Core-mantle Particles Research Articles

MODELING DUST IN THE MAGELLANIC CLOUDS

We model the extinction profiles observed in the Small and Large Magellanic clouds with a synthetic population of dust grains consisting by core-mantle particles and a collection of free-flying polycyclic aromatic hydrocarbons. All different flavors of the extinction curves observed in the Magellanic Clouds can be described by the present model, that has been previously (successfully) applied to a large sample of diffuse and translucent lines of sight in the Milky Way. We find that in the Magellanic Clouds the extinction produced by classic grains is generally larger than absorption by polycyclic aromatic hydrocarbons. Within this model, the non-linear far-UV rise is accounted for by polycyclic aromatic hydrocarbons, whose presence in turn is always associated to a gap in the size distribution of classical particles. This hints either a physical connection between (e.g., a common cause for) polycyclic aromatic hydrocarbons and the absence of middle-sized dust particles, or the need for an additional component in the model, that can account for the non-linear far-UV rise without contributing to the UV bump at $\sim$217 nm, e.g., nanodiamonds.

Construction of the Rayleigh Approximation for Axisymmetric Multilayered Particles, Making Use of Eigenfunctions of the Laplace Operator

The Rayleigh approximation is constructed from the solution of the electrostatic problem for axisymmetric multilayered particles. The approach used is based on the consideration of surface integral equations analogous to those applied in the extended boundary condition method (EBCM) for solving electromagnetic problems. The electric fields are connected with scalar potentials, which are represented by their expansions in terms of eigenfunctions of the Laplace operator written in spheroidal and spherical coordinates. The unknown expansion coefficients are derived from infinite linear algebraic equations. The explicit solution found in spheroidal coordinates for multilayered spheroids coincides with known solutions for homogeneous and core-mantle particles. Bibliography: 17 titles.

Modelling anomalous extinction using nanodiamonds

Modeling extinction along anomalous/non-CCM sightlines, which are characterized by a broad $217.5 nm$ bump and steep far-ultraviolet (FUV) rise, is reported. The extinction along these sightlines, viz. {HD 210121}, {HD 204827}, {HD 29647} and {HD 62542}, is difficult to reproduce using standard silicate and graphite grains. Very good match with the observed extinction is obtained by considering nanodiamond component as part of carbonaceous matter. Most of these sightlines are rich in carbon and are invariably backed by a young hot stellar object. Nanodiamond is taken as core within amorphous carbon and graphite. These core-mantle particles taken as additional components along with graphite and silicates lead to reduction in the silicate requirement. The abundance of carbonaceous matter is not affected as a very small fraction of nanodiamond is required. Extinction along sightlines that show steep FUV is also reported demonstrating the importance of nanodiamond component in all such regions.

Reconsidering the Origin of the 21 Micron Feature: Oxides in Carbon‐rich Protoplanetary Nebulae?

The origin of the so-called 21 μm feature that is especially prominent in the spectra of some carbon-rich protoplanetary nebulae (PPNs) is the matter of a lively debate. A large number of potential band carriers have been presented and discarded within the past decade. This paper gives an overview of the problems related to the hitherto proposed feature identifications, including the recently suggested candidate carrier silicon carbide. We also discuss the case for spectroscopically promising oxides. SiC is shown to produce a strong resonance band at 20-21 μm if coated by a layer of silicon dioxide. At low temperatures, core-mantle particles composed of SiC and amorphous SiO2 indeed have their strongest spectral signature at a position of 20.1 μm, which coincides with the position of the 21 μm emission band. The optical constants of another candidate carrier that has been relatively neglected so far—iron monoxide—are proven to permit a fairly accurate reproduction of the 21 μm feature profile as well, especially when low-temperature measurements of the infrared properties of FeO are taken into account. As candidate carrier of the 21 μm emission band, FeO has the advantage of being stable against further oxidation and reduction only in a narrow range of chemical and physical conditions, coinciding with the fact that this feature, too, is detected in a small group of objects only. However, it is unclear how FeO would form or survive, particularly in carbon-rich PPNs.

In situ observation of silicon carbide formation process using electron microscope

The silicon core and carbon mantle particles were produced by using the advanced carbon coating method which enables direct covering with the carbon layer using an electron microscope. The growth of SiC crystal was observed upon heating at 500°C in vacuum. The growth process of SiC on both the carbon layer and silicon particles was directly observed by in situ observation. The inward movement of carbon into silicon began at the twinned part. The growth rate of SiC on the carbon mantle layer was estimated from in situ images and found to be 8 times faster than the growth rate of silicon carbide in silicon particles.

