From interstellar abundances to grain composition: the major dust constituents Mg, Si and Fe

We analyse observational correlations for three elements entering into the composition of interstellar silicate and oxide grains. Using current solar abundances (Asplund et al. 2009), we convert the gas-phase abundances into dust-phase abundances for 196 sightlines. We deduce a sharp difference in abundances for sightlines located at low (|b|<30\degr) and high (|b|>30\degr) galactic latitudes. For high-latitude stars the ratios Mg/Si and Fe/Si in dust are close to 1.5. For disk stars they are reduced to ${\rm Mg/Si} \sim 1.2$ and ${\rm Fe/Si} \sim 1.05$. The derived numbers indicate that 1) the dust grains cannot be the mixture of silicates with olivine and pyroxene composition only and some amount of magnesium or iron (or both) should be in another population and 2) the destruction of Mg-rich grains in the warm medium is more effective than of Fe-rich grains. We reveal a decrease of dust-phase abundances and correspondingly an increase of gas-phase abundances with distance $D$ for stars with D\ga 400\,pc. We attribute this fact to an observational selection effect: a systematic trend toward smaller observed hydrogen column density for distant stars. We find differences in abundances for disk stars with low (E({\rm B-V}) \la 0.2) and high (E({\rm B-V}) \ga 0.2) reddenings which reflect the distinction between the sightlines passing through diffuse and translucent interstellar clouds. For Scorpius-Ophiuchus we detect an uniform increase of dust-phase abundances of Mg and Si with an increase of the ratio of total to selective extinction $R_{\rm V}$ and a decrease of the strength of the far-UV extinction. This is the first evidence for a growth of Mg-Si grains due to accretion in the interstellar medium.Comment: 16 pages, 16 figures, accepted for publication in Astronomy and Astrophysic

Asymptotic boundary layer method for unstable trajectories : Semiclassical expansions for individual scar wavefunctions.

We extend the asymptotic boundary layer (ABL) method, originally developed for stable resonator modes, to the description of individual wave functions localized around unstable periodic orbits. The formalism applies to the description of scar states in fully or partially chaotic quantum systems, and also allows for the presence of smooth and sharp potentials, as well as magnetic fields. We argue that the separatrix wave function provides the largest contribution to the scars on a single wave function. This agrees with earlier results on the wave-function asymptotics and on the quantization condition of the scar states. Predictions of the ABL formalism are compared with the exact numerical solution for a strip resonator with a parabolic confinement potential and a magnetic field

Counting statistics for mesoscopic conductors with internal degrees of freedom

We consider the transport of electrons passing through a mesoscopic device possessing internal dynamical quantum degrees of freedom. The mutual interaction between the system and the conduction electrons contributes to the current fluctuations, which we describe in terms of full counting statistics. We identify conditions where this discriminates coherent from incoherent internal dynamics, and also identify and illustrate conditions under which the device acts to dynamically bunch transmitted or reflected electrons, thereby generating super-Poissonian noise.Comment: 4 pages, 2 figure

Effective medium theories for irregular fluffy structures: aggregation of small particles

We study the extinction efficiencies as well as scattering properties of particles of different porosity. Calculations are performed for porous pseudospheres with small size (Rayleigh) inclusions using the discrete dipole approximation. Five refractive indices of materials covering the range from $1.20+0.00i$ to $1.75+0.58i$ were selected. They correspond to biological particles, dirty ice, silicate, amorphous carbon and soot in the visual part of spectrum. We attempt to describe the optical properties of such particles using Lorenz-Mie theory and a refractive index found from some effective medium theory (EMT) assuming the particle is homogeneous. We refer to this as the effective model. It is found that the deviations are minimal when utilizing the EMT based on the Bruggeman mixing rule. Usually the deviations in extinction factor do not exceed $\sim 5$ for particle porosity ${\cal P}=0 - 0.9$ and size parameters x_{\rm porous} = 2 \pi r_{\rm s, porous}/\lambda \la 25. The deviations are larger for scattering and absorption efficiencies and smaller for particle albedo and asymmetry parameter. Our calculations made for spheroids confirm these conclusions. Preliminary consideration shows that the effective model represents the intensity and polarization of radiation scattered by fluffy aggregates quite well. Thus, the effective models of spherical and non-spherical particles can be used to significantly simplify computations of the optical properties of aggregates containing only Rayleigh inclusions.Comment: 24 pages, 9 figures, accepted for publication in Applied Optic

