Optical properties of spheroidal particles

Вощинников, Н. В.; Фарафонов, В. Г.

doi:10.1007/bf00658095

Cited by 235 publications

(227 citation statements)

References 34 publications

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“…We apply the same dust parameters as for the linear polarisation spectrum described above. We note that V/I changes sign at wavelengths close to the position of maximum of the linear polarisation (Voshchinnikov 2004). This may explain many null detections of circular polarisation in the visual part of the spectrum because λ max ∼ 0.5 μm.…”

Section: Circular Polarisationmentioning

confidence: 70%

“…(7)−(9), with suffix TM for the transverse magnetic and TE for transverse electric modes of polarisation (Bohren & Huffman 1983), are defined as the ratios of the cross sections to the geometrical cross-section of the equal volume spheres, Q = C/πr 2 . The extinction and scattering efficiencies Q ext , Q sca , and phase lags Q pha of the two polarisation directions are computed with the program code provided by Voshchinnikov & Farafonov (1993). The average absorption and scattering cross sections C abs,sca are obtained similar to Eq.…”

Section: Cross Sections Of Spheroidsmentioning

confidence: 99%

“…More realistic particle shapes such as spheroids have been considered recently. The derivation of cross sections of spheroids is an elaborate task, and computer codes have been made available by Mishchenko (2000) and Voshchinnikov & Farafonov (1993). The influence of the type of spheroidal grain on the polarisation is discussed by Voshchinnikov (2004).…”

Section: Introductionmentioning

confidence: 99%

“…The derivation of cross sections of spheroids is an elaborate task, and computer codes have been made available by Mishchenko (2000) and Voshchinnikov & Farafonov (1993). The influence of the type of spheroidal grain on the polarisation is discussed by Voshchinnikov (2004). Dust models considering spheroidal particles that fit the average Galactic extinction and polarisation curves were presented by Gupta et al (2005); Draine & Allaf-Akbari (2006); and Draine & Fraisse (2009).…”

Section: Introductionmentioning

confidence: 99%

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Dust in the diffuse interstellar medium

Siebenmorgen

Вощинников

Bagnulo

2014

A&A

Self Cite

112

123

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We present a model for the diffuse interstellar dust that explains the observed wavelength-dependence of extinction, emission, and the linear and circular polarisation of light. The model is set up with a small number of parameters. It consists of a mixture of amorphous carbon and silicate grains with sizes from the molecular domain of 0.5 up to about 500 nm. Dust grains with radii larger than 6 nm are spheroids. Spheroidal dust particles have a factor 1.5-3 greater absorption cross section in the far-infrared than spherical grains of the same volume do. Mass estimates derived from submillimetre observations that ignore this effect are overestimated by the same amount. In the presence of a magnetic field, spheroids may be partly aligned and polarise light. We find that polarisation spectra help to determine the upper particle radius of the otherwise rather unconstrained dust size distribution. Stochastically heated small grains of graphite, silicates, and polycyclic aromatic hydrocarbons (PAHs) are included. We tabulate parameters for PAH emission bands in various environments. They show a trend with the hardness of the radiation field that can be explained by the ionisation state or hydrogenation coverage of the molecules. For each dust component its relative weight is specified so that absolute element abundances are not direct input parameters. The model is compared to the average properties of the Milky Way, which seem to represent dust in the solar neighbourhood. It is then applied to specific sight lines towards four particular stars, with one of them located in the reflection nebula NGC 2023. For these sight lines, we present ultra-high signal-to-noise linear and circular spectro-polarimetric observations obtained with FORS at the VLT. Using prolate rather than oblate grains gives a better fit to observed spectra; the axial ratio of the spheroids is typically two and aligned silicates are the dominant contributors to the polarisation.

