Interstellar extinction by spheroidal dust grains

The composite grain is made up of a host silicate spheroid and graphite inclusions. The extinction efficiencies of the composite spheroidal grains for three axial ratios are computed using the discrete dipole approximation (DDA). The interstellar extinction curve is evaluated in the spectral region 3.40--0.10$\mu m$ using the extinction efficiencies of the composite spheroidal grains. The model extinction curves are then compared with the average observed interstellar extinction curve. We also calculate the linear polarization for the spheroidal composite grains at three orientation angles and find the wavelength of maximum polarization. Further, we estimate the volume extinction factor, an important parameter from the point of view of cosmic abundance, for the composite grain models that reproduce the average observed interstellar extinction. The estimated abundances derived from the composite grain models for both carbon and silicon are found to be lower than that are predicted by the bare silicate/graphite grain models but these values are still higher than that are implied from the recent ISM values.

A composite dust grain model which simultaneously explains the observed interstellar extinction, polarization, IR emission and the abundance constraints, is required. We present a composite grain model, which is made up of a host silicate oblate spheroid and graphite inclusions. The interstellar extinction curve is evaluated in the spectral region 3.4-0.1$\mu m$ using the extinction efficiencies of the composite spheroidal grains for three axial ratios. Extinction curves are computed using the discrete dipole approximation (DDA). The model curves are subsequently compared with the average observed interstellar extinction curve and with an extinction curve derived from the IUE catalogue data.

We demonstrate that the method of regularization designed to resolve inverse problems may be successfully applied in analysis of interstellar extinction_ The absolute extinction curves of apparently single clouds, seen towards the stars HD 147165,179406 and 202904, have been derived and modelled using multicomponent bare spherical dust grain mixtures containing graphite, silicates, various types of amorphous carbon, SiC and water ice. We find that the grain size distributions are essentially different from Mathis, Rumpl & Nordsieck's (MRN) power law and may be multimodal. From the recent data about reduced (' 2/3 solar) cosmic abundances, it has been shown that a mixture of graphite, silicate and ice grains explains quite satisfactorily the extinction towards the stars under analysis whereas a traditional mixture of graphite and silicate grains fails.

The observed relation between the interstellar linear polarization curve parameters K and λ max characterizing the width and the wavelength of the polarization maximum, respectively, is interpreted quantitatively. We have considered 57 stars located in four dark clouds with evidence of star formation: in Taurus, Chamaeleon, around the stars ρ Oph and R CrA. In our modeling we have used the spheroidal dust grain model applied previously to simultaneously interpret the interstellar extinction and polarization curves in a wide wavelength range. The observed trend K ≈ 1.7λ max is shown to be most likely related to the growth of dust grains due to coagulation rather than accretion. The relationship of the interstellar polarization curve parameters K and λ max to the mean dust grain size is discussed.

view Abstract Citations (208) References (53) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Infrared extinction and polarization due to partially aligned spheroidal grains : models for the dust toward the BN object. Lee, H. M. ; Draine, B. T. Abstract Methods for computing the extinction and polarization due to partially aligned, precessing grains in the dipole aproximation regime are described. The methods are employed to develop theoretical models of the dust grains on the line of sight to the Becklin-Neugebauer (BN) object in the dense molecular cloud OMC-1. In the models, it is assumed that the dust grains consist of a mixture of graphite and silicate particles, and that some of the particles are coated with ice mantles. It is shown that the models are capable of reproducing all the spectroscopic and polarimetric features of the dust grains in the spectral range from about 2 to about 13 microns. The implications of the grain models for the sizes, shapes and compositions of grains on the line-of-sight to BN are discussed, and some possible mechanisms for grain alignment are identified. Publication: The Astrophysical Journal Pub Date: March 1985 DOI: 10.1086/162974 Bibcode: 1985ApJ...290..211L Keywords: Cosmic Dust; Infrared Astronomy; Molecular Clouds; Point Sources; Radiation Sources; Astronomical Models; Emission Spectra; Linear Polarization; Optical Properties; Orion Nebula; Shapes; Silicates; Astrophysics full text sources ADS | data products SIMBAD (3)

Numerical results of the floating potential of an isolated spherical dust grain or spherical Langmuir probe immersed in an electropositive or electronegative collisional plasma are presented. The positive ions are assumed to be cold so that the ion motion is radial; the electrons and negative ions are Maxwellian. Collisions between the positive ions and neutrals are modelled by a constant mobility. Curves of floating potential (in absolute magnitude) to electron temperature against dust (Langmuir probe) radius to electron Debye length are obtained over a wide range of collisionality and electronegativity. It is found that if there are sufficient collisions in the sheath the floating potential and surface charge are increased. Also, the floating potential increases with decreasing negative ion to electron density and temperature ratios. For typical conditions used in dusty plasma research, collisions are expected to be important for micron-sized dust grains for pressures greater than about 6 Pa in argon.

