Interstellar extinction and polarization by spheroidal dust grains

Using the recently available exact computations of the scattering efficiencies of spheroidal particles numerical calculations of the extinction and polarization curves have been made for a distribution of particle sizes, shapes and orientations. The results are presented and compared with the observed interstellar extinction and polarization. Possible models for interstellar dust with nonspherical grains have been discussed.     

Porous and fluffy grains in the regions of anomalous extinction

It has long been established that the ratio of total to selective extinction is anomalously large (>- 5) in certain regions of the interstellar medium. In these regions of anomalous extinction the dust grains are likely to be irregular in shape and fluffy in structure. Using discrete dipole approximation (DDA) we calculate the extinction for porous and fluffy grains. We apply DDA first to solid spheroidal particles assumed to be made of a certain (large) number of dipoles. Then we systematically reduce the number of dipoles to model the porous grains. The aggregates of these particles are suggested to form the fluffy grains. We study the extinction for these particles as a function of grain size, porosity and wavelength. We apply these calculations to interpret the observed extincttion data in the regions of star formation (e.g. the Orion complex).     

Optical properties of particulates

Optical properties of small particles of olivine (less than 0.1 μ) have been studied in the ultraviolet as an example of an insulating solid. Very little structure survives in the ultraviolet extinction curves for such small particles. By contrast ‘surface modes’, observed for graphite small particles in the ultraviolet and for olivine particles in the infrared, produce dominant and persistent structure in extinction. The general trend of optical properties of graphite is surprisingly similar to the behavior required to explain all features of the interstellar extinction and albedo curves from near visible to 1000 Å. Measured extinction of small olivine particles in the infrared agrees with calculations based on newly measured optical constants, but dominant sharp structure in the 10μ region still presents a bit of a problem in explaining ‘silicate’ features in astronomical data.     

Extinction of light by randomly-oriented non-spherical grains

Waterman'sT-matrix approach is used to derive a simple analytical expression for the extinction cross-section for randomly-oriented non-spherical grains. Numerical results are presented for randomly-oriented oblate and prolate spheroids and Chebyshev particles composed of astronomical silicate. These results are compared with those for spherical grains, and possible influence of the shape of dust grains on the value of interstellar extinction is considered. The range of validity of the Rayleigh approximation for computing extinction efficiency factors for randomly-oriented non-spherical grains is discussed.     

Unusual extinction profiles by RAYLEIGH scattering

The extinction efficiencies for small smooth metallic particles (granules) have been calculated using rigorous MIE theory of scattering. Strong size dependent resonances for spheres composed of sodium, potassium, and calcium have been found. The results seem to have relevance to the problems of interstellar grains especially with reference to structures in the observed extinction curve, diffuse bands and anomaly of cosmic abundances of elements.     

Absorption bands and extinction in small interstellar particles

The production of discrete line and broad-band extinction by small interstellar oxide and silicate particles is discussed quantitatively. Restrictions on particle size and refractive index that are required to produce ‘pure’ absorption features are reviewed. The relationship between optical depth in interstellar extinction and absorption coefficients for bulk materials is used to reach some general conclusions concerning the diffuse interstellar features, VUV extinction and the composition of interstellar dust. It is noted that charge transfer bands of ions such as Fe³⁺ may be detectable in the VUV spectrum of dust. Several effects that lead to the enhancement of oscillator strength by 10³–10⁴ in small particles are discussed.     

Detection of submillimeter size particles in the inner coma of a comet through their forward scattering

During the occultation of a star by the inner coma region of a comet, specific forward-scattering effects could be observable if submillimeter size particles contribute significantly to the net extinction of star light. In this paper we investigate the possibility of detecting the signature of such particles by observing the dependence of extinction on the angular size of the acceptance aperture used at the focal plane of the telescope. Calculations based on a simple model assuming spherically-symmetric and homogeneous coma are presented.     

The extinction of the mammoths

Two kinds of blanket of fine grains in the Earth's upper atmosphere are possible: a blanket of reflective dielectric particles and one of absorbing metallic particles. The first is a trigger for the onset of an ice-age, and the second a trigger for the disappearance of an ice-age. The facts support the view that it was a blanket of absorptive particles which caused the extinction of the mammoths.     

The shape of the interstellar absorption band

Profiles of the 4430 band are calculated for resonant absorbers distributed within graphite particles, silicate particles and solid H₂ grains. The sizes of grains adopted are those which give agreement with the interstellar extinction. Only in the latter two cases can satisfactory agreement be obtained with recent observational data.     

Properties of circumstellar grains in R coronae borealis

Extinction curves for core-mantle and multicored grains containing two allotropic carbons are compared with far-UV extinction observations of the variable star R CrB. A good agreement is reached by multicore grains; but it is found that only during deep minimum considered models of grain do not fit the observational UV extinction. It indicates the existence, in this phase, of another type of grains like those of Platt particles.     

A Cosmic Prevalence of Nanobacteria?