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Section: Circular Polarisationmentioning

confidence: 70%

Section: Cross Sections Of Spheroidsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Dust in the diffuse interstellar medium

Siebenmorgen

Вощинников

Bagnulo

2014

A&A

Self Cite

112

123

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“…ρ gr is the dust material mass density. The dust opacities are calculated from effective medium theory (Bruggeman 1935) and Mie theory (Miex from Wolf, according to Voshchinnikov 2002). Any uniform volume mix of solid materials with known optical constants can be used.…”

Section: Spectral Bands and Band-mean Quantitiesmentioning

confidence: 99%

Radiation thermo-chemical models of protoplanetary disks

Woitke

Kamp²,

Thi

2009

A&A

400

667

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Context. Emission lines from protoplanetary disks originate mainly in the irradiated surface layers, where the gas is generally warmer than the dust. Therefore, interpreting emission lines requires detailed thermo-chemical models, which are essential to converting line observations into understanding disk physics. Aims. We aim at hydrostatic disk models that are valid from 0.1 AU to 1000 AU to interpret gas emission lines from UV to sub-mm. In particular, our interest lies in interpreting far IR gas emission lines, such as will be observed by the Herschel observatory, related to the Gasps open time key program. This paper introduces a new disk code called ProDiMo. Methods. We combine frequency-dependent 2D dust continuum radiative transfer, kinetic gas-phase and UV photo-chemistry, ice formation, and detailed non-LTE heating & cooling with the consistent calculation of the hydrostatic disk structure. We include Fe ii and CO ro-vibrational line heating/cooling relevant to the high-density gas close to the star, and apply a modified escapeprobability treatment. The models are characterised by a high degree of consistency between the various physical, chemical, and radiative processes, where the mutual feedbacks are solved iteratively. Results. In application to a T Tauri disk extending from 0.5 AU to 500 AU, the models show that the dense, shielded and cold midplane (z/r < ∼ 0.1, T g ≈ T d ) is surrounded by a layer of hot (T g ≈ 5000 K) and thin (n H ≈ 10 7 to 10 8 cm −3 ) atomic gas that extends radially to about 10 AU and vertically up to z/r ≈ 0.5. This layer is predominantly heated by the stellar UV (e.g. PAH-heating) and cools via Fe ii semi-forbidden and Oi 630 nm optical line emission. The dust grains in this "halo" scatter the starlight back onto the disk, which affects the photochemistry. The more distant regions are characterised by a cooler flaring structure. Beyond r > ∼ 100 AU, T g decouples from T d even in the midplane and reaches values of about T g ≈ 2T d . Conclusions. Our models show that the gas energy balance is the key to understanding the vertical disk structure. Models calculated with the assumption T g = T d show a much flatter disk structure. The conditions in the close regions (<10 AU) with densities n H ≈ 10 8 to 10 15 cm −3 resemble those of cool stellar atmospheres and, thus, the heating and cooling is more like in stellar atmospheres. The application of heating and cooling rates known from PDR and interstellar cloud research alone can be misleading here, so more work needs to be invested to identify the leading heating and cooling processes.

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A Review of Elastic Light Scattering Theories

Wriedt

1998

Part. Part. Syst. Charact.

165

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Mie scattering is an important tool for diagnosing microparticles or aerosol particles in technical or natural environments. Mie theory is restricted to spherical, homogeneous, isotropic and non-magnetic particles in a non-absorbing medium. However, as microparticles are hardly ever spherical or homogeneous, there is much interest in more advanced scattering theories. During recent decades, scattering methods for non-spherical and non-homogeneous particles have been developed and even some computer codes are readily available. Extension of Mie theory covers coated spheres, stratified spheres and clustered spheres. For homogeneous non-spherical particles such as spheroids, ellipsoids and finite cylinders, surface discretization methods have been developed. Scattering by inhomogeneous particles may be computed by volume discretization methods.

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Optical properties of spheroidal particles

Cited by 235 publications

References 34 publications

Dust in the diffuse interstellar medium

Dust in the diffuse interstellar medium

Radiation thermo-chemical models of protoplanetary disks

A Review of Elastic Light Scattering Theories

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