Porous dust grains in the shells of Herbig Ae/Be stars

The transfer of polarized radiation in inhomogeneous circumstellar shells with a spheroidal spatial distribution of porous dust particles is computed. The grains are modeled by an MRN mixture of silicate and graphite particles. The optical properties of porous particles (considered separately in the Appendix) are computed by using effective medium theory and Mie theory. The following observational characteristics have been computed for WW Vul, a typical Herbig Ae star with Algol-like minima: the spectral energy distribution from the ultraviolet to the far infrared, the color-magnitude diagrams, the wavelength dependence of linear polarization, and the shell brightness distribution. The effect of grain porosity on the results is considered. It has been found that only moderate particle porosity (the volume fraction of matter is f ～0.5) can explain available observational data in terms of the approach used. Since radiation pressure must rapidly sweep submicron-sized grains out of the vicinity of Herbig Ae/Be stars, we briefly discuss how particle porosity can affect this process.     

Determination of dust properties and magnetic fields from polarimetric and photometric observations of stars (Review)

Verschiedene Modelle zur Darstellung der interstellaren Extinktion und Polarisation werden betrachtet. Dabei wird den Ausrichtungsmechanismen, wie dem Davies-Greenstein-Mechanismus und der Ausrichtung durch suprathermische Rotation, und ihren Näherungen besondere Aufmerksamkeit zu teil. Es wird geschlußfolgert, daß die Größe der Staubteilchen sowie die Starke und Richtung des interstellaren Magnetfeldes aus den Extinktions- und Polarisationsdaten abgeschätzt werden können. Die Anwendung der theoretischen Ergebnisse auf Sterne in der galaktischen Ebene wird erörtert.     

Investigation of the linear polarization in infrared absorption bands

We consider the effects of the grain size, shape, structure, and chemical composition as well as the angle between the grain rotation axis and the incident ray on the full widths at half maximum (FWHM) of the polarization bands in the two deepest infrared absorption bands observed in the spectra of protostars, the water-ice band centered at 3.1 μm and the silicate band centered at 9.7 μm, using a core—mantle confocal spheroid model with various axial ratios a/b and relative volumes of the core material. We have found that the observed polarization bands with FWHM_p < 0.3 μm in the water-ice absorption band can be explained only by oblate and prolate particles with r _v ≤ 0.35 μm and the polarization bands with FWHM_p ≈ 0.3 μm can be explained only by particles with r _v ≈ 0.35 μm. Broad silicate absorption bands (FWHM ≈ 3 μm) with broad polarization bands (FWHM_p ≈ 2.7 μm) can be explained by particles with r _v ≈ 0.35 μm. Narrow silicate absorption bands (FWHM ≤ 3 μm) with any FWHM of the polarization bands can be explained by a mixture of particles of two types of olivine. Narrow polarization bands (FWHM_p ≈ 2 μm) with broad absorption bands can be explained only by very small particles, r _v ≤ 0.1 μm. We have found the relationships between the effective polarization and extinction cross sections and estimated the ranges of observed polarizabilities that can be explained by particles of given shape and orientation in each of the bands independently. Independent studies of the observational data for each of the bands are shown to give a wider choice of particle model parameters.     

The distribution of interstellar dust in nine regions near the galactic plane

In dem vorliegenden Artikel werden die interstellare Extinktion und die Verteilung des interstellaren Staubes bis zu einer Entfernung von 8–10 kpc untersucht. Die Messungen erfolgten in der galaktischen Länge von 7° bis 222° in Richtung der offenen Sternhaufen NGC 6531, NGC 6866, Tr 35, NGC 7062, NGC 7654, IC 1805, NGC 1664, NGC 2251 und NGC 2323.     

The temperature of nonspherical circumstellar dust grains

The temperatures of prolate and oblate spheroidal dust grains in the envelopes of stars of various spectral types are calculated. Homogeneous particles with aspect ratios a/b≤10 composed of amorphous carbon, iron, dirty ice, various silicates, and other materials are considered. The temperatures of spherical and spheroidal particles were found to vary similarly with particle size, distance to the star, and stellar temperature. The temperature ratio T _d(spheroid)/T _d(sphere) depends most strongly on the grain chemical composition and shape. Spheroidal grains are generally colder than spherical particles of the same volume; only iron spheroids can be slightly hotter than iron spheres. At a/b≈2, the temperature differences do not exceed 10%. If a/b≥4, the temperatures can differ by 30–40%. For a fixed dust mass in the medium, the fluxes at wavelengths λ≥100 are higher if the grains are nonspherical, which gives overestimated dust masses from millimeter observations. The effect of grain shape should also be taken into account when modeling Galactic-dust emission properties, which are calculated when searching for fluctuations of the cosmic microwave background radiation in its Wien wing.     