Evolution of postglacial vegetation in the Western Laptev Sea region (Siberian Arctic)

On the basis of a detailed study of the pollen-spore spectra and a detailed radiocarbon chronology of a sediment core obtained from the western outer Laptev Sea shelf, the long-term and high-resolution changes of vegetation in the northwestern Laptev Sea region were reconstructed for the last 12.0 cal. ka. Three major phases in the development of paleoenvironments and vegetation on the surrounding hinterland and the exposed Laptev Sea shelf were recognized. The period between 12.0 and 10.3 cal. ka BP was characterized by predominance of grass-sedge and moss tundra. Rapid expansion of herbaceous tundra with dwarf birch and alder started at about 10.3 and lasted until 8.0 cal. ka. Pollen spectra from this time interval evidence the warmest and most favorable climate conditions. After 8.0 cal. ka mosses and lichen vegetation with scare herbs typical for the modern arctic tundra dominated. German: Auf der Grundlage detaillierter Pollen- und Sporenspektren aus einem 14C-datierten Sedimentkern vom äußeren Schelf des westlichen Laptewmeeres wurden die langfristigen und hochaufgelösten Veränderungen der Vegetation in den letzten 12 cal. ka in der nordwestlichen Laptewmeer-Region rekonstruiert. Es wurden drei Hauptphasen der Entwicklung von Umwelt und Vegetation im umgebenden Hinterland erkannt. Die Zeit zwischen 12,0 und 10,3 cal. ka BP war charakterisiert durch Riedgras- und Moos-Tundra. Rasche Ausdehnung der Kraut-Tundra mit Zwergbirke und Erle begann etwa um 10,3 cal. ka und dauerte bis 8,0 cal. ka. Pollenspektren aus diesem Zeitintervall beschreiben die wärmsten und besten Klimabedingungen. Nach 8,0 cal. ka dominierte die für die heutige arktische Tundra typische Moos- und Flechtenvegetation mit wenigen Kräutern

Dust extinction and absorption: the challenge of porous grains

In many models of dusty objects in space the grains are assumed to be composite or fluffy. However, the computation of the optical properties of such particles is still a very difficult problem. We analyze how the increase of grain porosity influences basic features of cosmic dust -- interstellar extinction, dust temperature, infrared bands and millimeter opacity. Porous grains can reproduce the flat extinction across the 3 - 8 \mkm wavelength range measured for several lines of sight by {\it ISO} and {\it Spitzer}. Porous grains are generally cooler than compact grains. At the same time, the temperature of very porous grains becomes slightly larger in the case of the EMT-Mie calculations in comparison with the results found from the layered-sphere model. The layered-sphere model predicts a broadening of infrared bands and a shift of the peak position to larger wavelengths as porosity grows. In the case of the EMT-Mie model variations of the feature profile are less significant. It is also shown that the millimeter mass absorption coefficients grow as porosity increases with a faster growth occurring for particles with Rayleigh/non-Rayleigh inclusions. As a result, for very porous particles the coefficients given by two models can differ by a factor of about 3. It is found that an increase of porosity leads to an increase of extinction cross sections at some wavelengths and a decrease at others depending on the grain model. However, this behaviour is sufficient to reproduce the extinction curve in the direction of the star $\sigma$ Sco using current solar abundances. In the case of the star $\zeta$ Oph our model requires larger amounts of carbon and iron in the dust-phase than is available.Comment: Astronomy and Astrophysics (accepted; 11 pages, 11 figures

Bose-Einstein correlations in thermal field theory

Two-particle correlation functions are calculated for bosons emitted from a localized thermal source (the ``glow'' of a ``hot spot''). In contrast to existing work, non-equilibrium effects up to first order in gradients of the particle distribution function are taken into account. The spectral width of the bosons is shown to be an important quantity: If it is too small, they do not equilibrate locally and therefore strongly increase the measured correlation radius. In memoriam of Eugene Wigner and Hiroomi Umezawa.Comment: Paper in LaTeX. Figures and complete paper available via anonymous ftp, ftp://tpri6c.gsi.de/pub/phenning/hhbr9