The data on the far-ultraviolet extinction of starlight in our galaxy and in external galaxies is interpreted in terms of the widespread occurrence of organic particles of optical refractive index 1.4 and radii less than or equal to 20~nm. Such particles are candidates for nanobacteria such as recently been found in abundance on the Earth.     

Mean free path limitation of conduction electrons and extinction efficiencies of graphite grains

An extended Drude theory taking account of the limited free path of conduction electrons in small particles is used to calculate size dependent complex refractive indices of graphite spheres from bulk refractive index data. The size dependent refractive indices are then used in conjunction with the Mie theory to compute mean extinction efficiencies of graphite spheres. Significant departures from earlier results are found.     

The Case Against Graphite Particles in Interstellar Space

Graphite flakes are excluded as a major component of interstellar grains because their ultraviolet extinction peaks occur at wavelengths longward of 2700 Å. Mie calculations for spherical graphite particles are shown to have no physical or astronomical relevance.     

Extinction and scattering by small planetesimal particles

Computations are presented for the extinction and scattering properties of small planetesimal particles consisting of iron cores and silicate mantles. The possible relevance of such calculations to the observed properties of T Tauri-like stars is discussed.     

The case against graphite particles in interstellar space

Graphite flakes are excluded as a major component of interstellar grains because their ultraviolet extinction peaks occur at wavelengths longward of 2700 Å. Mie calculations for spherical graphite particles are shown to have no physical or astronomical relevance.     

CO adsorption and the optical properties of interstellar grains

Laboratory data on the spectra of CO adsorbed on small MgO particles show that CO absorption leads to a weakening of the 220 nm band together with a shift of this band to shorter wavelengths. CO adsorption also results in the formation of a cyclic CO carbon ion that absorbs at 2.15 m^–1. It is shown that this band provides a close match to a major component of the very broad structure seen in interstellar extinction at the same energy. Effects of CO adsorption on the 220 nm band and VUV extinction are discussed in light of recent observational data on stars with peculiar extinction curves.     

Circumstellar absorption and emission in the post-AGB stars HR4049 and HD213985

The evolved C-rich low-mass stars HR 4049 and HD 213985 present variable circumstellar extinction and emission. Observations of HR 4049 strengthen the link between the far-UV extinction rise and the presence of small C-rich particles. Variability of the UV absorption bump in HD 213985 is reported here for the first time and is discussed.     

Diffuse band extinction and polarization in core-mantle grains

Diffuse band shapes in both extinction and polarization are calculated for interstellar coremantle particles for varying size distributions of mantle thickness. It is shown that no matter whether the source of the bands is in the silicate cores or the accreted icy mantles the polarization shapes are highly asymmetric for all mantel thicknesses. The extinction band shapes are significantly less asymmetric although the effect is clearly present. The only apparent possibility for producing symmetric band shapes in the dust grains is in the very small bare particles in interstellar space which, if they are aligned and produce the 2200 band, must exhibit a strong polarization effect in this region.Work supported in part by NASA Grant NGR-33-011-043.Paper presented at the Symposium on Solid State Astrophysics, held at the University College, Cardiff, Wales, between 9–12 July, 1974.     

The material of the interstellar dust and the surface of asteroids: A comparison of the spectral properties in the 0.3–1.1-μm range

From the data on bright stars of different spectral and luminosity classes from the 13-color photometry catalog, the selective extinction of light by the interstellar dust has been studied. The stars from the 1000-pc vicinity of the Sun were investigated. In the optical spectral range, the interstellar extinction curves show systematic deviations from the “λ^?1” law, which allows one to sort them into three types. The observed curves of the interstellar dust extinction were compared with the theoretical curves calculated from the reflectance spectra of the asteroids under the approximation of the Rayleigh particles. The calculated extinction curves for the surface material of D-type asteroids and the Tagish Lake carbonaceous chondrite agree rather well with the observed curves of the interstellar extinction of the first type.     

The anomalous extinction law

In this paper we present the results of the calculations of anomalous extinction laws characterized by different values of the ratio of the total to selective extinctionR=A _v/E(B-V). As far as we are aware, no such theoretical extinction laws have been published before. The calculation is based on the Mie theory of light scattering by small particles assuming a two-grain dust model of Mathiset al. (1977) and by adopting theri particle size (a) distribution, given byn(a)=a ^–3.5, in which the minimum and maximum size is taken to be 0.005 and 0.22 m, respectively. Furthermore, we use the dielectric functions for the two types of grains, graphite and astronomical silicate, as derived by Draine and Lee (1984), published in tabular form by Draine (1985). Following Mathis and Wallenhorst (1981), we then obtained the anomalous extinction laws by changing the upper size cutoff of the particles.A comparison shows that the calculated anomalous extinction laws agree quite well with the corresponding laws derived observationally using the colour-difference method. Finally, a trial and error method for the determination of the anomalous extinction law based on a comparison of the observed extinction-free spectral energy distribution with the corresponding theoretical Kurucz (1979) model, is explained.