13,253 research outputs found
The effect of internal gravity waves on cloud evolution in sub-stellar atmospheres
Context. Sub-stellar objects exhibit photometric variability which is believed to be caused by a number of processes such as magnetically-driven spots or inhomogeneous cloud coverage. Recent sub-stellar models have shown that turbulent flows and waves, including internal gravity waves, may play an important role in cloud evolution.Aims. The aim of this paper is to investigate the effect of internal gravity waves on dust cloud nucleation and dust growth, and whether observations of the resulting cloud structures could be used to recover atmospheric density information.Methods. For a simplified atmosphere in two dimensions, we numerically solve the governing fluid equations to simulate the effect on dust nucleation and mantle growth as a result of the passage of an internal gravity wave. Furthermore, we derive an expression that relates the properties of the wave-induced cloud structures to observable parameters in order to deduce the atmospheric density.Results. Numerical simulations show that the density, pressure and temperature variations caused by gravity waves lead to an increase of dust nucleation by up to a factor 20, and dust mantle growth rate by up to a factor 1:6, compared to their equilibrium values. Through an exploration of the wider sub-stellar parameter space, we show that in absolute terms, the increase in dust nucleation due to internal gravity waves is stronger in cooler (T dwarfs) and TiO2-rich sub-stellar atmospheres. The relative increase however is greater in warm(L dwarf) and TiO2-poor atmospheres due to conditions less suited for efficient nucleation at equilibrium. These variations lead to banded areas in which dust formation is much more pronounced, and lead to banded cloud structures similar to those observed on Earth. Conclusions. Using the proposed method, potential observations of banded clouds could be used to estimate the atmospheric density of sub-stellar objects
PORTFOLIOS OF AGRICULTURAL MARKET ADVISORY SERVICES: HOW MUCH DIVERSIFICATION IS ENOUGH?
Agricultural market advisory services offer specific advice to farmers on how to market their commodities. Farmers can subscribe to one or more of these services and follow their advice as a way of managing price risk. According to portfolio theory, a combination of these services may have risk/return benefits compared to individual services. This report analyzes the potential risk reduction gains from naïve diversification among market advisory services for corn and soybeans. Results show that increasing the number of (equal-weighting) services reduces portfolio expected risk, but the marginal decrease in risk from adding a new service decreases rapidly with portfolio size. The risk reduction benefits of naïve diversification among advisory services is relatively small compared to the results obtained in previous studies for stock portfolios, and this is mainly because advisory prices, on average, are highly correlated. A one service portfolio has only a 20%, 16% and 32% higher expected standard deviation than the minimum risk naïve portfolio for corn, soybeans and 50/50 revenue, respectively. Most risk reduction benefits are achieved with small portfolios. For instance, a four service portfolio has only 5%, 4% and 9% higher expected standard deviation than the minimum risk naïve portfolio for corn, soybeans and 50/50 revenue, respectively. Based on these results, there does not appear to be strong justification for farmers adopting portfolios with a large number of advisory services. Farmers may well choose portfolios with as few as two or three programs, since the relatively high total subscription costs associated with larger portfolios can be avoided while obtaining most of the benefits from diversification.Marketing,
On measuring alpha in B(t)-> rho^\pm pi^\mp
Defining a most economical parametrization of time-dependent B-> rho^\pm
pi^\mp decays, including a measurable phase alpha_{eff} which equals the weak
phase alpha in the limit of vanishing penguin amplitudes, we propose two ways
for determining alpha in this processes. We explain the limitation of one
method, assuming only that two relevant tree amplitudes factorize and that
their relative strong phase, delta_t, is negligible. The other method, based on
broken flavor SU(3), permits a determination of alpha in B^0-> rho^\pm pi^\mp
in an overconstrained system using also rate measurements of B^{0,+}-> K^* pi
and B^{0,+}->rho K. Current data are shown to restrict two ratios of penguin
and tree amplitudes, r_\pm, to a narrow range around 0.2, and to imply an upper
bound |alpha_{eff} - alpha| < 15 degrees. Assuming that delta_t is much smaller
than 90 degrees, we find alpha =(93\pm 16) degrees and (102 \pm 20) degrees
using BABAR and BELLE results for B(t)-> rho^\pm pi^mp. Avoiding this
assumption for completeness, we demonstrate the reduction of discrete
ambiguities in alpha with increased statistics, and show that SU(3) breaking
effects are effectively second order in r_\pm.Comment: 23 pages, 2 figures, data and references updated, to be published in
Phys. Rev.
Charged inclusion in nematic liquid crystals
We present a general theory of liquid crystals under inhomogeneous electric
field in a Ginzburg-Landau scheme. The molecular orientation can be deformed by
electric field when the dielectric tensor is orientation-dependent. We then
investigate the influence of a charged particle on the orientation order in a
nematic state. The director is aligned either along or perpendicular to the
local electric field around the charge, depending on the sign of the dielectric
anisotropy. The deformation becomes stronger with increasing the ratio ,
where is the charge and is the radius of the particle. Numerical
analysis shows the presence of defects around the particle for large .
