67,491 research outputs found
Lensing Properties of Cored Galaxy Models
A method is developed to evaluate the magnifications of the images of
galaxies with lensing potentials stratified on similar concentric ellipses. A
simple contour integral is provided which enables the sums of the
magnifications of even parity or odd parity or the central image to be easily
calculated. The sums for pairs of images vary considerably with source
position, while the signed sums can be remarkably uniform inside the tangential
caustic in the absence of naked cusps. For a family of models in which the
potential is a power-law of the elliptic radius, the number of visible images
is found as a function of flattening, external shear and core radius. The
magnification of the central image depends on the core radius and the slope of
the potential. For typical source and lens redshifts, the missing central image
leads to strong constraints; the mass distribution in the lensing galaxy must
be nearly cusped, and the cusp must be isothermal or stronger. This is in
accord with the cuspy cores seen in high resolution photometry of nearby,
massive, early-type galaxies, which typically have the surface density falling
like distance^{-1.3} outside a break radius of a few hundred parsecs. Cuspy
cores by themselves can provide an explanation of the missing central images.
Dark matter at large radii may alter the slope of the projected density;
provided the slope remains isothermal or steeper and the break radius remains
small, then the central image remains unobservable. The sensitivity of the
radio maps must be increased fifty-fold to find the central images in
abundance.Comment: 42 pages, 11 figures, ApJ in pres
New observations regarding deterministic, time reversible thermostats and Gauss's principle of least constraint
Deterministic thermostats are frequently employed in non-equilibrium
molecular dynamics simulations in order to remove the heat produced
irreversibly over the course of such simulations. The simplest thermostat is
the Gaussian thermostat, which satisfies Gauss's principle of least constraint
and fixes the peculiar kinetic energy. There are of course infinitely many ways
to thermostat systems, e.g. by fixing . In
the present paper we provide, for the first time, convincing arguments as to
why the conventional Gaussian isokinetic thermostat () is unique in this
class. We show that this thermostat minimizes the phase space compression and
is the only thermostat for which the conjugate pairing rule (CPR) holds.
Moreover it is shown that for finite sized systems in the absence of an applied
dissipative field, all other thermostats () perform work on the system
in the same manner as a dissipative field while simultaneously removing the
dissipative heat so generated. All other thermostats () are thus
auto-dissipative. Among all -thermostats, only the Gaussian
thermostat permits an equilibrium state.Comment: 27 pages including 10 figures; submitted for publication Journal of
Chemical Physic
The Capture of Centaurs as Trojans
Large scale simulations of Centaurs have yielded vast amounts of data, the
analysis of which allows interesting but uncommon scenarios to be studied. One
such rare phenomenon is the temporary capture of Centaurs as Trojans of the
giant planets. Such captures are generally short (10 kyr to 100 kyr), but occur
with sufficient frequency (about 40 objects larger than 1 km in diameter every
Myr) that they may well contribute to the present-day populations. Uranus and
Neptune seem to have great difficulty capturing Centaurs into the 1:1
resonance, while Jupiter captures some, and Saturn the most (80 %). We
conjecture that such temporary capture from the Centaur population may be the
dominant delivery route into the Saturnian Trojans. Photometric studies of the
Jovian Trojans may reveal outliers with Centaur-like as opposed to asteroidal
characteristics, and these would be prime candidates for captured Centaurs.Comment: 5 pages, 2 figures, submitted to MNRAS (Letters
Quantum tunneling of the Neel vector in antiferromagnetic [3 x 3] grid molecules
Based on numerical calculations it is shown that the antiferromagnetic grid
molecule Mn-[3 x 3] is a very promising candidate to experimentally detect the
phenomenon of quantum tunneling of the Neel vector.Comment: 4 pages, 3 figures, REVTEX 4, to appear in PR
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