Ab initio Quantum and ab initio Molecular Dynamics of the Dissociative Adsorption of Hydrogen on Pd(100)

The dissociative adsorption of hydrogen on Pd(100) has been studied by ab initio quantum dynamics and ab initio molecular dynamics calculations. Treating all hydrogen degrees of freedom as dynamical coordinates implies a high dimensionality and requires statistical averages over thousands of trajectories. An efficient and accurate treatment of such extensive statistics is achieved in two steps: In a first step we evaluate the ab initio potential energy surface (PES) and determine an analytical representation. Then, in an independent second step dynamical calculations are performed on the analytical representation of the PES. Thus the dissociation dynamics is investigated without any crucial assumption except for the Born-Oppenheimer approximation which is anyhow employed when density-functional theory calculations are performed. The ab initio molecular dynamics is compared to detailed quantum dynamical calculations on exactly the same ab initio PES. The occurence of quantum oscillations in the sticking probability as a function of kinetic energy is addressed. They turn out to be very sensitive to the symmetry of the initial conditions. At low kinetic energies sticking is dominated by the steering effect which is illustrated using classical trajectories. The steering effects depends on the kinetic energy, but not on the mass of the molecules. Zero-point effects lead to strong differences between quantum and classical calculations of the sticking probability. The dependence of the sticking probability on the angle of incidence is analysed; it is found to be in good agreement with experimental data. The results show that the determination of the potential energy surface combined with high-dimensional dynamical calculations, in which all relevant degrees of freedon are taken into account, leads to a detailed understanding of the dissociation dynamics of hydrogen at a transition metal surface.Comment: 15 pages, 9 figures, subm. to Phys. Rev.

First detection of Edwardsiella ictaluri (Proteobacteria: Enterobacteriaceae) in wild Australian catfish

The bacterium Edwardsiella ictaluri is considered to be one of the most significant pathogens of farmed catfish in the United States of America and has also caused mortalities in farmed and wild fishes in many other parts of the world. E. ictaluri is not believed to be present in wild fish populations in Australia, although it has previously been detected in imported ornamental fishes held in quarantine facilities. In an attempt to confirm freedom from the bacterium in Australian native fishes, we undertook a risk-based survey of wild catfishes from 15 sites across northern Australia. E. ictaluri was detected by selective culturing, followed by DNA testing, in Wet Tropics tandan (Tandanus tropicanus) from the Tully River, at a prevalence of 0.40 (95% CI 0.21–0.61). The bacterium was not found in fishes sampled from any of the other 14 sites. This is the first report of E. ictaluri in wild fishes in Australia

Study of hybrid Х pinches in different conditions

A standard X-pinch consists of two or more fine wires that cross at a single point as the load of a pulsed power generator. To simplify the X-pinch load, a hybrid configuration consisting of solid conical electrodes connected by a wire, has been suggested and tested on four generators ranging in current from 200 kA to 1 MA and risetimes varying from 45 to 170 ns. The experiments have shown that for each generator, a wire material, diameter and length can be found for which the X-pinches generate a single intense burst of soft x-rays and develop a single hot spot. Also they generated less hard x-ray intensity than that measured in comparable standard X-pinches.Стандартные Х-пинчи состоят из двух или более проволочек, перекрещенных в диоде сильноточного генератора. Чтобы упростить конфигурацию Х-пинчей, была предложена и протестирована на четырех сильноточных генераторах с токами от 200 кА до 1 МА и временем нарастания импульса от 45 до 170 нс нагрузка в виде гибридного Х-пинча. Гибридные Х-пинчи состоят из двух твердотельных тугоплавких электродов, соединенных проволочкой. Проведенные эксперименты показали, что для каждого генератора можно подобрать материал проволочки, диаметр и длину, при которых гибридные Х-пинчи образуют единичную горячую точку и излучают интенсивную вспышку мягкого рентгеновского излучения. При этом зарегистрированный уровень жесткого рентгеновского излучения был значительно ниже, чем в стандартных Х-пинчах.Стандартні Х-пінчі складаються з двох або більше дротиків, перехрещених у діоді потужнострумового генератора. Щоб спростити конфігурацію Х-пінча, була запропонована і протестована на чотирьох потужнострумових генераторах із струмами від 200 кА до 1 МА та часом наростання імпульсу від 45 до 170 нс навантаження у вигляді гібридного Х-пінча. Гібридні Х-пінчі складаються з двох твердотільних тугоплавких електродів, з'єднаних дротиком. Проведені експерименти показали, що для кожного генератора можна підібрати матеріал дротика, діаметр і довжину, при яких гібридні Х-пінчі утворюють одиничну гарячу краплю та випромінюють інтенсивний спалах м'якого рентгенівського випромінювання. При цьому зареєстрований рівень жорсткого рентгенівського випромінювання був значно нижче, ніж у стандартних Х-пінчах