Recent data on the interstellar extinction curve and on the diffuse galactic light may be accounted for by a mixture of graphite, iron and silicate grains.

For the investigation of collisions among protoplanetesimal dust aggregates, we performed microgravity experiments in which the impacts of high-porosity millimeter-sized dust aggregates into 2.5 cm high-porosity dust aggregates can be studied. The dust aggregates consisted either of monodisperse spherical, quasi-monodisperse irregular, or polydisperse irregular micrometer-sized dust grains and were produced by random ballistic deposition with porosities between 85% and 93%. Impact velocities ranged from ~0.1 to ~3 m s−1, and impact angles were almost randomly distributed. In addition to the smooth surfaces of the target aggregates formed in our experiments, we "molded" target aggregates such that the radii of the local surface curvatures corresponded to the projectile radii, decreasing the targets' porosities to 80%-85%. The experiments showed that impacts into the highest porosity targets almost always led to sticking, whereas for the less porous dust aggregates, consisting of monodisperse spherical dust grains, the collisions with intermediate velocities and high impact angles resulted in the bouncing of the projectile with a mass transfer from the target to the projectile aggregate. Sticking probabilities for the impacts into the "molded" target aggregates were considerably decreased. For the impacts into smooth targets, we measured the depth of intrusion and the crater volume, and were able to derive some interesting dynamical properties which can help to derive a collision model for protoplanetesimal dust aggregates. Future models of the aggregate growth in protoplanetary disks should take into account noncentral impacts, impact compression, the influence of the local radius of curvature on the collisional outcome, and the possible mass transfer between the target and projectile agglomerates in nonsticking collisions.

Modelling the circumnuclear coma of comets: objectives, methods and recent results

DULEY' second1 criticism of our hypothesis of graphite-iron-silicate grain mixtures2,3 contains further errors and misrepresentations of our statements. We wish to refer here to a few crucial facts which seem to have been overlooked by Duley1,4 and which invalidate his contention that the interstellar extinction curve does not provide additional evidence to support either graphite or silicate grains. The suggestion that a fit to the observed extinction curve on the basis of graphite, iron and silicate grains is confined to the adoption of the special set of values 0.06, 0.02 and 0.15 µm for their respective mean radii is incorrect. In our original paper2 we stated clearly that r (graphite)=0.045–0.07 µm, r (iron)≤0.02 µm and r (silicate)=0.15–0.18 µm are the requirements necessary to obtain good agreement with the observations. A wide range of relative proportions of the three grain species and several different types of size distributions2,3,5,6 have been shown to provide satisfactory fits to the available astronomical data. It should be stressed that the optical constants of graphite are such that the observed optical extinction curve and the ultraviolet hump at λ2200 A could be reproduced for a wide range of particle sizes. Iron and silicates in our model play a less important part at optical wavelengths so that constraints on their sizes and proportions relative to graphite are less rigorously specified by the optical extinction data.

view Abstract Citations (28) References (36) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Interstellar Amorphous Carbon Bussoletti, E. ; Colangeli, L. ; Orofino, V. Abstract Amorphous carbon grains are discussed as possible candidates for cosmic dust. Particles obtained in the laboratory do not correctly reproduce the portion of the interstellar extinction curve commonly attributed to graphite. Amorphous carbon grains with a 40-A mean radius respect the carbon cosmic abundance constraints; however, they cannot explain the interstellar UV extinction curve because they show a peak at 2350 A and give a contribution which is too high to the visual extinction. The present experimental results have been used to construct an 'interstellar amorphous carbon' (IAC) whose properties are able to resolve the above-mentioned difficulties. Extrapolations of the laboratory data show that a size distribution with a 10-A average radius has an extinction efficiency with the peak at the right position, satisfactorily matching in shape the interstellar hump. The peak to visual extinction ratio, Gamma = 3.8, justifies the existence of other dielectric materials which may account for the observed linear polarization. IAC requires no more than 18 percent of the available carbon to produce the hump. The results are discussed in terms of evolution of carbon grains in space showing that, in this picture, the presence of graphite is no longer necessary to account for most of the solid carbonaceous material. Publication: The Astrophysical Journal Pub Date: October 1987 DOI: 10.1086/185011 Bibcode: 1987ApJ...321L..87B Keywords: Amorphous Materials; Carbon; Cosmic Dust; Interstellar Matter; Abundance; Astronomical Models; Graphite; Interstellar Extinction; Microcrystals; Astrophysics; INTERSTELLAR: GRAINS full text sources ADS |

