189,297 research outputs found
Systematic Density Expansion of the Lyapunov Exponents for a Two-dimensional Random Lorentz Gas
We study the Lyapunov exponents of a two-dimensional, random Lorentz gas at
low density. The positive Lyapunov exponent may be obtained either by a direct
analysis of the dynamics, or by the use of kinetic theory methods. To leading
orders in the density of scatterers it is of the form
, where and are
known constants and is the number density of scatterers expressed
in dimensionless units. In this paper, we find that through order
, the positive Lyapunov exponent is of the form
. Explicit numerical values of the new constants
and are obtained by means of a systematic analysis. This takes into
account, up to , the effects of {\it all\/} possible
trajectories in two versions of the model; in one version overlapping scatterer
configurations are allowed and in the other they are not.Comment: 12 pages, 9 figures, minor changes in this version, to appear in J.
Stat. Phy
Genes involved in carotene synthesis and mating in Blakeslea trispora
Mating of Blakeslea trispora and other molds of the order Mucorales requires the interaction of mycelia of opposite sex, (+) and (-), leading to the development of specialized structures and to an enhanced accumulation of beta-carotene. Industry obtains beta-carotene by co-cultivating appropriate strains of Blakeslea (mated cultures). Gene transcription in single and mated cultures was assayed by cDNA-AFLP, a technique to observe the differential expression of subsets of mRNA fragments. Overexpression in mated cultures is about ten times more frequent than underexpression. We obtained and sequenced fragments of 97 candidate genes that appeared to be overexpressed during mating and confirmed four of them by reverse transcription and real-time PCR. Comparisons with gene sequences from other organisms suggest functions in carotene biosynthesis (4 genes), energy metabolism (8), cell wall synthesis (1), transfer of acetyl groups (1), and regulatory processes (10). Sodium acetate inhibited sexual overexpression in about two-thirds of the candidate genes and acted as a signal with broad effects on the metabolism and the morphology of mated cultures. Our work offers new materials for the study of carotene biosynthesis and its regulation and for the improvement of carotene production with Mucorales
Probing quantum-mechanical level repulsion in disordered systems by means of time-resolved selectively-excited resonance fluorescence
We argue that the time-resolved spectrum of selectively-excited resonance
fluorescence at low temperature provides a tool for probing the
quantum-mechanical level repulsion in the Lifshits tail of the electronic
density of states in a wide variety of disordered materials. The technique,
based on detecting the fast growth of a fluorescence peak that is red-shifted
relative to the excitation frequency, is demonstrated explicitly by simulations
on linear Frenkel exciton chains.Comment: 4 pages, 4 figures, to appear in Phys. Rev. Let
Dark matter as integration constant in Horava-Lifshitz gravity
In the non-relativistic theory of gravitation recently proposed by Horava,
the Hamiltonian constraint is not a local equation satisfied at each spatial
point but an equation integrated over a whole space. The global Hamiltonian
constraint is less restrictive than its local version, and allows a richer set
of solutions than in general relativity. We show that a component which behaves
like pressureless dust emerges as an "integration constant" of dynamical
equations and momentum constraint equations. Consequently, classical solutions
to the infrared limit of Horava-Lifshitz gravity can mimic general relativity
plus cold dark matter.Comment: 16 pages; (non-)conservation equation for "dark matter" added (v2);
note added to comment on some recent preprints (v3); version accepted for
publication in PRD (v4
Detection of Keplerian dynamics in a disk around the post-AGB star AC Her
So far, only one rotating disk has been clearly identified and studied in AGB
or post-AGB objects (in the Red Rectangle), by means of observations with high
spectral and spatial resolution. However, disks are thought to play a key role
in the late stellar evolution and are suspected to surround many evolved stars.
We aim to extend our knowledge on these structures.
We present interferometric observations of CO J=2-1 emission from the nebula
surrounding the post-AGB star AC Her, a source belonging to a class of objects
that share properties with the Red Rectangle and show hints of Keplerian disks.
We clearly detect the Keplerian dynamics of a second disk orbiting an evolved
star. Its main properties (size, temperature, central mass) are derived from
direct interpretation of the data and model fitting. With this we confirm that
there are disks orbiting the stars of this relatively wide class of post-AGB
objectsComment: 4 pages, 3 figure
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