5,018 research outputs found
Ultraviolet light curves of U Geminorum and VW Hydri
Ultraviolet light curves were obtained for the quiescent dwarf novae U Gem and VW Hyi. The amplitude of the hump associated with the accretion hot spot is much smaller in the UV than in the visible. This implies that the bright spot temperature is roughly 12000 K if it is optically thick. The flux distribution of U Gem in quiescence cannot be fitted by model spectra of steady state, viscous accretion disks. The absolute luminosity, the flux distribution, and the far UV spectrum suggest that the primary star is visible in the far UV. The optical UV flux distribution of VW Hyi can be matched roughly by the model accretion disks
ON NON-RIEMANNIAN PARALLEL TRANSPORT IN REGGE CALCULUS
We discuss the possibility of incorporating non-Riemannian parallel transport
into Regge calculus. It is shown that every Regge lattice is locally equivalent
to a space of constant curvature. Therefore well known-concepts of differential
geometry imply the definition of an arbitrary linear affine connection on a
Regge lattice.Comment: 12 pages, Plain-TEX, two figures (available from the author
Z_2-Regge versus Standard Regge Calculus in two dimensions
We consider two versions of quantum Regge calculus. The Standard Regge
Calculus where the quadratic link lengths of the simplicial manifold vary
continuously and the Z_2-Regge Model where they are restricted to two possible
values. The goal is to determine whether the computationally more easily
accessible Z_2 model still retains the universal characteristics of standard
Regge theory in two dimensions. In order to compare observables such as average
curvature or Liouville field susceptibility, we use in both models the same
functional integration measure, which is chosen to render the Z_2-Regge Model
particularly simple. Expectation values are computed numerically and agree
qualitatively for positive bare couplings. The phase transition within the
Z_2-Regge Model is analyzed by mean-field theory.Comment: 21 pages, 16 ps-figures, to be published in Phys. Rev.
Many-body GW calculations of ground-state properties: Quasi-2D electron systems and van der Waals forces
We present GW many-body results for ground-state properties of two simple but very distinct families of inhomogeneous systems in which traditional implementations of density-functional theory (DFT) fail drastically. The GW approach gives notably better results than the well-known random-phase approximation, at a similar computational cost. These results establish GW as a superior alternative to standard DFT schemes without the expensive numerical effort required by quantum Monte Carlo simulations
Breast cancer risk in male twins: joint analyses of four twin cohorts in Denmark, Finland, Sweden and the United States
To test the hypothesis that in utero exposure to high levels of oestrogen increases the risk of male breast cancer, we followed 115 235 male twins for more than 3.5 million person-years at risk. We observed 11 cases of male breast cancer versus 16.16 expected based on national rates (standardized rate ratio 0.68, 95% confidence interval 0.34–1.22) and conclude that any adverse influence of in utero oestrogen exposure is likely to be small. © 2000 Cancer Research Campaig
Analytic solutions and Singularity formation for the Peakon b--Family equations
Using the Abstract Cauchy-Kowalewski Theorem we prove that the -family
equation admits, locally in time, a unique analytic solution. Moreover, if the
initial data is real analytic and it belongs to with , and the
momentum density does not change sign, we prove that the
solution stays analytic globally in time, for . Using pseudospectral
numerical methods, we study, also, the singularity formation for the -family
equations with the singularity tracking method. This method allows us to follow
the process of the singularity formation in the complex plane as the
singularity approaches the real axis, estimating the rate of decay of the
Fourier spectrum
A General Limitation on Monte Carlo Algorithms of Metropolis Type
We prove that for any Monte Carlo algorithm of Metropolis type, the
autocorrelation time of a suitable ``energy''-like observable is bounded below
by a multiple of the corresponding ``specific heat''. This bound does not
depend on whether the proposed moves are local or non-local; it depends only on
the distance between the desired probability distribution and the
probability distribution for which the proposal matrix satisfies
detailed balance. We show, with several examples, that this result is
particularly powerful when applied to non-local algorithms.Comment: 8 pages, LaTeX plus subeqnarray.sty (included at end),
NYU-TH-93/07/01, IFUP-TH33/9
Global generalized solutions for Maxwell-alpha and Euler-alpha equations
We study initial-boundary value problems for the Lagrangian averaged alpha
models for the equations of motion for the corotational Maxwell and inviscid
fluids in 2D and 3D. We show existence of (global in time) dissipative
solutions to these problems. We also discuss the idea of dissipative solution
in an abstract Hilbert space framework.Comment: 27 pages, to appear in Nonlinearit
Action functionals for relativistic perfect fluids
Action functionals describing relativistic perfect fluids are presented. Two
of these actions apply to fluids whose equations of state are specified by
giving the fluid energy density as a function of particle number density and
entropy per particle. Other actions apply to fluids whose equations of state
are specified in terms of other choices of dependent and independent fluid
variables. Particular cases include actions for isentropic fluids and
pressureless dust. The canonical Hamiltonian forms of these actions are
derived, symmetries and conserved charges are identified, and the boundary
value and initial value problems are discussed. As in previous works on perfect
fluid actions, the action functionals considered here depend on certain
Lagrange multipliers and Lagrangian coordinate fields. Particular attention is
paid to the interpretations of these variables and to their relationships to
the physical properties of the fluid.Comment: 40 pages, plain Te
String tension in gonihedric 3D Ising models
For the 3D gonihedric Ising models defined by Savvidy and Wegner the bare
string tension is zero and the energy of a spin interface depends only on the
number of bends and self-intersections, in antithesis to the standard
nearest-neighbour 3D Ising action. When the parameter kappa weighting the
self-intersections is small the model has a first order transition and when it
is larger the transition is continuous.
In this paper we investigate the scaling of the renormalized string tension,
which is entirely generated by fluctuations, using Monte Carlo simulations This
allows us to obtain an estimate for the critical exponents alpha and nu using
both finite-size-scaling and data collapse for the scaling function.Comment: Latex + postscript figures. 8 pages text plus 7 figures, spurious
extra figure now removed
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