Circular polarization by scattering from spheroidal dust grains

Large degrees of circular polarization at near-infrared wavelengths have been reported in the OMC1 star-forming region. This discovery, in combination with compelling evidence for the existence of non-spherical aligned grains in star formation regions, has prompted us to investigate scattering from spheroidal particles as a possible mechanism for the production of large circular polarization in reflection nebulae. We use a dipole calculation to model the small particle limit and a T -matrix code to treat arbitrarily sized particles. We find that size distributions of perfectly aligned spheroids, with only modest 2:1 axis ratios, are capable of producing circular polarization of up to 50 per cent when scattering unpolarized incident light. This is the case even for dielectric materials, such as 'astronomical silicate', as long as sufficient large particles are included in the size distribution. We consider the effects of particle alignment and find that spinning oblate spheroids should be much more efficient circular polarizers than equivalent prolate spheroids.     

Pyroxene glasses — candidates for the interstellar silicate dust component

Experimentelle Arbeiten, die von der Universitäts-Sternwarte Jena in Zusammenarbeit mit chemischen Institutionen durchgeführt wurden, haben gezeigt, daß Pyroxengläser vielversprechende Kandidaten zur Identifizierung von Silikatstaubteilchen in der Umgebung sehr junger Objekte in Molekülwolken, z. B. BN-Objekten und T-Tauri-Sternen, sind. Die Wichtigkeit dieser Experimente besteht darin, daß sie die zur Modellierung der Quellen benötigren optischen Daten liefern und daß sie es ermöglichen, die mineralogische Beschaffenheit des Teilchenmaterials zu verstehen. In dieser Arbeit werden die Schritle, die in der jüngsten Zeit von der Jenaer Staubarbeitsgruppe in beide Richtungen unternommen wurden, beschrieben.     

Properties of circumstellar silicate dust (Review)

Es wird eine Übersicht über den gegenwärtigen Stand des Wissens über die Eigenschaften des zirkumstellaren Silikatstaubes gegeben. Nach der Darstellung der Entdeckungsgeschichte werden die Objekte zusammengestellt, in deren Spektren Silikatbanden beobachtet werden. Der Zusam-menhang zwischen den optischen Eigenschaften der Silikatteilchen und der Struktur der zirkumstellaren Hüllen bei der Bestimmung der ausgesandten Infrarotstrahlung wird analysiert, und alle wichtigeren Untersuchungen von Staubhüllen mit Silikatteilchen werden zusammengetragen. Wir diskutieren die optischen Eigenschaften des zirkumstellaren Silikatstaubes, wobei die amorphe Struktur besonders betont wird. Abschließend wird auf die Bedeutung von Feinstrukturen innerhalb der 10-μm-Bande, die bei verschiedenen Objekten beobachtet wurden, eingegangen.     

Identification and physical retrieval of dust storm using three MODIS thermal IR channels

In this paper, the dust event on 7 April 2001 in northern China is investigated with three MODIS thermal infrared (IR) bands. It is found that for the dust cloud, the observed 11 μm minus 12 μm brightness temperature difference (BTD) is always negative, while the BTD of 8.5 μm minus 11 μm varies from positive to negative depending on the dust concentration. Based on these distinguishing properties, we develop a dust mask algorithm to identify the dust storm occurrence and spatial extent. The algorithm can be used successfully in both the daytime and nighttime. Using the Mie spherical scattering theory, the thermal radiation transfer through the single dust layer is performed with the widely used forward model DISTORT. Our calculations show that the dust-like aerosols can well explain the observed BTD although both of the complex refractive index and particle size of aerosols will significantly influence the BTD. When the complex refractive index is fixed (dust-like aerosols in this paper), then the dust optical thickness and effective radii of dust particles can be retrieved from the brightness temperature (BT) of the 11 μm channel and the BTD of 11 μm minus 12 μm channels, respectively. The integral dust column density can also be derived from the retrieved dust optical thickness and effective radius.     

The analysis of four cosmic dust particles

A set of four stratospheric particles was reanalysed. These particles registered in the NASA Cosmic Dust Catalogs (CDCs) were classified in the Houston Johnson Space Center (JSC) as the particles of cosmic origin (C). Present energy-dispersive X-ray (EDX) spectra confirmed previous classification and revealed additional features in the chemical composition of the cosmic dust particles.The particles are extremely fine-grained aggregates with bulk chondritic composition. Somewhat higher content of oxygen may indicate a presence of hydroxide containing minerals in their phase composition.     

