Search

Context.We recently reported the formation of metallic inclusions in an amorphous and/or crystalline silicate matrix by thermal annealing of thin films in reducing atmospheres. Experimentally, the obtained microstructures closely resemble those of the glass with embedded metal and sulphides (GEMS) found in chondritic porous interplanetary dust particles (CP IDPs). We present here the magnetic properties of these synthetic samples.

Aims.We present laboratory studies of the micro-structural evolution of an amorphous ferro-magnesian silicate, of olivine composition, following thermal annealing under vacuum.Methods.The amorphous silicate was prepared as a thin film on a diamond substrate. Annealing under vacuum was performed at temperatures ranging from 870 to 1020 K. After annealing the thin films were extracted from the substrate and analysed by transmission electron microscopy to infer their microstructural and compositional evolution.Results.Spheroidal metallic nano-particles (2-50 nm) are found within the silicate films, which are still amorphous after annealing at 870 K and partially crystallized into forsterite for annealing up to 1020 K. We interpret this microstructure in terms of a reduction of the initial amorphous silicate FeO component, because of the carbon-rich partial pressure in the furnace due to pumping mechanism. Annealing in a controlled oxygen-rich atmosphere confirms this interpretation. Conclusions.The observed microstructures closely resemble those of the GEMS (Glass with Embedded Metal and Sulphides) found in chondritic IDPs (Interplanetary Dust Particles). Since IDPs contain abundant carbonaceous matter, a solid-state reduction reaction may have occurred during heating in the hot inner regions of the proto-solar disc. Related to this, the presence of forsterite grains grown from the amorphous precursor material clearly demonstrates that condensation from gaseous species is not required to explain the occurrence of forsterite around young protostars and in comets. Forsterite grains in these environments can be formed directly in the solid phase by thermal annealing of amorphous ferro-magnesian silicates precursor under reducing conditions. Finally, locking iron as metallic particles within the silicates explains why astronomical silicates always appear observationally Mg-rich.

To study the evolution of silicate dust in different astrophysical environments we simulate, in the laboratory, interstellar and circumstellar ion irradiation and thermal annealing processes. An experimental protocol that follows different steps in the dust life-cycle was developed. Using the silicate 10 μm band as an indicator, the evolution of the structural properties of an ion-irradiated olivine-type silicate sample, as a function of temperature, is investigated and an activation energy for crystallization is determined. The obtained value of ${E_{\rm a}}/k$= 41 700 ±2400 K is in good agreement with previous determinations of the activation energies of crystallization reported for non-ion-irradiated, amorphous silicates. This implies that the crystallization process is independent of the history of the dust. In particular, the defect concentration due to irradiation appears not to play a major role in stimulating, or hindering, crystallization at a given temperature. This activation energy is an important thermodynamical parameter that must be used in theoretical models which aim to explain the dust evolution from its place of birth in late type stars to its incorporation into young stellar environments, proto-stellar discs and proto-planetary systems after long passage through the interstellar medium.

In this article we investigate the interaction of silicate grains with light atoms ionized and accelerated during the propagation of shock waves in the diffuse interstellar medium (ISM). Such an interaction which is equivalent to the irradiation of the grains with accelerated ions, is a potentially important process for silicate grain evolution in the ISM. We present the results of irradiation experiments aimed at simulating this process. The same crystalline silicates as those identified around evolved stars before their injection in the ISM, forsterite, enstatite and diopside, were irradiated with light ions (H+, He+) at low energies (10–50 keV). The IR spectroscopic characteristics of the samples were analyzed before and after the irradiation in the 2–35 μm range allowing us to study the structural modifications in the irradiated minerals. The experiments show that low energy H+(10 keV) and He+(≤50 keV) ions efficiently amorphize crystalline silicates with fluence ≤1018ions/cm2. Since these experimental conditions are compatible with interstellar environments, the interaction of grains with high velocity shock waves may be responsible for the absence of crystalline silicates in the ISM. The comparison of the IR spectra of the irradiated silicates with observations of the Galactic Center is presented. This comparison calls into question the classical assignment of the interstellar amorphous silicate bands.

We present the results of irradiation experiments on crystalline olivine with He+ions at energies of 4 and 10 keV and fluences varying from 5 1016to 1018ions/cm2. The aim of these experiments is to simulate ion implantation into interstellar grains in shocks in the ISM. Irradiated samples were analysed by transmission electron microscopy (TEM). The irradiation causes the amorphization of the olivine, at all He+fluences considered. The thickness of the amorphized region is $40 \pm 15$nm and $90 \pm 10$nm for the 4 keV and 10 keV experiments, respectively. The amorphization of the olivine occurs in conjunction with an increase in the porosity of the material due to the formation of bubbles. In addition, the amorphized layer is deficient in oxygen and magnesium. We find that the O/Si and Mg/Si ratios decrease as the He+fluence increases. These experiments show that the irradiation of dust in supernova shocks can efficiently alter the dust structure and composition. Our result are consistent with the lack of crystalline silicates in the interstellar medium and also with the compositional evolution observed from olivine-type silicates around evolved stars to pyroxene-type silicates around protostars.

