ALBERT

Description: The effects of carbonate concentration and the presence of iron hydroxide phases on the process of arsenic release from an ore material were investigated under experimental oxic conditions and in the pH range from 6.0 to 9.0. These experimental conditions are pertinent to arsenic leaching from tailings and mining wastes. The leaching tests lasted for 〈= 99 days and were performed with materials of five different particle sizes (〈= 2 mm). Carbonate ions were produced in-situ by dolomite dissolution or were contained in used waters (0 to 30 mM as HCO(3)(-)) Iron hydroxide phases were formed in situ by oxidative dissolution of metallic iron (Fe(0)) or pyrite (FeS(2)). Non-disturbed batch experiments and air-homogenized experiments were conducted with a constant amount (10 g/L) of an arsenic-bearing rock (ore material) of a given particle size and different types of water (deionized, tap and mineral water). For comparison, experiments were conducted with 0.1 M EDTA, 0.1 M Na(2)CO(3), and 0.1 M H(2)SO(4). Neither the use of dolomite nor the use of water containing various carbonate (HCO(3)(-)) concentrations could confirm the recent results on the favorable role of As(III)-carbonate complexes on the arsenic transport in the environment. On the other hand, iron hydroxide phases (from Fe(0) and FeS(2)) univocally delayed the As release in both experimental procedures. Furthermore, the theoretically expected effects of the particle size of the ore material was observed. If one takes into consideration that the used HCO(3)(-) concentrations were up to six times larger then those of natural surface waters (〈= 5.5 mM) but up to five times lower than those currently used in the literature (〉= 100 mM), it is concluded that the reported conflicting results for As leaching from sediments may be a misinterpretation of processes occurring in the sediment and yielding increased As release with increasing HCO(3)(-)/CO(3)(2-) concentration.

Description: Snow in the environment acts as a host to rich chemistry and provides a matrix for physical exchange of contaminants within the ecosystem. The goal of this review is to summarise the current state of knowledge of physical processes and chemical reactivity in surface snow with relevance to polar regions. It focuses on a description of impurities in distinct compartments present in surface snow, such as snow crystals, grain boundaries, crystal surfaces, and liquid parts. It emphasises the microscopic description of the ice surface and its link with the environment. Distinct differences between the disordered air–ice interface, often termed quasi-liquid layer, and a liquid phase are highlighted. The reactivity in these different compartments of surface snow is discussed using many experimental studies, simulations, and selected snow models from the molecular to the macro-scale. Although new experimental techniques have extended our knowledge of the surface properties of ice and their impact on some single reactions and processes, others occurring on, at or within snow grains remain unquantified. The presence of liquid or liquid-like compartments either due to the formation of brine or disorder at surfaces of snow crystals below the freezing point may strongly modify reaction rates. Therefore, future experiments should include a detailed characterisation of the surface properties of the ice matrices. A further point that remains largely unresolved is the distribution of impurities between the different domains of the condensed phase inside the snowpack, i.e. in the bulk solid, in liquid at the surface or trapped in confined pockets within or between grains, or at the surface. While surface-sensitive laboratory techniques may in the future help to resolve this point for equilibrium conditions, additional uncertainty for the environmental snowpack may be caused by the highly dynamic nature of the snowpack due to the fast metamorphism occurring under certain environmental conditions. Due to these gaps in knowledge the first snow chemistry models have attempted to reproduce certain processes like the long-term incorporation of volatile compounds in snow and firn or the release of reactive species from the snowpack. Although so far none of the models offers a coupled approach of physical and chemical processes or a detailed representation of the different compartments, they have successfully been used to reproduce some field experiments. A fully coupled snow chemistry and physics model remains to be developed.

Description: This report deals with the theory of the vortex street which enables the reproduction of the mechanism of the form resistance with suitable approximation under stated conditions, although such a resistance is precluded in a fluid which is perfectly inviscid. Disregarding for the present the origination of the vortex, the stream attitude in the wake of the body may be described approximately correct by the representation of individual vortices, without transgressing the law governing the motion of such vortices in an ideal fluid. Another striking example is the theory of the induced drag of wings, which likewise shows the extent of applying the vortex equations without overstepping the bounds of the dynamics of ideal fluids.

Description: This paper presents a mathematical discussion of the laminar boundary layer, which was developed with a view of facilitating the investigation of those boundary layers in particular for which the phenomenon of separation occurs. The treatment starts with a slight modification of the form of the boundary layer equation first published by Von Mises. Two approximate solutions of this equation are found, one of which is exact at the outer edge of the boundary layer while the other is exact at the wall. The final solution is obtained by joining these two solutions at the inflection points of the velocity profiles. The final solution is given in terms of a series of universal functions for a fairly broad class of potential velocity distributions outside of the boundary layer. Detailed calculations of the boundary layer characteristics are worked out for the case in which the potential velocity is a linear function of the distance from the upstream stagnation point. Finally, the complete separation point characteristics are determined for the boundary layer associated with a potential velocity distribution made up of two linear functions of the distance from the stagnation point. It appears that extensions of the detailed calculations to more complex potential flows can be fairly easily carried out by using the explicit formulae given in the paper. (author)

