Towards a fully self-consistent spectral function of the nucleon in nuclear matter

We present a calculation of nuclear matter which goes beyond the usual quasi-particle approximation in that it includes part of the off-shell dependence of the self-energy in the self-consistent solution of the single-particle spectrum. The spectral function is separated in contributions for energies above and below the chemical potential. For holes we approximate the spectral function for energies below the chemical potential by a $\delta$-function at the quasi-particle peak and retain the standard form for energies above the chemical potential. For particles a similar procedure is followed. The approximated spectral function is consistently used at all levels of the calculation. Results for a model calculation are presented, the main conclusion is that although several observables are affected by the inclusion of the continuum contributions the physical consistency of the model does not improve with the improved self-consistency of the solution method. This in contrast to expectations based on the crucial role of self-consistency in the proofs of conservation laws.Comment: 26 pages Revtex with 4 figures, submitted to Phys. Rev.

Dynamical Expansion of Ionization and Dissociation Front around a Massive Star. II. On the Generality of Triggered Star Formation

We analyze the dynamical expansion of the HII region, photodissociation region, and the swept-up shell, solving the UV- and FUV-radiative transfer, the thermal and chemical processes in the time-dependent hydrodynamics code. Following our previous paper, we investigate the time evolutions with various ambient number densities and central stars. Our calculations show that basic evolution is qualitatively similar among our models with different parameters. The molecular gas is finally accumulated in the shell, and the gravitational fragmentation of the shell is generally expected. The quantitative differences among models are well understood with analytic scaling relations. The detailed physical and chemical structure of the shell is mainly determined by the incident FUV flux and the column density of the shell, which also follow the scaling relations. The time of shell-fragmentation, and the mass of the gathered molecular gas are sensitive tothe ambient number density. In the case of the lower number density, the shell-fragmentation occurs over a longer timescale, and the accumulated molecular gas is more massive. The variations with different central stars are more moderate. The time of the shell-fragmentation differs by a factor of several with the various stars of M_* = 12-101 M_sun. According to our numerical results, we conclude that the expanding HII region should be an efficient trigger for star formation in molecular clouds if the mass of the ambient molecular material is large enough.Comment: 49 pages, including 17 figures ; Accepted for publication in Ap

A new lab facility for measuring bidirectional reflectance/emittance distribution functions of soils and canopies

Recently, a laboratory measurement facility has been realized for assessing the anisotropic reflectance and emittance behaviour of soils, leaves and small canopies under controlled illumination conditions. The facility consists of an ASD FieldSpec 3 spectroradiometer covering the spectral range from 350 – 2500 nm at 1 nm spectral sampling interval. The spectroradiometer is deployed using a fiber optic cable with either a 1°, 8° or 25° instantaneous field of view (IFOV). These measurements can be used to assess the plant pigment (chlorophyll, xanthophyll, etc.) and non-pigment system (water, cellulose, lignin, nitrogen, etc.). The thermal emittance is measured using a NEC TH9100 Infrared Thermal Imager. It operates in a single band covering the spectral range from 8 – 14 mm with a resolution of 0.02 K. Images are 320 (H) by 240 (V) pixels with an IFOV of 1.2 mrad. A 1000 W Quartz Tungsten Halogen (QTH) lamp is used as illumination source, approximating the radiance distribution of the sun. This one is put at a fixed position during a measurement session. Multi-angular measurements are achieved by using a robotic positioning system allowing to perform either reflectance or emittance measurements over almost a complete hemisphere. The hemisphere can be sampled continuously between 0° and 80° from nadir and up to a few degrees from the hot-spot configuration (depending on the IFOV of the measurement device) for a backscattering target. Measurement distance to targets can be varied between 0.25 and 1 m, although with a distance of more than 0.6 m it is not possible to cover the full hemisphere. The goal is to infer the BRDF (bidirectional reflectance distribution function) and BTDF (bidirectional thermal distribution function) from these multi-angular measurements for various surface types (like soils, agricultural crops, small tree canopies and artificial objects) and surface roughness. The steering of the robotic arm and the reading of the spectroradiometer and the thermal camera are all fully automated

QuantMig:The use of migration scenarios in future characterisations: A systematic review and typology

BackgroundMigration plays an increasingly important role in shaping the demographic profiles of developed countries and receives ample attention in society at large as well as among policymakers. To understand how migration flows might evolve in the future, the QuantMig project set the goal of producing migration scenarios to support European migration policy. To do so, we need to make clear with what purpose scenarios are developed, how they are developed, and on which flows they focus. Other questions concern whether they are designed to describe the most likely future or a possible future, whether they are extrapolating trends observed in the past (assuming no fundamental changes in policies), or whether they are designed to describe desirable futures (migration as a panacea for ageing societies) or undesirable futures (massive inflow of immigration from developing countries). To produce the best possible migration scenarios, it is essential to get an overview of the literature. Migration scenarios have been used in a variety of future characterisations including forecasts, projections, and foresights. However, the term migration scenario is rarely well-defined or used consistently. Before developing a set of own scenarios, this document takes the necessary step of providing an overview of the existing literature and provide a definition and typology of migration scenarios. Based on this work, alternative ways of exploring the future of migration (for example in a vignette survey) will be discussed that lay out the bases for the extension of the work in the next deliverables of the work package. MethodsThis document looks at how migration scenarios are used in the literature presenting characterisations of societies’ futures. Relevant documents are systematically retrieved and assigned to one of six categories part of a pre-established typology. The typology rests on the focus (either migration or another aspect of societies influenced by migration) and purpose (either to predict the future, explore the future, or establish how a specific target can be reached) of each future characterisation. Subsequently, the techniques used for generating migration scenarios are described in terms of the approach taken (quantitative or qualitative) and how data is generated and transformed into meaningful output. Finally, the specific geographical context and characteristics of migration and migrants included in the scenarios are explored. ResultsA total of 107 documents were analysed. More than half presented migration scenarios that were developed to answer questions not about migration itself, but about its influence on a population’s future growth, age composition, or economic performance, among others. Future characterisations had most often prediction as purpose, being for example population forecasts, while many others had exploration as purpose, where the sensitivity of a given phenomenon to different migration assumptions is assessed. Most scenarios rest on a quantitative approach rather than on a narrative, but the latter has clearly expanded in the last years. Migration scenarios that follow a quantitative approach often rest on past migration trends to characterise the future, but seldom provide likelihoods that a given scenario will realise. Migration scenarios that follow a qualitative approach, on the other hand, often rest on experts and stakeholders’ views for input, or rely on previously developed storylines. Finally, quantitative scenarios often concentrate on net migration figures inside of a single, usually economically developed country, while qualitative scenarios are more likely to consider bidirectional flows between two (world) regions. ConclusionsThere was an increase over time in the use of qualitative scenarios to characterise the future of migration. However, these scenarios were seldom used to translate storylines into quantitative outputs that specifically aim at predicting future migration flows. Ways to achieve this are discussed, including more advanced data collection techniques among experts and stakeholders, and the consideration of multiple types of migration