Effects of shock processing on Serkowski polarization curves

This paper presents a semi-empirical model for variations of interstellar polarization curves based upon the Serkowski-Wilking law for optical and near-infrared wavebands. The model assumes that nonspherical dust grains producing interstellar polarization are core-mantle particles shaped like oblate spheroids. The physical picture is one in which large (a0 ≈ 0.1µm) particles in the dense cloud phase are deposited into the diffuse cloud medium and thereafter undergo mantle processing by galactic shocks and UV starlight. It is shown that polarization curves vary their widths mainly as a consequence of the nonthermal sputtering of mantles by low-velocity shocks. Mantle sputtering by shocks in low density clouds tends to broaden the curves, whereas mantle sputtering by shocks in denser clouds produce narrow curves. Hence, shock processing of grain mantles can explain the observed correlation between the width of polarization curves and the dust grain environment.

Where is the ice in comets

JHKL photometry of comets P/Tuttle, Meier 1980q, P/Stephan-Oterma, and Bowell 1980b has yielded J-H and H-K colors, uncontaminated by thermal emission, which are nearly identical for all four comets. The colors are inconsistent with the reflection spectrum from a cloud of icy particles, if the particles are composed primarily of any of the ices (H2O, CO2, CH4, or NH3) commonly assumed to be present in the nuclei of comet. Circular variable filter spectra of comet Stephan-Oterma confirm the absence of any absorption features due to these ices. The reflection spectra of these comets appear most like those of Cand S-type asteroids and the ring of Jupiter. Mie calculations for core-mantle particles suggest that ices could be present as thin mantles on inherently reddish, refractory cores, such as magnetite.

Pre-stellar interstellar dust

Before we can properly build a theory for the origin of the solar system we must know what interstellar gas and dust look like in the pre-stellar stage. In this paper we have concentrated on the dust. We arrive at a description of this pre-stellar dust on which the limiting criteria are developed out of evolution from average dust properties. We identify two populations of dust grains: large core-mantle grains and very small bare particles. The cores are presumed to be silicate of size ∼0.05 μm while the mantles consist of a size distribution of accreted thicknesses of C, N, O molecules and radicals which have been photoprocessed and an outer layer of molecules (mostly CO) and other species accreted in the later stages of cloud contraction. Most of the mantles must contain stored energy, some perhaps with enough to blow off the mantle if sufficiently disturbed. The maximum mean size of the core-mantle particles is ∼0.21 μm. The bare particles have not yet been positively identified but are ∼0.005 μm in size and outnumber the core-mantle particles by ∼3000–4000 while the remaining hydrogen in the cloud is ∼1012 times more abundant than the core-mantle grains. Physical arguments are presented to explain the narrow size distribution of these two types of particles.

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 λ 2200 band, must exhibit a strong polarization effect in this region.

Considerations about the absorption efficiency of dust particles in the infrared

Analytical approximations used often in the literature for calculating energy rates emitted by dust grains in infrared are discussed. Comparisons with correct complete formulations are made for three grain models: (1) pure graphite, (2) ice mantle-graphite core, (3) silicates. λ-1 and λ-1 dependences for the average effective emissivity of such grains are used. We find that for silicate and graphite grains the simplified approximations are valid only when accuracies between 10% and 50% are required and only for grain temperatures higher than 80 K. At lower temperatures the validity of the approximations fails for the graphite particle while it is variable for the silicate dust grain. The ice core mantle particles can instead be treated with approximated formulae without introducing appreciable errors.

The Chemical Composition and Distribution of Interstellar Grains

The chemical composition of interstellar grains is derived here on the basis of (1) the cosmic abundance of the elements; (2) the wavelength dependence of extinction and polarization; (3) the average total extinction; (4) the ratio of polarization to extinction; (5) the predominantly dielectric character of grains in the visible spectral region; and (6) infrared spectral characteristics of grains. It is indicated that the major portion of the grains, by mass, consist of core-mantle particles in the 0.1-µm size range, whose cores consist largely of silicates and whose mantles are a solid mixture of O, C, and N with H in a heterogeneous combination of simple and complex molecules with frozen free radicals. A minor constituent of the solid particles exist in the form of very small uncoated particles generally less than 10-6 cm in size whose precise composition is not certain. Inferences of the core-mantle model with respect to spatial distribution are consistent with the proposition that growth of the mantles occurs in the galactic shock region predicted by the density-wave theory. Estimates of the total visual extinction toward the galactic center and the consequent estimates of the total amount of far infrared radiation are shown to depend critically on the grain model. Variations of the ratio of far ultraviolet to visual extinction are correlated with the conditions for growth of mantles on the bare small particles which are generally prevented from accreting mantles primarily because of their extreme temperature fluctuations produced by the ultraviolet photons in the radiation field.