Occlusion and Motion Reasoning for Long-Term Tracking

International audienceObject tracking is a reoccurring problem in computer vision. Tracking-by-detection approaches, in particular Struck (Hare et al., 2011), have shown to be competitive in recent evaluations. However, such approaches fail in the presence of long-term occlusions as well as severe viewpoint changes of the object. In this paper we propose a principled way to combine occlusion and motion reasoning with a tracking-by-detection approach. Occlusion and motion reasoning is based on state-of-the-art long-term trajectories which are labeled as object or background tracks with an energy-based formulation. The overlap between labeled tracks and detected regions allows to identify occlusions. The motion changes of the object between consecutive frames can be estimated robustly from the geometric relation between object trajectories. If this geometric change is significant, an additional detector is trained. Experimental results show that our tracker obtains state-of-the-art results and handles occlusion and viewpoints changes better than competing tracking methods

Origins of the Ambient Solar Wind: Implications for Space Weather

The Sun's outer atmosphere is heated to temperatures of millions of degrees, and solar plasma flows out into interplanetary space at supersonic speeds. This paper reviews our current understanding of these interrelated problems: coronal heating and the acceleration of the ambient solar wind. We also discuss where the community stands in its ability to forecast how variations in the solar wind (i.e., fast and slow wind streams) impact the Earth. Although the last few decades have seen significant progress in observations and modeling, we still do not have a complete understanding of the relevant physical processes, nor do we have a quantitatively precise census of which coronal structures contribute to specific types of solar wind. Fast streams are known to be connected to the central regions of large coronal holes. Slow streams, however, appear to come from a wide range of sources, including streamers, pseudostreamers, coronal loops, active regions, and coronal hole boundaries. Complicating our understanding even more is the fact that processes such as turbulence, stream-stream interactions, and Coulomb collisions can make it difficult to unambiguously map a parcel measured at 1 AU back down to its coronal source. We also review recent progress -- in theoretical modeling, observational data analysis, and forecasting techniques that sit at the interface between data and theory -- that gives us hope that the above problems are indeed solvable.Comment: Accepted for publication in Space Science Reviews. Special issue connected with a 2016 ISSI workshop on "The Scientific Foundations of Space Weather." 44 pages, 9 figure

GRB 171010A/SN 2017htp: a GRB-SN at z = 0.33

The number of supernovae known to be connected with long-duration gamma-ray bursts (GRBs) is increasing and the link between these events is no longer exclusively found at low redshift (z ≲ 0.3) but is well established also at larger distances. We present a new case of such a liaison at z = 0.33 between GRB 171010A and SN 2017htp. It is the second closest GRB with an associated supernova of only three events detected by Fermi-LAT. The supernova is one of the few higher redshift cases where spectroscopic observations were possible and shows spectral similarities with the well-studied SN 1998bw, having produced a similar Ni mass (⁠ M Ni =0.33±0.02 M ⊙ MNi=0.33±0.02 M⊙ ⁠) with slightly lower ejected mass (⁠ M ej =4.1±0.7 M ⊙ Mej=4.1±0.7 M⊙ ⁠) and kinetic energy (⁠ E K =8.1±2.5× 10 51 erg EK=8.1±2.5×1051 erg ⁠). The host-galaxy is bigger in size than typical GRB host galaxies, but the analysis of the region hosting the GRB revealed spectral properties typically observed in GRB hosts and showed that the progenitor of this event was located in a very bright H ii region of its face-on host galaxy, at a projected distance of ∼ 10 kpc from its galactic centre. The star-formation rate (SFRGRB ∼ 0.2 M⊙ yr−1) and metallicity (12 + log(O/H) ∼8.15 ± 0.10) of the GRB star-forming region are consistent with those of the host galaxies of previously studied GRB–SN systems