comment: 23 pages, + 7 figures, accepted for publication in Ap
Magnetars as cooling neutron stars with internal heating
We study thermal structure and evolution of magnetars as cooling neutron
stars with a phenomenological heat source in a spherical internal layer. We
explore the location of this layer as well as the heating rate that could
explain high observable thermal luminosities of magnetars and would be
consistent with the energy budget of neutron stars. We conclude that the heat
source should be located in an outer magnetar's crust, at densities rho < 5e11
g/cm^3, and should have the heat intensity of the order of 1e20 erg/s/cm^3.
Otherwise the heat energy is mainly emitted by neutrinos and cannot warm up the
surface.Comment: 8 pages, 5 figures, submitted to MNRA
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