Searching in Unstructured Overlays Using Local Knowledge and Gossip

This paper analyzes a class of dissemination algorithms for the discovery of distributed contents in Peer-to-Peer unstructured overlay networks. The algorithms are a mix of protocols employing local knowledge of peers' neighborhood and gossip. By tuning the gossip probability and the depth k of the k-neighborhood of which nodes have information, we obtain different dissemination protocols employed in literature over unstructured P2P overlays. The provided analysis and simulation results confirm that, when properly configured, these schemes represent a viable approach to build effective P2P resource discovery in large-scale, dynamic distributed systems.Comment: A revised version of the paper appears in Proc. of the 5th International Workshop on Complex Networks (CompleNet 2014) - Studies in Computational Intelligence Series, Springer-Verlag, Bologna (Italy), March 201

Evolution of Migrating Planets Undergoing Gas Accretion

We analyze the orbital and mass evolution of planets that undergo run-away gas accretion by means of 2D and 3D hydrodynamic simulations. The disk torque distribution per unit disk mass as a function of radius provides an important diagnostic for the nature of the disk-planet interactions. We first consider torque distributions for nonmigrating planets of fixed mass and show that there is general agreement with the expectations of resonance theory. We then present results of simulations for mass-gaining, migrating planets. For planets with an initial mass of 5 Earth masses, which are embedded in disks with standard parameters and which undergo run-away gas accretion to one Jupiter mass (Mjup), the torque distributions per unit disk mass are largely unaffected by migration and accretion for a given planet mass. The migration rates for these planets are in agreement with the predictions of the standard theory for planet migration (Type I and Type II migration). The planet mass growth occurs through gas capture within the planet's Bondi radius at lower planet masses, the Hill radius at intermediate planet masses, and through reduced accretion at higher planet masses due to gap formation. During run-away mass growth, a planet migrates inwards by only about 20% in radius before achieving a mass of ~1 Mjup. For the above models, we find no evidence of fast migration driven by coorbital torques, known as Type III migration. We do find evidence of Type III migration for a fixed mass planet of Saturn's mass that is immersed in a cold and massive disk. In this case the planet migration is assumed to begin before gap formation completes. The migration is understood through a model in which the torque is due to an asymmetry in density between trapped gas on the leading side of the planet and ambient gas on the trailing side of the planet.Comment: 26 pages, 29 figures. To appear in The Astrophysical Journal vol.684 (September 20, 2008 issue

Modelling the evolution of planets in disks

To explain important properties of extrasolar planetary systems (eg. close-in hot Jupiters, resonant planets) an evolutionary scenario which allows for radial migration of planets in disks is required. During their formation protoplanets undergo a phase in which they are embedded in the disk and interact gravitationally with it. This planet-disk interaction results in torques (through gravitational forces) acting on the planet that will change its angular momentum and result in a radial migration of the planet through the disk. To determine the outcome of this very important process for planet formation, dedicated high resolution numerical modeling is required. This contribution focusses on some important aspects of the numerical approach that we found essential for obtaining successful results. We specifically mention the treatment of Coriolis forces, Cartesian grids, and the FARGO method.Comment: Talk given at JENAM meeting, Vienna 200

Evolution of inclined planets in three-dimensional radiative discs

While planets in the solar system only have a low inclination with respect to the ecliptic there is mounting evidence that in extrasolar systems the inclination can be very high, at least for close-in planets. One process to alter the inclination of a planet is through planet-disc interactions. Recent simulations considering radiative transport have shown that the evolution of migration and eccentricity can strongly depend on the thermodynamic state of the disc. We extend previous studies to investigate the planet-disc interactions of fixed and moving planets on inclined and eccentric orbits. We also analyse the effect of the disc's thermodynamic properties on the orbital evolution of embedded planets in detail. The protoplanetary disc is modelled as a viscous gas where the internally produced dissipation is transported by radiation. For locally isothermal discs, we confirm previous results and find inclination damping and inward migration for planetary cores. For low inclinations i < 2 H/r, the damping is exponential, while di/dt is proportional to i^-2 for larger i. For radiative discs, the planetary migration is very limited, as long as their inclination exceeds a certain threshold. If the inclination is damped below this threshold, planetary cores with a mass up to approximately 33 Earth masses start to migrate outwards, while larger cores migrate inwards right from the start. The inclination is damped for all analysed planet masses. In a viscous disc an initial inclination of embedded planets will be damped for all planet masses. This damping occurs on timescales that are shorter than the migration time. If the inclination lies beneath a certain threshold, the outward migration in radiative discs is not handicapped. Outward migration is strongest for circular and non-inclined orbits

