4,194 research outputs found
Two-dimensional models of layered protoplanetary discs - II. The effect of a residual viscosity in the dead zone
We study axisymmetric models of layered protoplanetary discs taking radiative
transfer effects into account, and allowing for a residual viscosity in the
dead zone. We also explore the effect of different viscosity prescriptions. In
addition to the ring instability reported in the first paper of the series we
find an oscillatory instability of the dead zone, accompanied by variations of
the accretion rate onto the central star. We provide a simplified analytical
description explaining the mechanism of the oscillations. Finally, we find that
the residual viscosity enables stationary accretion in large regions of layered
discs. Based on results obtained with the help of a simple 1-D hydrocode we
identify these regions, and discuss conditions in which layered discs can give
rise to FU~Orionis phenomena.Comment: 9 pages, 5 figures, accepted for publication in MNRA
Dielectric function and plasmons in graphene
The electromagnetic response of graphene, expressed by the dielectric
function, and the spectrum of collective excitations are studied as a function
of wave vector and frequency. Our calculation is based on the full band
structure, calculated within the tight-binding approximation. As a result, we
find plasmons whose dispersion is similar to that obtained in the single-valley
approximation by Dirac fermions. In contrast to the latter, however, we find a
stronger damping of the plasmon modes due to inter-band absorption. Our
calculation also reveals effects due to deviations from the linear Dirac
spectrum as we increase the Fermi energy, indicating an anisotropic behavior
with respect to the wave vector of the external electromagnetic field
Elementos conceituais para a representação de sistemas agrÃcolas.
bitstream/item/44877/1/documento-299.pd
Exploring GLIMPSE Bubble N107: Multiwavelength Observations and Simulations
Context. Bubble N107 was discovered in the infrared emission of dust in the
Galactic Plane observed by the Spitzer Space Telescope (GLIMPSE survey: l ~
51.0 deg, b ~ 0.1 deg). The bubble represents an example of shell-like
structures found all over the Milky Way Galaxy.
Aims. We aim to analyse the atomic and molecular components of N107, as well
as its radio continuum emission. With the help of numerical simulations, we aim
to estimate the bubble age and other parameters which cannot be derived
directly from observations.
Methods. From the observations of the HI (I-GALFA) and 13CO (GRS) lines we
derive the bubble's kinematical distance and masses of the atomic and molecular
components. With the algorithm DENDROFIND, we decompose molecular material into
individual clumps. From the continuum observations at 1420 MHz (VGPS) and 327
MHz (WSRT), we derive the radio flux density and the spectral index. With the
numerical code ring, we simulate the evolution of stellar-blown bubbles similar
to N107.
Results. The total HI mass associated with N107 is 5.4E3 Msun. The total mass
of the molecular component (a mixture of cold gasses of H2, CO, He and heavier
elements) is 1.3E5 Msun, from which 4.0E4 Msun is found along the bubble
border. We identified 49 molecular clumps distributed along the bubble border,
with the slope of the clump mass function of -1.1. The spectral index of -0.30
of a strong radio source located apparently within the bubble indicates
nonthermal emission, hence part of the flux likely originates in a supernova
remnant, not yet catalogued. The numerical simulations suggest N107 is likely
less than 2.25 Myr old. Since first supernovae explode only after 3 Myr or
later, no supernova remnant should be present within the bubble. It may be
explained if there is a supernova remnant in the direction towards the bubble,
however not associated with it.Comment: 15 pages, 11 figure
Dynamical polarization, screening, and plasmons in gapped graphene
The one-loop polarization function of graphene has been calculated at zero
temperature for arbitrary wavevector, frequency, chemical potential (doping),
and band gap. The result is expressed in terms of elementary functions and is
used to find the dispersion of the plasmon mode and the static screening within
the random phase approximation. At long wavelengths the usual square root
behaviour of plasmon spectra for two-dimensional (2D) systems is obtained. The
presence of a small (compared to a chemical potential) gap leads to the
appearance of a new undamped plasmon mode. At greater values of the gap this
mode merges with the long-wavelength one, and vanishes when the Fermi level
enters the gap. The screening of charged impurities at large distances differs
