1,624 research outputs found
Carbonates in space - The challenge of low temperature data
Carbonates have repeatedly been discussed as possible carriers of stardust
emission bands. However, the band assignments proposed so far were mainly based
on room temperature powder transmission spectra of the respective minerals.
Since very cold calcite grains have been claimed to be present in protostars
and in Planetary Nebulae such as NGC 6302, the changes of their dielectric
functions at low temperatures are relevant from an astronomical point of view.
We have derived the IR optical constants of calcite and dolomite from
reflectance spectra - measured at 300, 200, 100 and 10K - and calculated small
particle spectra for different grain shapes, with the following results: i) The
absorption efficiency factors both of calcite and dolomite are extremely
dependent on the particle shapes. This is due to the high peak values of the
optical constants of CaCO3 and CaMg[CO3]2. ii) The far infrared properties of
calcite and dolomite depend also very significantly on the temperature. Below
200K, a pronounced sharpening and increase in the band strengths of the FIR
resonances occurs. iii) In view of the intrinsic strength and sharpening of the
44 mum band of calcite at 200-100K, the absence of this band -- inferred from
Infrared Space Observatory data -- in PNe requires dust temperatures below 45K.
iv) Calcite grains at such low temperatures can account for the '92' mum band,
while our data rule out dolomite as the carrier of the 60-65 mum band. The
optical constants here presented are publicly available in the electronic
database http://www.astro.uni-jena.de/Laboratory/OCDBComment: 20 pages, 10 figures, accepted by ApJ, corrected typo
Lyapunov instability of fluids composed of rigid diatomic molecules
We study the Lyapunov instability of a two-dimensional fluid composed of
rigid diatomic molecules, with two interaction sites each, and interacting with
a WCA site-site potential. We compute full spectra of Lyapunov exponents for
such a molecular system. These exponents characterize the rate at which
neighboring trajectories diverge or converge exponentially in phase space.
Quam. These exponents characterize the rate at which neighboring trajectories
diverge or converge exponentially in phase space. Qualitative different degrees
of freedom -- such as rotation and translation -- affect the Lyapunov spectrum
differently. We study this phenomenon by systematically varying the molecular
shape and the density. We define and evaluate ``rotation numbers'' measuring
the time averaged modulus of the angular velocities for vectors connecting
perturbed satellite trajectories with an unperturbed reference trajectory in
phase space. For reasons of comparison, various time correlation functions for
translation and rotation are computed. The relative dynamics of perturbed
trajectories is also studied in certain subspaces of the phase space associated
with center-of-mass and orientational molecular motion.Comment: RevTeX 14 pages, 7 PostScript figures. Accepted for publication in
Phys. Rev.
Remarks on NonHamiltonian Statistical Mechanics: Lyapunov Exponents and Phase-Space Dimensionality Loss
The dissipation associated with nonequilibrium flow processes is reflected by
the formation of strange attractor distributions in phase space. The
information dimension of these attractors is less than that of the equilibrium
phase space, corresponding to the extreme rarity of nonequilibrium states. Here
we take advantage of a simple model for heat conduction to demonstrate that the
nonequilibrium dimensionality loss can definitely exceed the number of
phase-space dimensions required to thermostat an otherwise Hamiltonian system.Comment: 5 pages, 2 figures, minor typos correcte
Time-reversal focusing of an expanding soliton gas in disordered replicas
We investigate the properties of time reversibility of a soliton gas,
originating from a dispersive regularization of a shock wave, as it propagates
in a strongly disordered environment. An original approach combining
information measures and spin glass theory shows that time reversal focusing
occurs for different replicas of the disorder in forward and backward
propagation, provided the disorder varies on a length scale much shorter than
the width of the soliton constituents. The analysis is performed by starting
from a new class of reflectionless potentials, which describe the most general
form of an expanding soliton gas of the defocusing nonlinear Schroedinger
equation.Comment: 7 Pages, 6 Figure
Emergence of order in selection-mutation dynamics
We characterize the time evolution of a d-dimensional probability
distribution by the value of its final entropy. If it is near the
maximally-possible value we call the evolution mixing, if it is near zero we
say it is purifying. The evolution is determined by the simplest non-linear
equation and contains a d times d matrix as input. Since we are not interested
in a particular evolution but in the general features of evolutions of this
type, we take the matrix elements as uniformly-distributed random numbers
between zero and some specified upper bound. Computer simulations show how the
final entropies are distributed over this field of random numbers. The result
is that the distribution crowds at the maximum entropy, if the upper bound is
unity. If we restrict the dynamical matrices to certain regions in matrix
space, for instance to diagonal or triangular matrices, then the entropy
distribution is maximal near zero, and the dynamics typically becomes
purifying.Comment: 8 pages, 8 figure
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