13,578 research outputs found
CB17: Inferring the dynamical history of a prestellar core with chemo-dynamical models
We present a detailed theoretical study of the isolated Bok globule CB17
(L1389) based on spectral maps of CS, HCO, CO, CS, and
HCO lines. A phenomenological model of prestellar core evolution, a
time-dependent chemical model, and a radiative transfer simulation for
molecular lines are combined to reconstruct the chemical and kinematical
structure of this core. We developed a general criterion that allows to
quantify the difference between observed and simulated spectral maps. By
minimizing this difference, we find that very high and very low values of the
effective sticking probability are not appropriate for the studied
prestellar core. The most probable value for CB17 is 0.3--0.5. The spatial
distribution of the intensities and self-absorption features of optically thick
lines is indicative of UV irradiation of the core. By fitting simultaneously
optically thin and optically thick transitions, we isolate the model that
reproduces all the available spectral maps to a reasonable accuracy. The line
asymmetry pattern in CB17 is reproduced by a combination of infall, rotation,
and turbulent motions with velocities km s, km
s, and km s, respectively. These parameters corresponds
to energy ratios , , and (the rotation
parameters are determined for ). The chemical age of the core is
about 2 Myrs. In particular, this is indicated by the central depletion of CO,
CS, and HCO. Based on the angular momentum value, we argue that the core is
going to fragment, i.e., to form a binary (multiple) star. (abridged)Comment: ApJ, in pres
A Unified Monte Carlo Treatment of Gas-Grain Chemistry for Large Reaction Networks. I. Testing Validity of Rate Equations in Molecular Clouds
In this study we demonstrate for the first time that the unified Monte Carlo
approach can be applied to model gas-grain chemistry in large reaction
networks. Specifically, we build a time-dependent gas-grain chemical model of
the interstellar medium, involving about 6000 gas-phase and 200 grain surface
reactions. This model is used to test the validity of the standard and modified
rate equation methods in models of dense and translucent molecular clouds and
to specify under which conditions the use of the stochastic approach is
desirable.
We found that at temperatures 25--30 K gas-phase abundances of HO,
NH, CO and many other gas-phase and surface species in the stochastic model
differ from those in the deterministic models by more than an order of
magnitude, at least, when tunneling is accounted for and/or diffusion energies
are 3x lower than the binding energies. In this case, surface reactions,
involving light species, proceed faster than accretion of the same species. In
contrast, in the model without tunneling and with high binding energies, when
the typical timescale of a surface recombination is greater than the timescale
of accretion onto the grain, we obtain almost perfect agreement between results
of Monte Carlo and deterministic calculations in the same temperature range. At
lower temperatures ( K) gaseous and, in particular, surface abundances
of most important molecules are not much affected by stochastic processes.Comment: 33 pages, 9 figures, 1 table. Accepted for publication in Ap
The Electron Glass in a Switchable Mirror: Relaxation, Aging and Universality
The rare earth hydride YH can be tuned through the
metal-insulator transition both by changing and by illumination with
ultraviolet light. The transition is dominated by strong electron-electron
interactions, with transport in the insulator sensitive to both a Coulomb gap
and persistent quantum fluctuations. Via a systematic variation of UV
illumination time, photon flux, Coulomb gap depth, and temperature, we
demonstrate that polycrystalline YH serves as a model system for
studying the properties of the interacting electron glass. Prominent among its
features are logarithmic relaxation, aging, and universal scaling of the
conductivity
Levitation of the quantum Hall extended states in the 0 limit
We investigate the fate of the quantum Hall extended states within a
continuum model with spatially correlated disorder potentials. The model can be
projected onto a couple of the lowest Landau bands. Levitation of the
critical states is observed if at least the two lowest Landau bands are
considered. The dependence on the magnetic length and
on the correlation length of the disorder potential is combined into a
single dimensionless parameter . This enables us to study
the behavior of the critical states for vanishing magnetic field. In the two
Landau band limit, we find a disorder dependent saturation of the critical
states' levitation which is in contrast to earlier propositions, but in accord
with some experiments.Comment: 7 pages, 9 figures. Replaced with published versio
Association of Organochlorine Pesticides with Peripheral Neuropathy in Patients with Diabetes or Impaired Fasting Glucose
OBJECTIVE—Recent epidemiological studies have shown that background exposure to persistent organic pollutants (POPs)—xenobiotics accumulated in adipose tissue—is strongly associated with type 2 diabetes. Hyperglycemia is the cause of long-term complications of diabetes as well as diabetes itself, and POPs are well-known neurotoxicants. This study was performed to explore whether POPs are associated with peripheral neuropathy, a common long-term complication of diabetes, in people with glucose abnormalities
"Optical conductance fluctuations: diagrammatic analysis in Landauer approach and non-universal effects"
The optical conductance of a multiple scattering medium is the total
transmitted light of a diffuse incoming beam. This quantity, very analogous to
the electronic conductance, exhibits universal conductance fluctuations. We
perform a detailed diagrammatic analysis of these fluctuations. With a
Kadanoff-Baym technique all the leading diagrams are systematically generated.
