Molecular Hydrogen Emission from Protoplanetary Disks: Effects of X-ray Irradiation and Dust Evolution

Detailed models for the density and temperature profiles of gas and dust in protoplanetary disks are constructed by taking into account X-ray and UV irradiation from a central T Tauri star, as well as dust size growth and settling toward the disk midplane. The spatial and size distributions of dust grains are numerically computed by solving the coagulation equation for settling dust particles, with the result that the mass and total surface area of dust grains per unit volume of the gas in the disks are very small, except at the midplane. The H2 level populations and line emission are calculated using the derived physical structure of the disks. X-ray irradiation is the dominant heating source of the gas in the inner disk and in the surface layer, while the UV heating dominates otherwise. If the central star has strong X-ray and weak UV radiation, the H2 level populations are controlled by X-ray pumping, and the X-rayinduced transition lines could be observable. If the UV irradiation is strong, the level populations are controlled by thermal collisions or UV pumping, depending on the dust properties. As the dust particles evolve in the disks, the gas temperature at the disk surface drops because the grain photoelectric heating becomes less efficient. This makes the level populations change from LTE to non-LTE distributions, which results in changes to the line ratios. Our results suggest that dust evolution in protoplanetary disks could be observable through the H2 line ratios. The emission lines are strong from disks irradiated by strong UV and X-rays and possessing small dust grains; such disks will be good targets in which to observe H2 emission.

Turbulent diffusion and mixing in interstellar dark clouds with adsorption onto grains

This paper reports the results of models of dark cloud chemistry incorporating a depth dependent density distribution with diffusive mixing and adsorption onto grains. The model is based on the approach taken by Xie et al. (1995), with the addition of grain accretion effects. Without diffusion, the central regions of the cloud freeze out in less than 10(7) years. Freeze-out time is dependent on density, so the diffuse outer region of the cloud remains abundant in gas for about an order of magnitude longer. We find that fairly small amounts of diffusive mixing can delay freeze-out at the centre of the model cloud for a time up to an order of magnitude greater than without diffusion, due to material diffusing inward from the edges of the cloud. The gas-phase lifetime of the cloud core can thus be increased by up to an order of magnitude or more by this process. We have run three different grain models with various diffusion coefficients to investigate the effects of changing the sticking parameters.

Per una genealogia della giustizia penale internazionale: ambiguita' e paradossi

This article aims to consider the role for a critical criminology outside the national dimension, highlighting its continuities with studies in the critical tradition which have suggested the need to address State criminality and criminogenic structures more in general, but also suggesting a critique of international criminal law as a governmentality project.It reconstructs the genealogy of the international criminal justice system, following on from Schmitt and other well known authors, but it focuses in specific on its paradoxes, contradictions and ambiguities rather than its purely political effect. The authors argue that critical criminologists should engage with the international dimension of crime and control and approach this venture of a international criminal justice system as the possibility of “telling the truth” about State atrocities without missing on using strategically the category of human rights and law to bring to the fore minoritarian interests which are
usually denied by power discourses and economic forces.

Localized excitations of charged dust grains in dusty plasma lattices

The nonlinear aspects of charged dust grain motion in a one-dimensional dusty plasma (DP) monolayer are discussed. Both horizontal (longitudinal, acoustic mode) and vertical (transverse, optic mode) displacements are considered, and various types of localized excitations are reviewed, in a continuum approximation. Dust crystals are shown to support nonlinear kink-shaped supersonic longitudinal solitary excitations, as well as modulated envelope (either longitudinal or transverse) localized modes. The possibility for Discrete Breather (DB-) type excitations (Intrinsic Localized Modes, ILMs) to occur is investigated, from first principles. These highly localized excitations owe their existence to lattice discreteness, in combination with the interaction and/or
substrate (sheath) potential nonlinearity. This possibility may open new directions in DP- related research. The relation to previous results on atomic chains as well as to experimental results on strongly-coupled dust layers in gas discharge plasmas is discussed.

ON THE ORIGIN OF NH IN DIFFUSE INTERSTELLAR CLOUDS

The observation by Meyer & Roch of NH in the interstellar clouds towards zeta Per and HD 27778 cannot be explained with conventional gas-phase chemistry models. A simple non-equilibrium model for the zeta Per cloud, which incorporates the grain-surface production of NH and OH or, alternatively, NH3 and H2O, is able to reproduce the abundances of all observed species (except CH+) quite accurately. Moreover, chemical models which include grain-surface reactions can reproduce the observed abundance not only of NH but also of CN, which is efficiently formed at low temperatures, initiated by the reaction of NH with C+. Pure gas-phase models and cloud interface models, in which NH and CH+ are formed in a warm and tenuous environment, fail to explain the observed high abundance of CN. Hence the observation of NH in zeta Per and HD 27778 provides evidence for the presence of grain-surface reactions leading to molecules other than H-2. It is predicted that NH2 and NH3 should have abundances not much below that of NH if NH3 instead of NH is formed on grains. With or without surface reactions, the column densities of H2O and C2H are expected to be about 10(13) cm-2, and these molecules may be detectable in the zeta Per cloud.