星际尘埃研究现状与进展

由于星际尘埃的广泛存在和其在恒星与行星系统的形成、星系以及整个宇宙演化中的重要作用,星际尘埃的研究成为当今天体物理领域的热点前沿课题。该文从尘埃与电磁场相互作用的观测证据出发,系统地介绍了星际消光(包括吸收和散射)、星际红外辐射、星际偏振等的研究现状,讨论了星际元素减损,以及行星际尘埃和陨石中的前太阳尘埃等问题。从相应的观测证据中,可以得到关于星际尘埃的丰度、化学组成、尺寸和形状的信息。该文还对当前比较流行的三种尘埃模型(硅酸盐-石墨-PAHs模型、硅酸盐核-碳有机耐熔质壳层模型和多孔尘埃模型)进行了讨论与比较,对该研究领域中待解决的问题也作了简要的概括。     

Nearby galaxies. III. Gas–to–dust ratio in the interstellar medium of spiral and dwarf irregular galaxies

The gas-to-dust ratio in the interstellar medium of nearby spiral systems and dwarf irregular galaxies has been obtained by using mainly the compiled data by Schmidt and Boller (1992a). The gas-to-dust ratio of dwarf irregulars is larger by about one order of magnitude than for spiral galaxies, on the average. A relation between the gas-to-dust ratio and the metallicity, represented by the abundance of oxygen, has been detected.     

Probing the surfaces of interstellar dust grains: the adsorption of CO at bare grain surfaces

A solid-state feature was detected at around 2175 cm⁻¹ towards 30 embedded young stellar objects in spectra obtained using the Infrared Spectrometer and Array Camera at the European Southern Observatory Very Large Telescope. We present results from laboratory studies of CO adsorbed at the surface of zeolite wafers, where absorption bands were detected at 2177 and 2168 cm⁻¹ (corresponding to CO chemisorbed at the zeolite surface) and 2130 cm⁻¹ (corresponding to CO physisorbed at the zeolite surface), providing an excellent match to the observational data. We propose that the main carrier of the 2175-band is CO chemisorbed at bare surfaces of dust grains in the interstellar medium. This result provides the first direct evidence that gas–surface interactions do not have to result in the formation of ice mantles on interstellar dust. The strength of the 2175-band is estimated to be  ∼4 × 10⁻¹⁹ cm  molecule⁻¹. The abundance of CO adsorbed at bare grain surfaces ranges from 0.06 to 0.16 relative to H₂O ice, which is, at most, half of the abundance (relative to H₂O ice) of CO residing in H₂O-dominated ice environments. These findings imply that interstellar grains have a large (catalytically active) surface area, providing a refuge for interstellar species. Consequently, the potential exists for heterogeneous chemistry to occur involving CO molecules in unique surface chemistry pathways not currently considered in gas grain models of the interstellar medium.     

The influence of the brightness of the asteroidal dust bands on the gegenschein

The position and shape of the Gegenschein’s maximum brightness provide information on the structure of the interplanetary dust cloud. We show that the asteroidal dust bands, extended near the anti-solar point, play an important role in determining both the position of the maximum brightness and the shape of the Gegenschein. After removing the asteroidal dust bands from an observation of the Gegenschein on November 2, 1997, it was found that the maximum brightness point shifted −0.4° in ecliptic latitude, i.e., to the south of the ecliptic plane, at an ecliptic longitude of 180°, in contrast to a latitude value of +0.1° when the dust bands were included. Furthermore, the part of the Gegenschein to the south of the ecliptic plane was brighter than the northern part at the time of observation. Referring to the cloud model of T. Kelsall et al. (1998, Astrophy. J. 508, 44-73), it can be estimated that the ascending node of the symmetry plane of the dust cloud is 57°^−3°_+7° when its inclination is 2.03° ? 0.50°.     

Diffuse component of the cosmic far UV radiation and interstellar dust grains

The diffuse far UV radiation ( 1350–1480 Å) observed in the sky region ofl ^II180°, 0°b ^II40° is analyzed in connection with the distributions of stars and dust grains as well as with optical properties of grains. Its intensity (starlight+scattered light) is about 6×10^–7 erg cm^–2 sec^–1 sr^–1 Å^–1 in the direction ofb ^II0° andl ^II180°. The latitude dependence of the intensity is in approximate agreement with the plane parallel slab model of the galaxy with a reasonable set of parameters. The interstellar scattering gives an albedo close to unity and forward phase function of about 0.6, which are not inconsistent with the model of interstellar grains of Wickramasinghe. The upper limit of the extragalactic UV is 2×10^–8 erg cm^–2 sec^–1 sr^–1 Å^–1 in the same region of wave-length.     

