150 research outputs found
Weak selection and stability of localized distributions in Ostwald ripening
We support and generalize a weak selection rule predicted recently for the
self-similar asymptotics of the distribution function (DF) in the
zero-volume-fraction limit of Ostwald ripening (OR). An asymptotic perturbation
theory is developed that, when combined with an exact invariance property of
the system, yields the selection rule, predicts a power-law convergence towards
the selected self-similar DF and agrees well with our numerical simulations for
the interface- and diffusion-controlled OR.Comment: 4 pages, 2 figures, submitted to PR
The effect of parallel static and microwave electric fields on excited hydrogen atoms
Motivated by recent experiments we analyse the classical dynamics of a
hydrogen atom in parallel static and microwave electric fields. Using an
appropriate representation and averaging approximations we show that resonant
ionisation is controlled by a separatrix, and provide necessary conditions for
a dynamical resonance to affect the ionisation probability.
The position of the dynamical resonance is computed using a high-order
perturbation series, and estimate its radius of convergence. We show that the
position of the dynamical resonance does not coincide precisely with the
ionisation maxima, and that the field switch-on time can dramatically affect
the ionisation signal which, for long switch times, reflects the shape of an
incipient homoclinic. Similarly, the resonance ionisation time can reflect the
time-scale of the separatrix motion, which is therefore longer than
conventional static field Stark ionisation. We explain why these effects should
be observed in the quantum dynamics.
PACs: 32.80.Rm, 33.40.+f, 34.10.+x, 05.45.Ac, 05.45.MtComment: 47 pages, 20 figure
Control of star formation by supersonic turbulence
Understanding the formation of stars in galaxies is central to much of modern
astrophysics. For several decades it has been thought that stellar birth is
primarily controlled by the interplay between gravity and magnetostatic
support, modulated by ambipolar diffusion. Recently, however, both
observational and numerical work has begun to suggest that support by
supersonic turbulence rather than magnetic fields controls star formation. In
this review we outline a new theory of star formation relying on the control by
turbulence. We demonstrate that although supersonic turbulence can provide
global support, it nevertheless produces density enhancements that allow local
collapse. Inefficient, isolated star formation is a hallmark of turbulent
support, while efficient, clustered star formation occurs in its absence. The
consequences of this theory are then explored for both local star formation and
galactic scale star formation. (ABSTRACT ABBREVIATED)Comment: Invited review for "Reviews of Modern Physics", 87 pages including 28
figures, in pres
Fibroblasts from patients with major depressive disorder show distinct transcriptional response to metabolic stressors
Major depressive disorder (MDD) is increasingly viewed as interplay of environmental stressors and genetic predisposition, and recent data suggest that the disease affects not only the brain, but the entire body. As a result, we aimed at determining whether patients with major depression have aberrant molecular responses to stress in peripheral tissues. We examined the effects of two metabolic stressors, galactose (GAL) or reduced lipids (RL), on the transcriptome and miRNome of human fibroblasts from 16 pairs of patients with MDD and matched healthy controls (CNTR). Our results demonstrate that both MDD and CNTR fibroblasts had a robust molecular response to GAL and RL challenges. Most importantly, a significant part (messenger RNAs (mRNAs): 26-33%; microRNAs (miRNAs): 81-90%) of the molecular response was only observed in MDD, but not in CNTR fibroblasts. The applied metabolic challenges uncovered mRNA and miRNA signatures, identifying responses to each stressor characteristic for the MDD fibroblasts. The distinct responses of MDD fibroblasts to GAL and RL revealed an aberrant engagement of molecular pathways, such as apoptosis, regulation of cell cycle, cell migration, metabolic control and energy production. In conclusion, the metabolic challenges evoked by GAL or RL in dermal fibroblasts exposed adaptive dysfunctions on mRNA and miRNA levels that are characteristic for MDD. This finding underscores the need to challenge biological systems to bring out disease-specific deficits, which otherwise might remain hidden under resting conditions
Low temperature and low pressure CO oxidation on gold clusters supported on MgO(100)
CO oxidation has been investigated on Au/MgO(100) model catalysts at RT and
low pressure. The gold particles prepared by UHV evaporation on clean MgO surfaces are characterized by HRTEM. The gold particles are FCC single
crystals or multiple twins with five-fold symmetry. Infrared spectroscopy
indicates that two types of adsorption sites are present which correspond to
loosely and strongly bound CO. The equilibrium CO coverage for the strongly
bound CO is smaller than 0.1 ML. CO titration experiments show that oxygen
does not dissociate on the gold nanoparticles. The CO oxidation reaction is
studied at RT by molecular beam methods. A steady state CO reaction
probability up to 0.50 is measured, for the first time at low pressure, for
gold particles with a mean size of 1.5 nm. A reaction mechanism is proposed
in which CO adsorbed on low coordinated gold atoms reacts with oxygen
adsorbed molecularly, possibly at the Au/MgO interface.
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