42,336 research outputs found
An N-body Integrator for Gravitating Planetary Rings, and the Outer Edge of Saturn's B Ring
A new symplectic N-body integrator is introduced, one designed to calculate
the global 360 degree evolution of a self-gravitating planetary ring that is in
orbit about an oblate planet. This freely-available code is called epi_int, and
it is distinct from other such codes in its use of streamlines to calculate the
effects of ring self-gravity. The great advantage of this approach is that the
perturbing forces arise from smooth wires of ring matter rather than discreet
particles, so there is very little gravitational scattering and so only a
modest number of particles are needed to simulate, say, the scalloped edge of a
resonantly confined ring or the propagation of spiral density waves.
The code is applied to the outer edge of Saturn's B ring, and a comparison of
Cassini measurements of the ring's forced response to simulations of Mimas'
resonant perturbations reveals that the B ring's surface density at its outer
edge is 195+-60 gm/cm^2 which, if the same everywhere across the ring would
mean that the B ring's mass is about 90% of Mimas' mass.
Cassini observations show that the B ring-edge has several free normal modes,
which are long-lived disturbances of the ring-edge that are not driven by any
known satellite resonances. Although the mechanism that excites or sustains
these normal modes is unknown, we can plant such a disturbance at a simulated
ring's edge, and find that these modes persist without any damping for more
than ~10^5 orbits or ~100 yrs despite the simulated ring's viscosity of 100
cm^2/sec. These simulations also indicate that impulsive disturbances at a ring
can excite long-lived normal modes, which suggests that an impact in the recent
past by perhaps a cloud of cometary debris might have excited these
disturbances which are quite common to many of Saturn's sharp-edged rings.Comment: 55 pages, 13 figures, accepted for publication in the Astrophysical
Journa
A Relativistic Symmetry in Nuclei: Its origins and consequences
We review the status of quasi-degenerate doublets in nuclei, called
pseudospin doublets, which were discovered about thirty years ago and the
origins of which have remained a mystery, until recently. We show that
pseudospin doublets originate from an SU(2) symmetry of the Dirac Hamiltonian
which occurs when the sum of the scalar and vector potentials is a constant.
Furthermore, we survey the evidence that pseudospin symmetry is approximately
conserved in nuclear spectra and eigenfunctions and in nucleon-nucleus
scattering for a Dirac Hamiltonian with realistic nuclear scalar and vector
potentials.Comment: Invited Talk for "Nuclei and Nucleons", Darmstadt, Germany, Oct.
11-13,2000; International Symposium on the occasion of Achim Richter's 60th
Birthda
Managing the Faustian bargain: monetary autonomy in the pursuit of development in Eastern Europe and Latin America
International capital markets have grown to be a major force shaping today's world economy, presenting a range of opportunities and threats to developing countries. Capital market liberalization created large pools of much-needed capital that developing economies could access, but tapping these funds often came at the cost of increasing economic vulnerability, lost policy-making autonomy and a range of structural distortions that could ultimately undermine development in the long-term. As the potential threats of integrating one's country into global capital markets has become apparent, countries have devised a range of strategies to buffer themselves from the strains of global capital markets. This article considers the pursuit of monetary autonomy with reference to a typology of the strategies that policy-makers can use to open their markets to international capital, while simultaneously attempting to buffer themselves from the economic and political pressures of global financial integration. Such autonomy can be purchased in a myriad of ways, and a discussion of the choices facing Latin American and Eastern European countries is presented.capital market liberalization; monetary autonomy; financial policy; financial system stability
Critical Points in Nuclei and Interacting Boson Model Intrinsic States
We consider properties of critical points in the interacting boson model,
corresponding to flat-bottomed potentials as encountered in a second-order
phase transition between spherical and deformed -unstable nuclei. We
show that intrinsic states with an effective -deformation reproduce the
dynamics of the underlying non-rigid shapes. The effective deformation can be
determined from the the global minimum of the energy surface after projection
onto the appropriate symmetry. States of fixed and good O(5) symmetry
projected from these intrinsic states provide good analytic estimates to the
exact eigenstates, energies and quadrupole transition rates at the critical
point.Comment: 10 pages, 3 figures, Proc. Int. Conf. on "Symmetry in Physics", March
23-30, 2003, Erice, Ital
All-optical photochromic spatial light modulators based on photoinduced electron transfer in rigid matrices
A single material (not a multi-element structure) spatial light modulator may be written to, as well as read out from, using light. The device has tailorable rise and hold times dependent on the composition and concentration of the molecular species used as the active components. The spatial resolution of this device is limited only by light diffraction as in volume holograms. The device may function as a two-dimensional mask (transmission or reflection) or as a three-dimensional volume holographic medium. This device, based on optically-induced electron transfer, is able to perform incoherent to coherent image conversion or wavelength conversion over a wide spectral range (ultraviolet, visible, or near-infrared regions)
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