Description: Report deals, first with the theory of the laminar friction flow, where the basic concepts of Prandtl's boundary layer theory are represented from mathematical and physical points of view, and a method is indicated by means of which even more complicated cases can be treated with simple mathematical means, at least approximately. An attempt is also made to secure a basis for the computation of the turbulent friction by means of formulas through which the empirical laws of the turbulent pipe resistance can be applied to other problems on friction drag. (author)

Description: Context. Debris disks are the intrinsic by-products of the star and planet formation processes. Most likely due to instrumental limitations and their natural faintness, little is known about debris disks around low mass stars, especially when it comes to spatially resolved observations. Aims. We present new VLT/SPHERE IRDIS dual-polarization imaging (DPI) observations in which we detect the dust ring around the M2 spectral type star TWA 7. Combined with additional angular differential imaging observations we aim at a fine characterization of the debris disk and setting constraints on the presence of low-mass planets. Methods. We modeled the SPHERE DPI observations and constrain the location of the small dust grains, as well as the spectral energy distribution of the debris disk, using the results inferred from the observations, and performed simple N-body simulations. Results. We find that the dust density distribution peaks at ~0.72 (25 au), with a very shallow outer power-law slope, and that the disk has an inclination of ~13 with a position angle of ~91 east of north. We also report low signal-to-noise ratio detections of an outer belt at a distance of ~1.5 (~52 au) from the star, of a spiral arm in the southern side of the star, and of a possible dusty clump at 0.11. These findings seem to persist over timescales of at least a year. Using the intensity images, we do not detect any planets in the close vicinity of the star, but the sensitivity reaches Jovian planet mass upper limits. We find that the SED is best reproduced with an inner disk at ~0.2 (~7 au) and another belt at 0.72 (25 au). Conclusions. We report the detections of several unexpected features in the disk around TWA 7. A yet undetected 100 Solar Mass planet with a semi-major axis at 2030 au could possibly explain the outer belt as well as the spiral arm. We conclude that stellar winds are unlikely to be responsible for the spiral arm.

Description: We report the first extragalactic detection of the complex organic molecules (COMs) dimethyl ether (CH3OCH3) and methyl formate (CH3OCHO) with the Atacama Large Millimeter/submillimeter Array (ALMA). These COMs, together with their parent species methanol (CH3OH), were detected toward two 1.3 mm continuum sources in the N 113 star-forming region in the low-metallicity Large Magellanic Cloud (LMC). Rotational temperatures (Trot approx. 130 K) and total column densities (Nrot 10 approx. 16 cm2) have been calculated for each source based on multiple transitions of CH3OH. We present the ALMA molecular emission maps for COMs and measured abundances for all detected species. The physical and chemical properties of two sources with COMs detection, and the association with H2O and OH maser emission, indicate that they are hot cores. The fractional abundances of COMs scaled by a factor of 2.5 to account for the lower metallicity in the LMC are comparable to those found at the lower end of the range in Galactic hot cores. Our results have important implications for studies of organic chemistry at higher redshift.

Description: There has been under development for the high-speed wind tunnel of the LFA an optical measuring arrangement for the qualitative and quantitative investigation of flow. By the use of interference measurements, the determination of density at the surface of the bodies being tested in the air stream and in the vicinity of these bodies can be undertaken. The results obtained so far in the simple preliminary investigations show that it is possible, even at a low Reynolds number, to obtain the density field in the neighborhood of a test body by optical means. Simple analytical expressions give the relation between density, pressure, velocity, and temperature. In addition to this, the interference measurement furnishes valuable data on the state of the boundary layer, that is, the sort of boundary layer (whether laminar or turbulent), as well as the temperature and velocity distribution.

Description: We present an intensive monitoring of high-resolution spectra of the Ca II K line in the A7IV shell star Phi Leonis at very short (minutes, hours), short (night to night), and medium (weeks, months) timescales. The spectra show remarkable variable absorptions on timescales of hours, days, and months. The characteristics of these sporadic events are very similar to most that are observed toward the debris disk host star Beta Pictoris, which are commonly interpreted as signs of the evaporation of solid, comet-like bodies grazing or falling onto the star. Therefore, our results suggest the presence of solid bodies around Phi Leonis. To our knowledge, with the exception of Beta Pictoris, our monitoring has the best time resolution at the mentioned timescales for a star with events attributed to exocomets. Assuming the cometary scenario and considering the timescales of our monitoring, our results indicate that Phi Leonis presents the richest environment with comet-like events known to date, second only to Beta Pictoris.

Description: In this paper, we investigate the opportunities provided by the James Webb Space Telescope (JWST) for significant scientific advances in the study of Solar System bodies and rings using stellar occultations. The strengths and weaknesses of the stellar occultation technique are evaluated in light of JWST's unique capabilities. We identify several possible JWST occultation events by minor bodies and rings and evaluate their potential scientific value. These predictions depend critically on accurate a priori knowledge of the orbit of JWST near the SunEarth Lagrange point 2 (L2). We also explore the possibility of serendipitous stellar occultations by very small minor bodies as a byproduct of other JWST observing programs. Finally, to optimize the potential scientific return of stellar occultation observations, we identify several characteristics of JWST's orbit and instrumentation that should be taken into account during JWST's development.