Characterisation of global flow and local fluctuations in 3D SPH simulations of protoplanetary discs

A complete and detailed knowledge of the structure of the gaseous component in protoplanetary discs is essential to the study of dust evolution during the early phases of pre-planetesimal formation. The aim of this paper is to determine if three-dimensional accretion discs simulated by the Smoothed Particle Hydrodynamics (SPH) method can reproduce the observational data now available and the expected turbulent nature of protoplanetary discs. The investigation is carried out by setting up a suite of diagnostic tools specifically designed to characterise both the global flow and the fluctuations of the gaseous disc. The main result concerns the role of the artificial viscosity implementation in the SPH method: in addition to the already known ability of SPH artificial viscosity to mimic a physical-like viscosity under specific conditions, we show how the same artificial viscosity prescription behaves like an implicit turbulence model. In fact, we identify a threshold for the parameters in the standard artificial viscosity above which SPH disc models present a cascade in the power spectrum of velocity fluctuations, turbulent diffusion and a mass accretion rate of the same order of magnitude as measured in observations. Furthermore, the turbulence properties observed locally in SPH disc models are accompanied by meridional circulation in the global flow of the gas, proving that the two mechanisms can coexist.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical Society. 21 pages, 25 figure

Planet gaps in the dust layer of 3D proto-planetary disks: Observability with ALMA

Among the numerous known extrasolar planets, only a handful have been imaged directly so far, at large orbital radii and in rather evolved systems. The Atacama Large Millimeter/submillimeter Array (ALMA) will have the capacity to observe these wide planetary systems at a younger age, thus bringing a better understanding of the planet formation process. Here we explore the ability of ALMA to detect the gaps carved by planets on wide orbits.Comment: 2 pages, 2 figures, to appear in the Proceedings of IAU Symp. 299: Exploring the Formation and Evolution of Planetary Systems (Victoria, Canada

Growing dust grains in protoplanetary discs - I. Radial drift with toy growth models

In a series of papers, we present a comprehensive analytic study of the global motion of growing dust grains in protoplanetary discs, addressing both the radial drift and the vertical settling of the particles. Here we study how the radial drift of dust particles is affected by grain growth. In a first step, toy models in which grain growth can either be constant, accelerate or decelerate are introduced. The equations of motion are analytically integrable and therefore the grains dynamics is easy to understand. The radial motion of growing grains is governed by the relative efficiency of the growth and migration processes which is expressed by the dimensionless parameter Lambda, as well as the exponents for the gas surface density and temperature profiles, denoted p and q respectively. When Lambda is of order unity, growth and migration are strongly coupled, providing the most efficient radial drift. For the toy models considered, grains pile up when -p+q+1/2<0. Importantly, we show the existence of a second process which can help discs to retain their solid materials. For accelerating growth, grains end up their migration at a finite radius, thus avoiding being accreted onto the central star.Comment: 12 pages, 9 figures. Accepted for publication in MNRAS. v2: typos correcte

Fiscal policy challenges in oil-exporting countries – a review of key issues.

Fiscal policy choices have a particularly significant impact on economic performance in oil-exporting countries, owing to the importance of the oil sector in the economy and the fact that in most countries oil revenues accrue to the government. At the same time, fiscal policy in oil-centred economies s facing specific challenges, both in the long run, as regards intergenerational equity and fiscal sustainability, and in the short run, as regards macroeconomic stabilisation and fiscal planning. Institutional responses to the specific fiscal challenges in oil-exporting countries involve conservative oil price assumptions in the budget, the establishment of oil stabilisation and savings funds and fiscal rules. Fiscal policy in most oil-exporting countries has been expansionary over the past years in the wake of high oil prices. Fiscal expansion has added to inflationary pressure, and monetary policy has been constrained in tackling inflation as a result of prevailing exchange rate regimes. While, in this context, fiscal policy is the major tool for macroeconomic stabilisation, it has faced competing objectives and considerations. Cyclical considerations would have warranted fiscal restraint, but, in times of high oil prices, pressures to increase public spending have been mounting. Such pressures stem from primarily distribution-related considerations, development-related spending needs (e.g. in the areas of physical and social infrastructure) and international considerations in the context of, for example, global imbalances. The sharp fall in oil prices since mid-2008 has brought to the fore a different question – whether oil exporters can sustain spending levels reached in previous years. JEL Classification: E62, E63, H30, H60, Q32, Q38.Fiscal policy, oil-exporting countries, inflation, global imbalances.

The Host Stars of Extrasolar Planets Have Normal Lithium Abundances

The lithium abundances of planet-harbouring stars have been compared with those of open clusters and field stars. Young (chromospherically active) and subgiant stars have been eliminated from the comparison because they are at different stages of evolution and Li processing to the planet-harbouring stars, and hence have systematically higher Li abundances. The analysis showed that the Li abundances of the planet-harbouring stars are indistinguishable from those of non-planet-harbouring stars of the same age, temperature, and composition. This conclusion is opposite to that arrived at by Gonzalez & Laws (2000); it is believed that the field star sample used by them contained too wide a range of ages, evolutionary types, and temperatures to be accommodated by the model they adopted to describe the dependence on parameters. Li does not appear set to provide key insights into the formation and evolution of planetary systems.Comment: 2000, MNRAS, in press (5 pages including 2 figures and 2 tables