Abstract—We discuss the relationship between large cosmic dust that represents the main source of extraterrestrial matter presently accreted by the Earth and samples from comet 81P/Wild 2 returned by the Stardust mission in January 2006. Prior examinations of the Stardust samples have shown that Wild 2 cometary dust particles contain a large diversity of components, formed at various heliocentric distances. These analyses suggest large‐scale radial mixing mechanism(s) in the early solar nebula and the existence of a continuum between primitive asteroidal and cometary matter. The recent collection of CONCORDIA Antarctic micrometeorites recovered from ultra‐clean snow close to Dome C provides the most unbiased collection of large cosmic dust available for analyses in the laboratory. Many similarities can be found between Antarctic micrometeorites and Wild 2 samples, in terms of chemical, mineralogical, and isotopic compositions, and in the structure and composition of their carbonaceous matter. Cosmic dust in the form of CONCORDIA Antarctic micrometeorites and primitive IDPs are preferred samples to study the asteroid‐comet continuum.

Aims. Olivine surfaces have been subjected to low-energy ion irradiation with H+, He+and Ar+at energies within the keV range in order to simulate the effects of energetic gas-grain interactions within shocked regions of the interstellar medium.Methods. The induced modifications in the chemical composition and the bonding configuration of the upper and the near surface regions were monitored in?situ by X-ray photoelectron spectroscopy (XPS). The associated morphological evolution of the samples was studied by atomic force microscopy (AFM).Results. Results show that the surface chemistry evolves during irradiation with a noticeable Mg?enrichment relative to Si. This evolution is interpreted as coming from magnesium atom diffusion driven by the electric field caused by the positive ion implantation. The iron valence state is also strongly affected by irradiation, with reduction occurring at relatively high fluxes. However, at low fluxes the iron is found to oxidise from Fe2+to Fe3+due to a charge transfer between the incident positive ions and the iron in the sample. The atomic force microscopy results show that the surface roughness tends to increase with irradiation and that this roughness influences the surface chemical reactivity of the grains, as shown by the enhanced formation of carbonate on the surfaces when they are exposed to a CO2?atmosphere. The implications for the evolution of interstellar dust include an enhanced dust catalytic activity. These effects would arise from modification under irradiation of the surface reactivity and an increase in the available grain surface area.

Abstract—Aluminum foils of the Stardust cometary dust collector are peppered with impact features of a wide range of sizes and shapes. By comparison to laboratory shots of known particle dimensions and density, using the same velocity and incidence geometry as the Stardust Wild 2 encounter, we can derive size and mass of the cometary dust grains. Using scanning electron microscopy (SEM) of foil samples (both flown on the mission and impacted in the laboratory) we have recognized a range of impact feature shapes from which we interpret particle density and internal structure. We have documented composition of crater residues, including stoichiometric material in 3 of 7 larger craters, by energy dispersive X‐ray microanalysis. Wild 2 dust grains include coarse (>10 μm) mafic silicate grains, some dominated by a single mineral species of density around 3–4 g cm−3(such as olivine). Other grains were porous, low‐density aggregates from a few nanometers to 100 μm, with an overall density that may be lower than 1 g cm−3, containing mixtures of silicates and sulfides and possibly both alkali‐rich and mafic glass. The mineral assemblage is very similar to the most common species reported from aerogel tracks. In one large aggregate crater, the combined diverse residue composition is similar to CI chondrites. The foils are a unique collecting substrate, revealing that the most abundant Wild 2 dust grains were of sub‐micrometer size and of complex internal structure. Impact residues in Stardust foil craters will be a valuable resource for future analyses of cometary dust.

Abstract: The nature of the amorphous regions and their recovery processes in two natural metamict zircon samples from Sri Lanka have been studied by high resolution and analytical transmission electron microscopy. Samples untreated and annealed at different temperatures were investigated. Nanoprobe analyses on untreated samples and samples annealed at 1000 K show that within experimental uncertainties, no chemical segregation occurred. In samples annealed at higher temperatures (≥1100 K) recovery occurs in a two-stage process and leads to different microstructures, which depended on the initial amount of metamictization. In highly amorphized samples, recrystallization starts at 1200 K. Randomly oriented ZrO2 grains embedded in a silica-rich matrix are detected. At higher temperature (16 h at 1600 K), the assemblage transforms into a polygonal texture of small zircon grains. Some untransformed zirconia grains and pockets of silica-rich glass are still present, however. In partially metamict samples, recovery starts at 1100 K. The small surviving oriented zircon domains grow at the expense of the surrounding amorphous material. At 1200 K, new zirconia grains nucleate with random orientations. After 1 h annealing at 1400 K, the zircon structure is restored and the microstructure coarse-grained. The proportion of crystalline zirconia and silica-rich glass has dramatically decreased.