from that in gapless graphene by slower decay of Friedel oscillations (
instead of ), similarly to conventional 2D systems.Comment: 8 pages, 8 figures, v2: to match published versio
Fostering institutionalisation? The impact of the EU accession process on state–civil society relations in Serbia
In the framework of its enlargement policy, the EU has placed considerable emphasis on supporting civil society organisations (CSOs) both as domestic drivers of change and as a means to foster new, more participatory modes of governance. Our research examines the impact of the EU accession process on state–civil society relations in the Western Balkans and assesses the extent to which new forms of interaction are becoming institutionalised. Comparing minority rights and environmental regulation in Serbia, we find that enlargement negotiations lead to increased dialogue and more formalised interactions between government and CSOs. However, the institutionalisation of state–CSO cooperation remains partial and is hampered by a lack of political will. Whereas civil servants are generally open to civil society input, political officials frequently resort to façade cooperation in response to external pressures. We conclude that the emerging governance model is nothing like the ‘double weakness’ or agency capture found in earlier studies, but instead consists of strong hierarchy and a narrow group of highly professional CSOs engaged at the margins
Oceanic tracer and proxy time scales revisited
Quantifying time-responses of the ocean to tracer
input is important to the interpretation of paleodata from sediment
cores – because surface-injected tracers do not instantaneously
spread throughout the ocean. To obtain insights
into the time response, a computationally efficient statetransition
matrix method is demonstrated and used to compute
successive states of passive tracer concentrations in the
global ocean. Times to equilibrium exceed a thousand years
for regions of the global ocean outside of the injection and
convective areas and concentration gradients give time-lags
from hundreds to thousands of years between the Atlantic
and Pacific abyss, depending on the injection region and the
nature of the boundary conditions employed. Equilibrium
times can be much longer than radiocarbon ages – both because
the latter are strongly biased towards the youngest fraction
of fluid captured in a sample, and because they represent
distinct physical properties. Use of different boundary conditions
– concentration, or flux – produces varying response
times, with the latter depending directly upon pulse duration.
With pulses, the sometimes very different transient approach
to equilibrium in various parts of the ocean generates event
identification problems.National Science Foundation (U.S.) (Grant OCE-0824783)United States. National Aeronautics and Space Administration (Award NNX08AF09G
Wavenumber-explicit parametric holomorphy of Helmholtz solutions in the context of uncertainty quantification
A crucial role in the theory of uncertainty quantification (UQ) of PDEs is played by the regularity of the solution with respect to the stochastic parameters; indeed, a key property one seeks to establish is that the solution is holomorphic with respect to (the complex extensions of) the parameters. In the context of UQ for the high-frequency Helmholtz equation, a natural question is therefore: how does this parametric holomorphy depend on the wavenumber k? The recent paper [35] showed for a particular nontrapping variable-coefficient Helmholtz problem with affine dependence of the coefficients on the stochastic parameters that the solution operator can be analytically continued a distance \simk - 1 into the complex plane. In this paper, we generalize the result in [35] about k-explicit parametric holomorphy to a much wider class of Helmholtz problems with arbitrary (holomorphic) dependence on the stochastic parameters; we show that in all cases the region of parametric holomorphy decreases with k and show how the rate of decrease with k is dictated by whether the unperturbed Helmholtz problem is trapping or nontrapping. We then give examples of both trapping and nontrapping problems where these bounds on the rate of decrease with k of the region of parametric holomorphy are sharp, with the trapping examples coming from the recent results of [31]. An immediate implication of these results is that the k-dependent restrictions imposed on the randomness in the analysis of quasi-Monte Carlo methods in [35] arise from a genuine feature of the Helmholtz equation with k large (and not, for example, a suboptimal bound).</p
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