Quartic double solids with ordinary singularities

We study the mixed Hodge structure on the third homology group of a threefold which is the double cover of projective three-space ramified over a quartic surface with a double conic. We deal with the Torelli problem for such threefolds.Comment: 14 pages, presented at the Conference Arnol'd 7

QuantMig:The use of migration scenarios in future characterisations: A systematic review and typology

BackgroundMigration plays an increasingly important role in shaping the demographic profiles of developed countries and receives ample attention in society at large as well as among policymakers. To understand how migration flows might evolve in the future, the QuantMig project set the goal of producing migration scenarios to support European migration policy. To do so, we need to make clear with what purpose scenarios are developed, how they are developed, and on which flows they focus. Other questions concern whether they are designed to describe the most likely future or a possible future, whether they are extrapolating trends observed in the past (assuming no fundamental changes in policies), or whether they are designed to describe desirable futures (migration as a panacea for ageing societies) or undesirable futures (massive inflow of immigration from developing countries). To produce the best possible migration scenarios, it is essential to get an overview of the literature. Migration scenarios have been used in a variety of future characterisations including forecasts, projections, and foresights. However, the term migration scenario is rarely well-defined or used consistently. Before developing a set of own scenarios, this document takes the necessary step of providing an overview of the existing literature and provide a definition and typology of migration scenarios. Based on this work, alternative ways of exploring the future of migration (for example in a vignette survey) will be discussed that lay out the bases for the extension of the work in the next deliverables of the work package. MethodsThis document looks at how migration scenarios are used in the literature presenting characterisations of societies’ futures. Relevant documents are systematically retrieved and assigned to one of six categories part of a pre-established typology. The typology rests on the focus (either migration or another aspect of societies influenced by migration) and purpose (either to predict the future, explore the future, or establish how a specific target can be reached) of each future characterisation. Subsequently, the techniques used for generating migration scenarios are described in terms of the approach taken (quantitative or qualitative) and how data is generated and transformed into meaningful output. Finally, the specific geographical context and characteristics of migration and migrants included in the scenarios are explored. ResultsA total of 107 documents were analysed. More than half presented migration scenarios that were developed to answer questions not about migration itself, but about its influence on a population’s future growth, age composition, or economic performance, among others. Future characterisations had most often prediction as purpose, being for example population forecasts, while many others had exploration as purpose, where the sensitivity of a given phenomenon to different migration assumptions is assessed. Most scenarios rest on a quantitative approach rather than on a narrative, but the latter has clearly expanded in the last years. Migration scenarios that follow a quantitative approach often rest on past migration trends to characterise the future, but seldom provide likelihoods that a given scenario will realise. Migration scenarios that follow a qualitative approach, on the other hand, often rest on experts and stakeholders’ views for input, or rely on previously developed storylines. Finally, quantitative scenarios often concentrate on net migration figures inside of a single, usually economically developed country, while qualitative scenarios are more likely to consider bidirectional flows between two (world) regions. ConclusionsThere was an increase over time in the use of qualitative scenarios to characterise the future of migration. However, these scenarios were seldom used to translate storylines into quantitative outputs that specifically aim at predicting future migration flows. Ways to achieve this are discussed, including more advanced data collection techniques among experts and stakeholders, and the consideration of multiple types of migration

Star formation in disk galaxies driven by primordial H_2

We show that gaseous \HI disks of primordial composition irradiated by an external radiation field can develop a multiphase medium with temperatures between 10^2 and 10^4 K due to the formation of molecular hydrogen. For a given \HI column density there is a critical value of the radiation field below which only the cold \HI phase can exist. Due to a time decreasing quasar background, the gas starts cooling slowly after recombination until the lowest stable temperature in the warm phase is reached at a critical redshift $z=z_{cr}$. Below this redshift the formation of molecular hydrogen promotes a rapid transition towards the cold \HI phase. We find that disks of protogalaxies with 10^{20}\simlt N_{HI}\simlt 10^{21} cm^{-2} are gravitationally stable at $T\sim 10^4$ K and can start their star formation history only at z \simlt z_{cr}\sim 2, after the gas in the central portion of the disk has cooled to temperatures T\simlt 300 K. Such a delayed starbust phase in galaxies of low gas surface density and low dynamical mass can disrupt the disks and cause them to fade away. These objects could contribute significantly to the faint blue galaxy population.Comment: 16 pages (LaTeX), 2 Figures to be published in Astrophysical Journal Letter