A closer examination of interstellar reddening on the basis of graphite grains has been undertaken. The Mie formulae have been computed for values of the complex refractive index corresponding to graphite and accurate extinction curves constructed for graphite spheres of various radii. The reddening law is found to be essentially the same as that given by Hoyle and Wickramasinghe provided the radii are less than ∼ 5 × 10 −6 cm. For slightly larger particles, with radii ∼ 8 × 10 −6 cm, the reddening law already begins to deviate markedly from this curve. Particles of radii 8 × 10 −6 cm are found to have an albedo y ≅ 0.4 at a wavelength λ = 0.4 μ , whereas smaller particles, which fit the reddening law better, possess somewhat lower albedos. Grains of radius ∼ 5 × 10 −6 cm have an albedo of ∼ 0.3 at a wavelength of 0.4 μ . Albedo considerations lead us to examine the possibility that graphite grains, in certain circumstances, may become coated with ice. Rigorous computations have also been done to obtain extinction and scattering cross-sections for graphite spheres covered with concentric spherical shells of ice. It is found that graphite cores, which themselves come near to satisfying the reddening law, do not lose this property when they become coated with ice. The outer radius of a grain may increase to as much as thrice the core radius. The effect of the ice is to produce as light steepening of the extinction curve in the blue, whilst also enhancing the blue albedo. In a typical case the albedo at λ = 0.4 μ is raised to about 0.8. Expressions have been derived for the extinction cross-sections of graphite grains in the form of small oblate spheroids with anisotropic conductivity. It is shown that provided 2 πa / λ ≪ 1, the extinction cross-section for light with electric vector perpendicular to the axis of symmetry is ∼ 10 2 times that for light with electric vector parallel to the axis of symmetry. For larger grains with 2 πa / λ ∼ 1, this ratio may not be much smaller than ∼ 10 at visual wavelengths. These results do not appear to depend strongly on the ratio of axes of the spheroid, and are essentially due to the anisotropic conductivity of graphite. Normalized reddening curves have been constructed for small oblate spheroidal grains which are aligned by a magnetic field. It is shown that the variability of the reddening law observed by Wampler cannot be produced by fluctuations in the alignment of graphite grains. Such an effect may, however, be caused by variations in the amount of ice round graphite grains.

We investigate the effect of solar wind and solar electromagnetic radiation on the dynamics of spherical cosmic dust particles. We also consider the non-radial component of the solar wind velocity, in the reference frame of the Sun. We apply the equation of motion to the motion of dust grains near commensurability resonances with a planet – mean motion orbital resonance (MMR; a particle is in resonance with a planet when the ratio of their mean motions is approximately the ratio of two small integers) – and possible capture of the grains in the resonances. Up to now, only nonspherical grains, under action of the electromagnetic radiation of the central star, were known to exhibit an increase of semimajor axis before capture into the MMR. This paper shows that the same result can be generated by the non-radial component of the solar wind even for spherical dust particles. Spherical dust grains enable the treatment of the problem in an analytic way (at least partially), which is not the case for the effect of electromagnetic radiation on nonspherical dust grains. The situation treated in the paper presents the second known case when resonant trapping of a cosmic body occurs for diverging orbits. The paper presents the first case of secular evolution of the eccentricity of a body captured in the resonance derived in an analytic way for a body characterized by a diverging orbit.

The charging process of nonspherical dust grains in an unmagnetized plasma as well as in the presence of a magnetic field is studied. It is shown that unlike the spherical dust grain, due to nonhomogeneity of charge distribution on the spheroidal dust surface, the resultant electric forces on electrons and ions are different. This process produces some surface charge density gradient on the nonspherical grain surface. Effects of a magnetic field and other plasma parameters on the properties of the dust particulate are studied. It has been shown that the alignment direction could be changed or even reversed with the magnetic field and plasma parameters. Finally, the charge distribution on the spheroidal grain surface is studied for different ambient parameters including plasma temperature, neutral collision frequency, and the magnitude of the magnetic field.

The extinction curves from 800 to 130 nm (1.25-7.7/micron) of amorphous silicate smokes nominally of olivine and pyroxene composition, carbon smokes, and crystalline SiC smokes are presented. The SiC smoke occurred in the low-temperature (beta) cubic structural form. The SiC smoke showed an absorption edge which occurred at significantly longer wavelengths than the calculated extinction profile of the hexagonal SiC form previously used to calculate the interstellar extinction profile. Neither SiC nor amorphous silicates show an extinction band similar to the observed 6.6/micron astronomical extinction band. The infrared absorption peaks for the silicate and SiC samples near 10 microns and 11-13 microns, respectively, were also measured. The ultraviolet to infrared extinction ratio for the amorphous silicate samples is similar to the observed astronomical extinction ratio. The measured extinction ratios for SiC smokes are significantly below the interstellar extinction ratio. The extinction peak of the carbon smokes occurred at 4.0 and 4.25/micron, for samples of mean radii 13 and 6 nm, respectively. The extinction profile is distinctly different from that predicted for graphite grains of the same size, and is similar to that predicted for glassy carbon grains.