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 agrícolas.

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 ($1/r^2$ instead of $1/r^3$), 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

Double Quantum Dots in Carbon Nanotubes

We study the two-electron eigenspectrum of a carbon-nanotube double quantum dot with spin-orbit coupling. Exact calculation are combined with a simple model to provide an intuitive and accurate description of single-particle and interaction effects. For symmetric dots and weak magnetic fields, the two-electron ground state is antisymmetric in the spin-valley degree of freedom and is not a pure spin-singlet state. When double occupation of one dot is favored by increasing the detuning between the dots, the Coulomb interaction causes strong correlation effects realized by higher orbital-level mixing. Changes in the double-dot configuration affect the relative strength of the electron-electron interactions and can lead to different ground state transitions. In particular, they can favor a ferromagnetic ground state both in spin and valley degrees of freedom. The strong suppression of the energy gap can cause the disappearance of the Pauli blockade in transport experiments and thereby can also limit the stability of spin-qubits in quantum information proposals. Our analysis is generalized to an array of coupled dots which is expected to exhibit rich many-body behavior.Comment: 14 pages, 11 pages and 1 table. Typos in text and Figs.4 and 6 correcte