A cancellation of the short distance divergencies occurs, that yields a well
behaved theory. The analytical form of the fluctuations is calculated and
applied to optical systems. Absorption and internal reflections reduce the
fluctuations significantly.Comment: 25 pages Revtex 3.0, 18 seperate postscript figure
Ultrafast phase-change logic device driven by melting processes.
The ultrahigh demand for faster computers is currently tackled by traditional methods such as size scaling (for increasing the number of devices), but this is rapidly becoming almost impossible, due to physical and lithographic limitations. To boost the speed of computers without increasing the number of logic devices, one of the most feasible solutions is to increase the number of operations performed by a device, which is largely impossible to achieve using current silicon-based logic devices. Multiple operations in phase-change-based logic devices have been achieved using crystallization; however, they can achieve mostly speeds of several hundreds of nanoseconds. A difficulty also arises from the trade-off between the speed of crystallization and long-term stability of the amorphous phase. We here instead control the process of melting through premelting disordering effects, while maintaining the superior advantage of phase-change-based logic devices over silicon-based logic devices. A melting speed of just 900 ps was achieved to perform multiple Boolean algebraic operations (e.g., NOR and NOT). Ab initio molecular-dynamics simulations and in situ electrical characterization revealed the origin (i.e., bond buckling of atoms) and kinetics (e.g., discontinuouslike behavior) of melting through premelting disordering, which were key to increasing the melting speeds. By a subtle investigation of the well-characterized phase-transition behavior, this simple method provides an elegant solution to boost significantly the speed of phase-change-based in-memory logic devices, thus paving the way for achieving computers that can perform computations approaching terahertz processing rates.This is the author's accepted manuscript. The final version is published by PNAS here: http://www.pnas.org/content/early/2014/08/27/1407633111.full.pdf+html?with-ds=yes
Investigating cerebral oedema using poroelasticity
Cerebral oedema can be classified as the tangible swelling produced by expansion of the interstitial fluid volume. Hydrocephalus can be succinctly described as the abnormal accumulation of cerebrospinal fluid (CSF) within the brain which ultimately leads to oedema within specific sites of parenchymal tissue. Using hydrocephalus as a test bed, one is able to account for the necessary mechanisms involved in the interaction between oedema formation and cerebral fluid production, transport and drainage. The current state of knowledge about integrative cerebral dynamics and transport phenomena indicates that poroelastic theory may provide a suitable framework to better understand various diseases. In this work, Multiple-Network Poroelastic Theory (MPET) is used to develop a novel spatio-temporal model of fluid regulation and tissue displacement within the various scales of the cerebral environment. The model is applied through two formats, a one-dimensional finite difference – Computational Fluid Dynamics (CFD) coupling framework, as well as a two-dimensional Finite Element Method (FEM) formulation. These are used to investigate the role of endoscopic fourth ventriculostomy in alleviating oedema formation due to fourth ventricle outlet obstruction (1D coupled model) in addition to observing the capability of the FEM template in capturing important characteristics allied to oedema formation, like for instance in the periventricular region (2D model)
Unhealthy weight control behaviours in adolescent girls: a process model based on self-determination theory
This study used self-determination theory (Deci, E.L., & Ryan, R.M. (2000). The 'what' and 'why' of goal pursuits: Human needs and the self-determination of behavior. Psychological Inquiry, 11, 227-268.) to examine predictors of body image concerns and unhealthy weight control behaviours in a sample of 350 Greek adolescent girls. A process model was tested which proposed that perceptions of parental autonomy support and two life goals (health and image) would predict adolescents' degree of satisfaction of their basic psychological needs. In turn, psychological need satisfaction was hypothesised to negatively predict body image concerns (i.e. drive for thinness and body dissatisfaction) and, indirectly, unhealthy weight control behaviours. The predictions of the model were largely supported indicating that parental autonomy support and adaptive life goals can indirectly impact upon the extent to which female adolescents engage in unhealthy weight control behaviours via facilitating the latter's psychological need satisfaction
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