Optical properties of dust grains

In this paper we present new and exact analytical and computational developments of Güttler's formulae for composite grains, thereafter applied for the two models:

On the size distribution and physical properties of interplanetary dust grains

This paper synthesizes information on the size distribution and physical properties of interplanetary dust grains obtained from analyses of lunar microcraters performed until 1979. The different aspects of these analyses (counting methods, simulation, calibrations) are summarized and a large amount of data is collected and discussed in order to clarify past contradictions. The number of small microcraters (D_c < 5 μm) is found to be higher than previously derived and the ratio P/D_c (depth to crater diameter) to depend upon their sizes. All results converge to a two-component dust population: Population 1 consists principally of large grains (d > 2 μm) with density typical of silicates while Population 2 consists of small grains (d < 2 μm) with higher density typical of iron, with a minor component of silicates. The conclusion appears to be further supported by spatial measurements and collection experiments. Fluffy grains of very low density (0.3 g/cm³) are probably not present to a large extent.     

Orientation of cosmic dust grains

Orientation of nonspherical cosmic dust grains is found in anisotropic corpuscular or radiative fluxes and in the presence of the magnetic field; cosmic grains being approximated by axially symmetric ellipsoids. A comparatively small twisting of grains is shown to cause differences in scattering right-hand and left-hand circularly polarized photons and growth of the angular momentum. If the period of the grain's angular momentum precession induced by the magnetic field is shorter than the time of orientation by corpuscular or radiative flux, distribution of grains' axes becomes symmetric relative to the magnetic lines. This orientation mechanism easily explains interstellar linear polarization observed in our Galaxy. The mechanisms of grains' orientation near the Becklin-Neugebauer infrared source, in the B 96 reflected cloud near RY Tau and in cometary heads are proposed.     

Effects of charged dust grains

Dust grains expelled by radiation pressure of stars are charged to potentials in the range 30–40 V in Hi clouds. These grains may be responsible for the following phenomena which are otherwise hardly explicable. (1) A considerable fraction of electrons knocked-out by charged grains of high speeds have energies around 15 eV and produce singly ionized ions but not doubly ionized ones in accord with an ultraviolet observation of interstellar atoms and ions. (2) Transverse momentum transferred to grains by Coulomb scattering of ambient electrons and protons is greater than that by multiple scattering of cosmic ray protons, thus the former being more effective for the grain alignment than the latter. (3) At a shock front charge separation due to a large inertial mass of grains produces an electric field, thus accelerating charged particles and causing a drift of interstellar matter.     

Relativistic and non-relativistic dust grains

Charged dust grains of radiia≃3×10⁻⁶∼3×10⁻⁵ cm could be a help in understanding high energy particles in extensive air showers (EAS). It is suggested that the dust grains in the intergalactic medium may attain relatistic energy (≥10²⁰ eV), and may be responsible for the apparent ‘bump’ in the energy spectrum. The relativistic and non-relativistic dust grains may help to understand the anomalies in the energy spectrum as regards the slope and intensity.     

Far-infrared properties of Virgo spirals

Using a sample of 109 spiral galaxies in the Virgo cluster we show that Hi deficient galaxies have cooler far-infrared colour temperatures and lower 60 and 100 m fluxes than those with normal Hi content. We suggest that these results can be explained if the dust content of Hi deficient galaxies have been reduced by a factor of two compared to field galaxies.     

Temperature fluctuations in interstellar dust grains

Temperature fluctuations in interstellar dust grains caused by absorption of ultraviolet photons are discussed. Temperature probability distributions are presented for various assumptions about the grain specific heat and far infrared emissivity. Grains smaller than or about 0.01 may suffer large temperature fluctuations and would spend most of their time at very low temperatures. The equilibrium temperature is not a good measure for most average temperatures of such grains. It is shown that, although small grains spend only a small fraction of their time at the higher temperatures, the emitted far infrared spectrum is significantly shifted towards shorter wavelengths than predicted for grains assumed to be at the equilibrium temperature.Invited contribution to the Proceedings of a Workshop onThermodynamics and Kinetics of Dust Formation in the Space Medium held at the Lunar and Planetary Institute, Houston, 6–8 September, 1978.     

Far-infrared observations of main sequence stars surrounded by dust shells

We present 50 and 100µm photometry and size information for several main sequence stars surrounded by dust shells. The observations from NASA's Kuiper Airborne Observatory include the Vega-like stars, Beta Pic, Fomalhaut, as well as four stars suggested by Walker and Wolstencroft to belong possibly to the same class. The results of our observations are best interpreted as upper limits to the far-infrared sizes of the dust clouds around all of the stars except Fomalhaut and Beta Pic. We have also fit simple, optically thin models to the Beta Pic data to explore the range of shell parameters consistent with our limits and with previous observations.     