They are nanometer-scale defects for microscopic ions. If the dielectric
anisotropy is positive, a Saturn ring defect appears. If it is negative, a pair
of point defects appear apart from the particle surface, each being connected
to the surface by a disclination line segment.Comment: 12 figure
Inactivation of the Na,K-ATPase by radiation-induced free radicals Evidence for a radical-chain mechanism
AbstractFree radicals produced by water radiolysis were used to study the inactivation of the enzymatic activity of the Na,K-ATPase. A decrease of the activity to virtually zero with a mono-exponential dependence on the radiation dose was observed. The inactivation process is initiated by hydroxyl radicals. This was shown by the effect of appropriate radical scavengers such as t-butanol, formate and vitamin C. In all cases a significant increase in the characteristic D37 dose of inactivation was observed. Inactivation was found to show a so-called inverse dose-rate effect, i.e, the sensitivity of the enzyme to radical attack is increased if the dose rate is reduced. The data were found to agree with the relationship 1/D371̃/D1/2, which is known to be a strong indicator of a radical chain mechanism. This means that the inactivation, after initiation by single radicals, is amplified by a subsequent chain mechanism
Physical State of Molecular Gas in High Galactic Latitude Translucent Clouds
The rotational transitions of carbon monoxide (CO) are the primary means of
investigating the density and velocity structure of the molecular interstellar
medium. Here we study the lowest four rotational transitions of CO towards
high-latitude translucent molecular clouds (HLCs). We report new observations
of the J = (4-3), (2-1), and (1-0) transitions of CO towards eight
high-latitude clouds. The new observations are combined with data from the
literature to show that the emission from all observed CO transitions is
linearly correlated. This implies that the excitation conditions which lead to
emission in these transitions are uniform throughout the clouds. Observed
13CO/12CO (1-0) integrated intensity ratios are generally much greater than the
expected abundance ratio of the two species, indicating that the regions which
emit 12CO (1-0) radiation are optically thick. We develop a statistical method
to compare the observed line ratios with models of CO excitation and radiative
transfer. This enables us to determine the most likely portion of the physical
parameter space which is compatible with the observations. The model enables us
to rule out CO gas temperatures greater than 30K since the most likely
high-temperature configurations are 1 pc-sized structures aligned along the
line of sight. The most probable solution is a high density and low temperature
(HDLT) solution. The CO cell size is approximately 0.01 pc (2000 AU). These
cells are thus tiny fragments within the 100 times larger CO-emitting extent of
a typical high-latitude cloud. We discuss the physical implications of HDLT
cells, and we suggest ways to test for their existence.Comment: 19 pages, 13 figures, 2 tables, emulateapj To be published in The
Astrophysical Journa
Photon Channelling in Foams
Experiments by Gittings, Bandyopadhyay, and Durian [Europhys. Lett.\
\textbf{65}, 414 (2004)] demonstrate that light possesses a higher probability
to propagate in the liquid phase of a foam due to total reflection. The authors
term this observation photon channelling which we investigate in this article
theoretically. We first derive a central relation in the work of Gitting {\em
et al.} without any free parameters. It links the photon's path-length fraction
in the liquid phase to the liquid fraction . We then construct
two-dimensional Voronoi foams, replace the cell edges by channels to represent
the liquid films and simulate photon paths according to the laws of ray optics
using transmission and reflection coefficients from Fresnel's formulas. In an
exact honeycomb foam, the photons show superdiffusive behavior. It becomes
diffusive as soon as disorder is introduced into the foams. The dependence of
the diffusion constant on channel width and refractive index is explained by a
one-dimensional random-walk model. It contains a photon channelling state that
is crucial for the understanding of the numerical results. At the end, we
shortly comment on the observation that photon channelling only occurs in a
finite range of .Comment: 9 pages, minor change
Interaction and flocculation of spherical colloids wetted by a surface-induced corona of paranematic order
Particles dispersed in a liquid crystal above the nematic-isotropic phase
transition are wetted by a surface-induced corona of paranematic order. Such
coronas give rise to pronounced two-particle interactions. In this article, we
report details on the analytical and numerical study of these interactions
published recently [Phys. Rev. Lett. 86, 3915 (2001)]. We especially
demonstrate how for large particle separations the asymptotic form of a Yukawa
potential arises. We show that the Yukawa potential is a surprisingly good
description for the two-particle interactions down to distances of the order of
the nematic coherence length. Based on this fact, we extend earlier studies on
a temperature induced flocculation transition in electrostatically stabilized
colloidal dispersions [Phys. Rev. E 61, 2831 (2000)]. We employ the Yukawa
potential to establish a flocculation diagram for a much larger range of the
electrostatic parameters, namely the surface charge density and the Debye
screening length. As a new feature, a kinetically stabilized dispersion close
to the nematic-isotropic phase transition is found.Comment: Revtex v4.0, 16 pages, 12 Postscript figures. Accepted for
publication in Phys. Rev.
Emergence of geometrical optical nonlinearities in photonic crystal fiber nanowires
We demonstrate analytically and numerically that a subwavelength-core
dielectric photonic nanowire embedded in a properly designed photonic crystal
fiber cladding shows evidence of a previously unknown kind of nonlinearity (the
magnitude of which is strongly dependent on the waveguide parameters) which
acts on solitons so as to considerably reduce their Raman self-frequency shift.
An explanation of the phenomenon in terms of indirect pulse negative chirping
and broadening is given by using the moment method. Our conclusions are
supported by detailed numerical simulations.Comment: 5 pages, 3 figure
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