6,479 research outputs found
The Most Luminous Galaxies
Ultraluminous galaxies in the local universe (z0.2) emit the bulk of
their energy in the mid and far-infrared. The multiwavelength approach to these
objects has shown that they are advanced mergers of gas-rich spiral galaxies.
Galaxy-galaxy collisions took place on all cosmological time-scales, and nearby
mergers serve as local analogs to gain insight into the physical processes that
lead to the formation and trans-formation of galaxies in the more distant
universe. Here I review multiwavelength observations --with particular emphasis
on recent results obtained with ISO-- of mergers of massive galaxies driving
the formation of: 1) luminous infrared galaxies, 2) elliptical galaxy cores, 3)
luminous dust-enshrouded extranuclear starbursts, 4) symbiotic galaxies that
host AGNs, and 5) tidal dwarf galaxies. The most important implication for
studies on the formation of galaxies at early cosmological timescales is that
the distant analogs to the local ultraluminous infrared galaxies are invisible
in the ultraviolet and optical wavelength rest-frames and should be detected as
sub-millimeter sources with no optical counterparts.Comment: 12 pages, 6 figures. Invited Review at the Conference Lighthouses of
the universe. August 6-10, 2001 (Garching, Germany
Intrinsic rotation in tokamaks: theory
Self-consistent equations for intrinsic rotation in tokamaks with small
poloidal magnetic field compared to the total magnetic field are
derived. The model gives the momentum redistribution due to turbulence,
collisional transport and energy injection. Intrinsic rotation is determined by
the balance between the momentum redistribution and the turbulent diffusion and
convection. Two different turbulence regimes are considered: turbulence with
characteristic perpendicular lengths of the order of the ion gyroradius,
, and turbulence with characteristic lengths of the order of the
poloidal gyroradius, . Intrinsic rotation driven by gyroradius
scale turbulence is mainly due to the effect of neoclassical corrections and of
finite orbit widths on turbulent momentum transport, whereas for the intrinsic
rotation driven by poloidal gyroradius scale turbulence, the slow variation of
turbulence characteristics in the radial and poloidal directions and the
turbulent particle acceleration can be become as important as the neoclassical
and finite orbit width effects. The magnetic drift is shown to be indispensable
for the intrinsic rotation driven by the slow variation of turbulence
characteristics and the turbulent particle acceleration. The equations are
written in a form conducive to implementation in a flux tube code, and the
effect of the radial variation of the turbulence is included in a novel way
that does not require a global gyrokinetic formalism.Comment: 88 pages, 4 figure
Phase-space Lagrangian derivation of electrostatic gyrokinetics in general geometry
Gyrokinetic theory is based on an asymptotic expansion in the small parameter
, defined as the ratio of the gyroradius and the characteristic
length of variation of the magnetic field. In this article, this ordering is
strictly implemented to compute the electrostatic gyrokinetic phase-space
Lagrangian in general magnetic geometry to order . In particular, a
new expression for the complete second-order gyrokinetic Hamiltonian is
provided, showing that in a rigorous treatment of gyrokinetic theory magnetic
geometry and turbulence cannot be dealt with independently. The new phase-space
gyrokinetic Lagrangian gives a Vlasov equation accurate to order
and a Poisson equation accurate to order . The final expressions are
explicit and can be implemented into any simulation without further
computations.Comment: 55 pages. Version with typo in equation (135) corrected. The second
term in the second line of (135) was missing the subindex that indicates that
only the perpendicular component of the gradient enters this ter
Radial penetration of flux surface shaping in tokamaks
Using analytic calculations, the effects of the edge flux surface shape and
the toroidal current profile on the penetration of flux surface shaping are
investigated in a tokamak. It is shown that the penetration of shaping is
determined by the poloidal variation of the poloidal magnetic field on the
surface. This fact is used to investigate how different flux surface shapes
penetrate from the edge. Then, a technique to separate the effects of magnetic
pressure and tension in the Grad-Shafranov equation is presented and used to
calculate radial profiles of strong elongation for nearly constant current
profiles. Lastly, it is shown that more hollow toroidal current profiles are
significantly better at conveying shaping from the edge to the core.Comment: 11 pages, 13 figure
The origin of Scorpius X-1
We have used multi-wavelength observations of high precision to derive the
space velocity and compute the orbit around the Galactic Centre of the
prototype X-ray binary Scorpius X-1. An origin in the local spiral arm of the
Milky Way is ruled out. The galactocentric kinematics of Scorpius X-1 is
similar to that of the most ancient stars and globular clusters of the inner
Galactic halo. Most probably, this low-mass X-ray binary was formed by a close
encounter in a globular cluster. However, it cannot be ruled out that a natal
supernova explosion launched Scorpius X-1 into an orbit like this from a birth
place in the galactic bulge. In any case, the Galactocentric orbit indicates
that Scorpius X-1 was formed more than 30 Myrs ago.Comment: 4 pages, 1 figure. Animation and high resolution figures can be
retrived from the NRAO press release:
http://www.aoc.nrao.edu/epo/pr/2003/scox1
Turbulent momentum pinch of diamagnetic flows in a tokamak
The ion toroidal rotation in a tokamak consists of an flow due to
the radial electric field and a diamagnetic flow due to the radial pressure
gradient. The turbulent pinch of toroidal angular momentum due to the Coriolis
force studied in previous work is only applicable to the flow. In
this Letter, the momentum pinch for the rotation generated by the radial
pressure gradient is calculated and is compared with the Coriolis pinch. This
distinction is important for subsonic flows or the flow in the pedestal where
the two types of flows are similar in size and opposite in direction. In the
edge, the different pinches due to the opposite rotations can result in
intrinsic momentum transport that gives significant rotation peaking.Comment: 5 pages and 3 figure
Topological superconducting phase in helical Shiba chains
Recently, it has been suggested that topological superconductivity and
Majorana end states can be realized in a chain of magnetic impurities on the
surface of an s-wave superconductor when the magnetic moments form a spin helix
as a result of the RKKY interaction mediated by the superconducting substrate.
Here, we investigate this scenario theoretically by developing a tight-binding
Bogoliubov-de Gennes description starting from the Shiba bound states induced
by the individual magnetic impurities. While the resulting model Hamiltonian
has similarities with the Kitaev model for one-dimensional spinless p-wave
superconductors, there are also important differences, most notably the
long-range nature of hopping and pairing as well as the complex hopping
amplitudes. We use both analytical and numerical approaches to explore the
consequences of these differences for the phase diagram and the localization
properties of the Majorana end states when the Shiba chain is in a topological
superconducting phase.Comment: 14 pages, 9 figures, minor changes, references added; published
versio
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