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.     

Segregation of dust grains in dark clouds

Gravitational settling of dust grains in dark clouds has been considered. It has been shown that such a process gives rise to a modification of the grain size distribution. Starting with a simple model of uniform spherical cloud and normal interstellar grain size distribution for the dust we derive expressions for the modified grain size distribution function, average grain size and extinction as functions of distance from the cloud's center and the age of the cloud. The mean grain size increases towards the center of the cloud as does the extinction. Results of the numerical evaluation of these quantities have been discussed with their implications for the observations of anomalous reddening and polarization within dark clouds and Bok globules.     

Molecular hydrogen formation on porous dust grains

Recent laboratory experiments on interstellar dust analogues have shown that H₂ formation on dust-grain surfaces is efficient in a range of grain temperatures below 20 K. These results indicate that surface processes may account for the observed H₂ abundance in cold diffuse and dense clouds. However, high abundances of H₂ have also been observed in warmer clouds, including photon-dominated regions (PDRs), where grain temperatures may reach 50 K, making the surface processes extremely inefficient. It was suggested that this apparent discrepancy can be resolved by chemisorption sites. However, recent experiments indicate that chemisorption processes may not be efficient at PDR temperatures. Here we consider the effect of grain porosity on H₂ formation, and analyse it using a rate-equation model. It is found that porosity extends the efficiency of the recombination process to higher temperatures. This is because H atoms that desorb from the internal surfaces of the pores may re-adsorb many times and thus stay longer on the surface. However, this porosity-driven extension may enable efficient H₂ formation in PDRs only if porosity also contributes to significant cooling of the grains, compared to non-porous grains.     

Interstellar extinction by spheroidal absorbing dust grains

On the basis of exact computations of the extinction efficiencies of spheroidal absorbing particles, numerical calculations of the extinction curves have been made for a distribution of particle sizes. The results are presented and compared with the observed interstellar extinction. For the sake of comparison, results for the nonabsorbing particles are also given.     

Transition radiation from relativistic intergalactic dust grains

Transition radiation is emitted when a charged particle traverses a boundary separating two media. Such radiation must therefore be emitted as X-rays and ultraviolet radiation, from a medium where cosmic-ray electrons pass through dust grains. Calculation shows that transition radiation emitted in this way might account for soft X-rays emission from relativistic charged dust grains in the intergalactic medium (IGM). This could be a help in understanding the IGM as well as an indirect method of measuring the energies of high energy particles.     

Optical properties of the carbon dust grains in the envelopes around asymptotic giant branch stars

We have investigated the optical properties of the carbon dust grains in the envelopes around carbon-rich asymptotic giant branch stars, paying close attention to the infrared observations of the stars and the laboratory-measured optical data of the candidate dust grain materials. We have compared the radiative transfer model results with the observed spectral energy distributions of the stars including IRAS Point Source Catalog and IRAS Low Resolution Spectrograph data. We have deduced an opacity function of amorphous carbon dust grains from model fitting with infrared carbon stars. From the opacity function, we have derived the optical constants of the AMC grains. The optical constants satisfy the Kramers–Kronig relation and produce the opacity function that fits the observations of infrared carbon stars better than previous works in the wide wavelength range 1–1000 μm. We have used simple mixtures of the AMC and silicon carbide grains for modelling. We have compared the contributions that AMC and SiC grains make to the opacity for the cases of simple mixtures of them and spherical core–mantle type grains consisting of a SiC core and an AMC mantle .     

Production of metagalactic X-rays by relativistic dust grains

The relativistic dust grains which may be responsible for ultra-high energy cosmic rays, as suggested by the present author, interact with the cosmic black-body radiation. This results in the energy loss of the relativistic dust grains, so that their energy spectrum is cut-off at the Lorentz factor as large as 2×10³ (0.1/a), wherea is the grain radius. The black-body radiation is scattered and absorbed by the dust grains. The photons scattered and reemitted contribute to metagalactic X-rays. The X-ray intensity estimated is comparable to the observed one in the soft X-ray region.     

Ionization of interstellar matter by suprathermal dust grains

Suprathermal dust grains as suggested by Wickramasinghe produce electrons of energies not higher than 20 eV by Coulomb collisions with free electrons in an interstellar medium. These electrons are responsible for the production of singly ionized ions but not effective for that of highly ionized ones. This explains a general feature of the composition of atoms and ions as observed from the Copernicus satellite.