The classes of strongly prime, uniformly strongly prime and superprime hemirings are defined and shown to be special classes. Intersection properties of the radicals generated by these classes are studied and related to α-ideas. Hemiring radicals and ring radicals of rings are shown to be identical for these three radical concepts.

The effects of nitrogen supply on the growth and nitrogen contents of four cultivars of Zea mays L. of different origins were examined under water-culture conditions at the seedling stage. Seedlings of cultivars CNIA12, LG11, Tusa Finn, and UNPHU XC301 were grown under three different relative addition rates of nitrogen. Growth parameters were determined by means of functional growth analysis conducted on 10 to 19 d-old seedlings. No differences in relative growth rates were found among cultivars when nitrogen supply was high. However, at the lowest rate of nitrogen supply, Tusa Fina and LG11 showed lower relative growth rates than CNIA12 and UNPHU XC301, where relative growth rates were sustained even at the lowest rate of nitrogen supply, due to a higher unit leaf rate. The higher unit leaf rate of these two cultivars corresponds directly to higher leaf and plant nitrogen contents. High positive correlations were found between plant nitrogen contents and both relative growth rate and unit leaf rate. The results suggest a potential for selection of genotypes with improved performance under conditions where high rates of nitrogen-fertilizer application are too costly or not desirable.Copyright 1994, 1999 Academic Press

Abstract—The Ilafegh 009 meteorite is an impact melt rock from an EL‐chondritic parent body. Its mineralogic assemblage is the result of rapid crystallization after shock‐induced melting. We report here an analytical transmission electron microscopy (ATEM) study of the major minerals of this meteorite (enstatite, plagioclase, Fe‐Ni metal and sulfides). Based on this study, we discuss the crystallization sequence and the further evolution of the rock in the solid state.

1. An attempt has been made to demonstrate the influence of the thymus upon the adrenal in guinea-pigs by daily injection of unphysiological amounts of a highly purified thymus extract.2. Such injections caused a marked but transient increase in both the vitamin C and cholesterol content of the adrenal.3. Morphologically, there was a decrease in the weight of the adrenal cortex relative to that of the whole gland, while the relative weight of the medulla increased correspondingly. This change was not transient, but persisted up to 110 days of treatment.4. It is concluded that the effect of injecting thymus extract is opposite to that of injecting ACTH and gives rise to a resting condition of the adrenal cortex.

Verpleegkundig specialisten hebben een rechtszaak gewonnen van het Universitair Medisch Centrum Groningen (UMCG) waarin ze hetzelfde loon eisten als de physician assistants.

Ohne Zusammenfassung:

Context.Cold dust grains are responsible for the far-infrared and submillimetre (FIR/submm) emission observed by Herschel and Planck. Their thermal emission is usually expressed as a modified black body law in which the FIR/submm dust opacity, or mass absorption coefficient (MAC), is described by the MAC at a given wavelength κλ0and the temperature- and wavelength-independent emissivity spectral index β. However, numerous data from previous space and balloon-borne missions and recently from Herschel and Planck show that the dust emission is not well understood, as revealed for example by the observed anti-correlation of βwith the grain temperature.

Context.Interstellar silicate grains are thought to be amorphized by interaction with high- and low-energy particle interactions in astrophysical environments. In addition, low energy (a few keV) particles will implant atoms within the grains.

The crystallinity of silicate dust detected in protoplanetary disks contrasts with the dominantly amorphous nature of dust in the interstellar medium. The amorphous-to-crystal transition is therefore a valuable probe to constrain physical properties of disks such as temperature gradients or the extent of radial mixing. However, it requires a comprehensive knowledge of the behaviour of amorphous Mg silicates during thermal processing. In this respect, amorphous analogues of enstatite composition (MgSiO3) were thermally annealed around the glass transition temperature (Tg~  1040 K). We show that enstatite is not produced below Tg. Instead, the annealing leads to a mineralogical assemblage dominated by forsterite (Mg2SiO4). A sharp change is observed at Tg, and pyroxene becomes the dominant mineral species. The annealing conditions marginally change the mineralogical assemblages produced above Tg. A notable exception is the recovery of protoenstatite (the high-temperature polymorph), which appears to be easily quenched in dust analogues. Detection of this phase would then provide an excellent probe for fast thermal events in disks. Our data suggest that below  ~1000 K, the mineralogy of silicate dust of solar composition should naturally be dominated by olivine while above this temperature pyroxenes should dominate. They also show that silica polymorphs recently detected in cold regions of disks are the natural by-products of the formation of forsterite out of enstatite precursor. The low-temperature crystallization pathway reported, combined with the evaporation/condensation process, captures the essential features of the zoned mineralogy of protoplanetary disks.