The expulsion and retention of dust grains by galactic discs

We have constructed a numerical model of a galaxy that consists of a stellar, gas and dust disc imbedded within a dark halo. We have used this model to assess the radiation, gravitational and viscous forces on dust grains and to trace their motion through the interstellar medium over a period of 10⁹ yr. We conclude that the disc opacity is a crucial factor in understanding the motion of the grains. Large grains (≈0.1 μm) with low disc opacity will lead to dust expulsion from the stellar disc, while high opacity leads to dust retention. Reasonable disc opacities lead to the recycling of the larger grains from the outer to the inner regions of the galaxy. The larger grains travel at higher velocities than small grains (0.01−0.001 μm), and so the smaller grains remain relatively close to their formation sites. Dust can 'leak' out over the entire surface of the disc because of the imbalance of radiation and gravitational forces. The dust is dynamically coupled to the gas and so although the gas lags behind the dust it is carried along with it. This explains the close correlation between the far-infrared emission from dust and the gas column density. We use a simple analytical model to show how the dust mass of a galaxy may evolve with time and how a significant fraction (90 per cent) of the total dust mass produced may have been expelled into the intergalactic medium.     

On the problem of H2 formation

In the first part of the paper the framework for theoretical studies of the H₂ formation on interstellar grains will be shortly considered. In the second part the chemisorption of a hydrogen atom at a carbon grain will be examined and results of an ab initio perturbational approach are presented.     

Decreased values of cosmic dust number density estimates in the Solar System

Experiments to investigate the effect of impacts on side-walls of dust detectors such as the present NASA/ESA Galileo/Ulysses instrument are reported. Side walls constitute 27% of the internal area of these instruments, and increase field of view from 140° to 180°. Impact of cosmic dust particles onto Galileo/Ulysses Al side walls was simulated by firing Fe particles, 0.5-5 μm diameter, 2-50 km s⁻¹, onto an Al plate, simulating the targets of Galileo and Ulysses dust instruments. Since side wall impacts affect the rise time of the target ionization signal, the degree to which particle fluxes are overestimated varies with velocity. Side-wall impacts at particle velocities of 2-20 km s⁻¹ yield rise times 10-30% longer than for direct impacts, so that derived impact velocity is reduced by a factor of ∼2. Impacts on side wall at 20-50 km s⁻¹ reduced rise times by a factor of ∼10 relative to direct impact data. This would result in serious overestimates of flux of particles intersecting the dust instrument at velocities of 20-50 km s⁻¹. Taking into account differences in laboratory calibration geometry we obtain the following percentages for previous overestimates of incident particle number density values from the Galileo instrument [Grün et al., 1992. The Galileo dust detector. Space Sci. Rev. 60, 317-340]: 55% for 2 km s⁻¹ impacts, 27% at 10 km s⁻¹ and 400% at 70 km s⁻¹. We predict that individual particle masses are overestimated by ∼10-90% when side-wall impacts occur at 2-20 km s⁻¹, and underestimated by ∼10-¹⁰² at 20-50 km s⁻¹. We predict that wall impacts at 20-50 km s⁻¹ can be identified in Galileo instrument data on account of their unusually short target rise times. The side-wall calibration is used to obtain new revised values [Krüger et al., 2000. A dust cloud of Ganymede maintained by hypervelocity impacts of interplanetary micrometeoroids. Planet. Space Sci. 48, 1457-1471; 2003. Impact-generated dust clouds surrounding the Galilean moons. Icarus 164, 170-187] of the Galilean satellite dust number densities of 9.4×¹⁰⁻⁵, 9.9×¹⁰⁻⁵, 4.1×¹⁰⁻⁵, and 6.8×¹⁰⁻⁵ m⁻³ at 1 satellite radius from Io, Europa, Ganymede, and Callisto, respectively. Additionally, interplanetary particle number densities detected by the Galileo mission are found to be 1.6×¹⁰⁻⁴, 7.9×¹⁰⁻⁴, 3.2×¹⁰⁻⁵, 3.2×¹⁰⁻⁵, and 7.9×¹⁰⁻⁴ m⁻³ at heliocentric distances of 0.7, 1, 2, 3, and 5 AU, respectively. Work by Burchell et al. [1999b. Acceleration of conducting polymer-coated latex particles as projectiles in hypervelocity impact experiments. J. Phys. D: Appl. Phys. 32, 1719-1728] suggests that low-density “fluffy” particles encountered by Ulysses will not significantly affect our results—further calibration would be useful to confirm this.