584 research outputs found
A hybrid moment equation approach to gas-grain chemical modeling
[Context] The stochasticity of grain chemistry requires special care in
modeling. Previously methods based on the modified rate equation, the master
equation, the moment equation, and Monte Carlo simulations have been used.
[Aims] We attempt to develop a systematic and efficient way to model the
gas-grain chemistry with a large reaction network as accurately as possible.
[Methods] We present a hybrid moment equation approach which is a general and
automatic method where the generating function is used to generate the moment
equations. For large reaction networks, the moment equation is cut off at the
second order, and a switch scheme is used when the average population of
certain species reaches 1. For small networks, the third order moments can also
be utilized to achieve a higher accuracy. [Results] For physical conditions in
which the surface reactions are important, our method provides a major
improvement over the rate equation approach, when benchmarked against the
rigorous Monte Carlo results. For either very low or very high temperatures, or
large grain radii, results from the rate equation are similar to those from our
new approach. Our method is faster than the Monte Carlo approach, but slower
than the rate equation approach. [Conclusions] The hybrid moment equation
approach with a cutoff and switch scheme is applicable to large gas-grain
networks, and is accurate enough to be used for astrochemistry studies. The
layered structure of the grain mantle could also be incorporated into this
approach, although a full implementation of the grain micro-physics appears to
be difficult.Comment: 11 pages, 4 figures. Accepted for publication in Astronomy and
Astrophysic
Incorporation of stochastic chemistry on dust grains in the PDR code using moment equations
Unlike gas-phase reactions, chemical reactions taking place on interstellar
dust grain surfaces cannot always be modeled by rate equations. Due to the
small grain sizes and low flux,these reactions may exhibit large fluctuations
and thus require stochastic methods such as the moment equations.
We evaluate the formation rates of H2, HD and D2 molecules on dust grain
surfaces and their abundances in the gas phase under interstellar conditions.
We incorporate the moment equations into the Meudon PDR code and compare the
results with those obtained from the rate equations. We find that within the
experimental constraints on the energy barriers for diffusion and desorption
and for the density of adsorption sites on the grain surface, H2, HD and D2
molecules can be formed efficiently on dust grains.
Under a broad range of conditions, the moment equation results coincide with
those obtained from the rate equations. However, in a range of relatively high
grain temperatures, there are significant deviations. In this range, the rate
equations fail while the moment equations provide accurate results. The
incorporation of the moment equations into the PDR code can be extended to
other reactions taking place on grain surfaces
Short term fat feeding rapidly increases plasma insulin but does not result in dyslipidaemia
Although the association between obesity and hypertension is well known, the underlying mechanism remains elusive. Previously, we have shown that 3 week fat feeding in rabbits produces greater visceral adiposity, hypertension, tachycardia and elevated renal sympathetic nerve activity compared to rabbits on a normal diet. Because hyperinsulinaemia, hyperleptinemia and dyslipidaemia are independent cardiovascular risk factors associated with hypertension we compared plasma insulin, leptin and lipid profiles in male New Zealand White rabbits fed a normal fat diet (NFD 4.3% fat, n = 11) or high fat diet (HFD 13.4% fat, n = 13) at days 1, 2, 3 and weeks 1, 2, 3 of the diet. Plasma concentrations of diacylglyceride (DG), triacylglyceride (TG), ceramide and cholesteryl esters (CE) were obtained after analysis by liquid chromatography mass spectrometry. Plasma insulin and glucose increased within the first 3 days of the diet in HFD rabbits (P <0.05) and remained elevated at week 1 (P <0.05). Blood pressure and heart rate followed a similar pattern. By contrast, in both groups, plasma leptin levels remained unchanged during the first few days (P >0.05), increasing by week 3 in fat fed animals alone (P <0.05). Concentrations of total DG, TG, CE and Ceramide at week 3 did not differ between groups (P >0.05). Our data show plasma insulin increases rapidly following consumption of a HFD and suggests that it may play a role in the rapid rise of blood pressure. Dyslipidaemia does not appear to contribute to the hypertension in this animal model
Quantifying the connectivity of a network: The network correlation function method
Networks are useful for describing systems of interacting objects, where the
nodes represent the objects and the edges represent the interactions between
them. The applications include chemical and metabolic systems, food webs as
well as social networks. Lately, it was found that many of these networks
display some common topological features, such as high clustering, small
average path length (small world networks) and a power-law degree distribution
(scale free networks). The topological features of a network are commonly
related to the network's functionality. However, the topology alone does not
account for the nature of the interactions in the network and their strength.
Here we introduce a method for evaluating the correlations between pairs of
nodes in the network. These correlations depend both on the topology and on the
functionality of the network. A network with high connectivity displays strong
correlations between its interacting nodes and thus features small-world
functionality. We quantify the correlations between all pairs of nodes in the
network, and express them as matrix elements in the correlation matrix. From
this information one can plot the correlation function for the network and to
extract the correlation length. The connectivity of a network is then defined
as the ratio between this correlation length and the average path length of the
network. Using this method we distinguish between a topological small world and
a functional small world, where the latter is characterized by long range
correlations and high connectivity. Clearly, networks which share the same
topology, may have different connectivities, based on the nature and strength
of their interactions. The method is demonstrated on metabolic networks, but
can be readily generalized to other types of networks.Comment: 10 figure
Stochastic Analysis of Dimerization Systems
The process of dimerization, in which two monomers bind to each other and
form a dimer, is common in nature. This process can be modeled using rate
equations, from which the average copy numbers of the reacting monomers and of
the product dimers can then be obtained. However, the rate equations apply only
when these copy numbers are large. In the limit of small copy numbers the
system becomes dominated by fluctuations, which are not accounted for by the
rate equations. In this limit one must use stochastic methods such as direct
integration of the master equation or Monte Carlo simulations. These methods
are computationally intensive and rarely succumb to analytical solutions. Here
we use the recently introduced moment equations which provide a highly
simplified stochastic treatment of the dimerization process. Using this
approach, we obtain an analytical solution for the copy numbers and reaction
rates both under steady state conditions and in the time-dependent case. We
analyze three different dimerization processes: dimerization without
dissociation, dimerization with dissociation and hetero-dimer formation. To
validate the results we compare them with the results obtained from the master
equation in the stochastic limit and with those obtained from the rate
equations in the deterministic limit. Potential applications of the results in
different physical contexts are discussed.Comment: 10 figure
Reforming state-market relations in rural China 1
Because of the political importance of stable and affordable food prices, governments in transitional economies in Europe and Asia fkequently struggle to balance the desire to maintain state controls in the rural sector with the goal of promoting market development and privatizing or commercializing state agencies involved in rural trade or finance. This paper examines institutional change in China's rural state agencies during the reform period, focusing on the conflict between managerial incentives to maximize profits, on the one hand, and implement policy, on the other. We explain the reasons for changing contractual incentives and authority arrangements over time, assess the effects of new institutional forms on economic performance and policy implementation, and consider reform options.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71599/1/j.1468-0351.1998.tb00059.x.pd
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