15,009 research outputs found
Dense matter equation of state and neutron star properties from nuclear theory and experiment
The equation of state of dense matter determines the structure of neutron
stars, their typical radii, and maximum masses. Recent improvements in
theoretical modeling of nuclear forces from the low-energy effective field
theory of QCD has led to tighter constraints on the equation of state of
neutron-rich matter at and somewhat above the densities of atomic nuclei, while
the equation of state and composition of matter at high densities remains
largely uncertain and open to a multitude of theoretical speculations. In the
present work we review the latest advances in microscopic modeling of the
nuclear equation of state and demonstrate how to consistently include also
empirical nuclear data into a Bayesian posterior probability distribution for
the model parameters. Derived bulk neutron star properties such as radii,
moments of inertia, and tidal deformabilities are computed, and we discuss as
well the limitations of our modeling.Comment: 9 pages, 5 figures. To appear in the AIP Proceedings of the
Xiamen-CUSTIPEN Workshop on the EOS of Dense Neutron-Rich Matter in the Era
of Gravitational Wave Astronomy, Jan. 3-7, Xiamen, Chin
Hans Bethe: The Nuclear Many Body Problem
We discuss the work of Hans Bethe and others in formulating a theoretical
foundation for the nuclear shell model. Written for a general audience, this
article describes the evolution from Brueckner's reaction matrix theory to the
Moszkowski-Scott separation method and ultimately to the Reference Spectrum
method of Bethe, Brandow, and Petschek. We also discuss connections with the
recently developed low momentum nucleon-nucleon interactions.Comment: 25 pages, 15 figures, In "Hans Bethe and His Physics" (World
Scientific, Singapore, 2006
Proton pairing in neutron stars from chiral effective field theory
We study the proton pairing gap in beta-equilibrated neutron star
matter within the framework of chiral effective field theory. We focus on the
role of three-body forces, which strongly modify the effective proton-proton
spin-singlet interaction in dense matter. We find that three-body forces
generically reduce both the size of the pairing gap and the maximum density at
which proton pairing may occur. The pairing gap is computed within BCS theory,
and model uncertainties are estimated by varying the nuclear potential and the
choice of single-particle spectrum in the gap equation. We find that a
second-order perturbative treatment of the single-particle spectrum suppresses
the proton pairing gap relative to the use of a free spectrum. We
estimate the critical temperature for the onset of proton superconductivity to
be K, which is consistent with previous
theoretical results in the literature and marginally within the range deduced
from a recent Bayesian analysis of neutron star cooling observations.Comment: 8 pages, 9 figure
Chiral Fermi liquid approach to neutron matter
We present a microscopic calculation of the complete quasiparticle
interaction, including central as well as noncentral components, in neutron
matter from high-precision two- and three-body forces derived within the
framework of chiral effective field theory. The contributions from two-nucleon
forces are computed in many-body perturbation theory to first and second order
(without any simplifying approximations). In addition we include the
leading-order one-loop diagrams from the N2LO chiral three-nucleon force, which
contribute to all Fermi liquid parameters except those associated with the
center-of-mass tensor interaction. The relative-momentum dependence of the
quasiparticle interaction is expanded in Legendre polynomials up to L=2.
Second-order Pauli blocking and medium polarization effects act coherently in
specific channels, namely for the Landau parameters f_1, h_0 and g_0, which
results in a dramatic increase in the quasiparticle effective mass as well as a
decrease in both the effective tensor force and the neutron matter spin
susceptibility. For densities greater than about half nuclear matter saturation
density \rho_0, the contributions to the Fermi liquid parameters from the
leading-order chiral three-nucleon force scale in all cases approximately
linearly with the nucleon density. The largest effect of the three-nucleon
force is to generate a strongly repulsive effective interaction in the
isotropic spin-independent channel. We show that the leading-order chiral
three-nucleon force leads to an increase in the spin susceptibility of neutron
matter, but we observe no evidence for a ferromagnetic spin instability in the
vicinity of the saturation density \rho_0. This work sets the foundation for
future studies of neutron matter response to weak and electromagnetic probes
with applications to neutron star structure and evolution.Comment: 21 pages, 6 figures, 5 table
Job Reallocation, Unemployment and Hours in a New Keynesian Model
This paper focusses on the reallocation of labour resources in a New Keynesian environment with labour market search and endogenous separations. We show that introduction of variation in hours per worker alters the incentives for intertemporal substitution in a way that generates a more steeply downward sloping Beveridge curve and reduces the tendency to synchronise gross job flows. This also enables the New Keynesian model to capture the interaction of hours and employment at business cycle frequencies. We show that the impact of labour supply elasticity on the slope of the Beveridge curve and the correlation of gross job flows is determined primarily by variation in the response to monetary shocks. When hours variation is suppressed the comovement of job creation with job destruction and of unemployment with vacancies are strongly positive in response to monetary shocks. Whereas with variation in hours both measures of reallocation take on the correct negative sign. We also note that frictions in price adjustment make it possible to account a large part of the variation in unemployment observed in US data, despite the absence of wage rigidity or departures from the Hosios condition which have been proposed to resolve the unemployment variability puzzle identified by Shimer (2005).
Study of vortex valve for medium temperature solid propellants
Fluid state vortex valve secondary injection control system shows considerable promise for future application to solid propellant rocket engine thrust vector control. The single axis injection system tested would be capable of providing secondary injection thrust vector control using 2000 deg F gas
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