Unstable Planetary Systems Emerging Out Of Gas Disks

The discovery of over 400 extrasolar planets allows us to statistically test our understanding of formation and dynamics of planetary systems via numerical simulations. Traditional N-body simulations of multiple-planet systems without gas disks have successfully reproduced the eccentricity (e) distribution of the observed systems, by assuming that the planetary systems are relatively closely packed when the gas disk dissipates, so that they become dynamically unstable within the stellar lifetime. However, such studies cannot explain the small semi-major axes (a) of extrasolar planetary systems, if planets are formed, as the standard planet formation theory suggests, beyond the ice line. In this paper, we numerically study the evolution of three-planet systems in dissipating gas disks, and constrain the initial conditions that reproduce the observed semi-major axis and eccentricity distributions simultaneously. We adopt the initial conditions that are motivated by the standard planet formation theory, and self-consistently simulate the disk evolution, and planet migration by using a hybrid N-body and 1D gas disk code. We also take account of eccentricity damping, and investigate the effect of saturation of corotation resonances on the evolution of planetary systems. We find that the semi-major axis distribution is largely determined in a gas disk, while the eccentricity distribution is determined after the disk dissipation. We also find that there may be an optimum disk mass which leads to the observed a-e distribution. Our simulations generate a larger fraction of planetary systems trapped in mean-motion resonances (MMRs) than the observations, indicating that the disk's perturbation to the planetary orbits may be important to explain the observed rate of MMRs. We also find much lower occurrence of planets on retrograde orbits than the current observations of close-in planets suggest.Comment: 12 pages, 9 figures, accepted for publication in Ap

Internationalisation, cultural distance and country characteristics: a Bayesian analysis of SME's financial performance

Relying on the accounting data of a panel of 403 Italian manufacturing SMEs collected over a period of 5 years, we find results suggesting that multinationality per se does not impact on the economic performance of international small and medium sized firms. It is the characteristics of the country selected i.e. the political hazard, the financial stability and the economic performance that significantly influence SMEs financial performance. The management implication for small and medium sized firms selecting and entering new geographic markets is significant, since our results show that for SMEs it is the market selection process that really matters and not the degree of multinationality

On the migration of protogiant solid cores

The increase of computational resources has recently allowed high resolution, three dimensional calculations of planets embedded in gaseous protoplanetary disks. They provide estimates of the planet migration timescale that can be compared to analytical predictions. While these predictions can result in extremely short migration timescales for cores of a few Earth masses, recent numerical calculations have given an unexpected outcome: the torque acting on planets with masses between 5 M_Earth and 20 M_Earth is considerably smaller than the analytic, linear estimate. These findings motivated the present work, which investigates existence and origin of this discrepancy or ``offset'', as we shall call it, by means of two and three dimensional numerical calculations. We show that the offset is indeed physical and arises from the coorbital corotation torque, since (i) it scales with the disk vortensity gradient, (ii) its asymptotic value depends on the disk viscosity, (iii) it is associated to an excess of the horseshoe zone width. We show that the offset corresponds to the onset of non-linearities of the flow around the planet, which alter the streamline topology as the planet mass increases: at low mass the flow non-linearities are confined to the planet's Bondi sphere whereas at larger mass the streamlines display a classical picture reminiscent of the restricted three body problem, with a prograde circumplanetary disk inside a ``Roche lobe''. This behavior is of particular importance for the sub-critical solid cores (M <~ 15 M_Earth) in thin (H/r <~0.06) protoplanetary disks. Their migration could be significantly slowed down, or reversed, in disks with shallow surface density profiles.Comment: Accepted for publication in Ap

Dispersion enhancement and damping by buoyancy driven flows in 2D networks of capillaries

The influence of a small relative density difference on the displacement of two miscible liquids is studied experimentally in transparent 2D networks of micro channels. Both stable displacements in which the denser fluid enters at the bottom of the cell and displaces the lighter one and unstable displacements in which the lighter fluid is injected at the bottom and displaces the denser one are realized. Except at the lowest mean flow velocity U, the average $C(x,t)$ of the relative concentration satisfies a convection-dispersion equation. The dispersion coefficient is studied as function of the relative magnitude of fluid velocity and of the velocity of buoyancy driven fluid motion. A model is suggested and its applicability to previous results obtained in 3D media is discussed

Entangled Quantum Clocks for Measuring Proper-Time Difference

We report that entangled pairs of quantum clocks (non-degenerate quantum bits) can be used as a specialized detector for precisely measuring difference of proper-times that each constituent quantum clock experiences. We describe why the proposed scheme would be more precise in the measurement of proper-time difference than a scheme of two-separate-quantum-clocks. We consider possibilities that the proposed scheme can be used in precision test of the relativity theory.Comment: no correction, 4 pages, RevTe