This Comment is in response to an Erratum to the paper entitled "The shape and composition of interstellar silicate grains" (Min et al., A&A, 462, 667-676 (2007). The Erratum corrects erroneously cited data but upholds the paper's original conclusions that GEMS are not remnant interstellar grains and that most GEMS formed in the early solar system. In our Comment, we show that the correct data in fact strengthen the relationship between GEMS and interstellar silicates, contrary to the conclusions of Min et al. in their Erratum. We also explore the validity of the isotopic and bulk chemical bases for comparison and present additional indicators of past ISM radiation processing of silicates.
The abundance of both amorphous and crystalline silicates in very small grains is limited by the fact that the 10 micron silicate emission feature is not detected in the diffuse ISM. On the basis of the observed IR emission spectrum for the diffuse ISM, the observed ultraviolet extinction curve, and the 10 micron silicate absorption profile, we obtain upper limits on the abundances of ultrasmall (a < 15 Angstrom) amorphous and crystalline silicate grains. Contrary to previous work, as much as ~20% of interstellar Si could be in a < 15 Angstrom silicate grains without violating observational co…
We investigate the composition and shape distribution of silicate dust grains in the interstellar medium. The effect of the amount of magnesium in the silicate lattice is studied. We fit the spectral shape of the interstellar 10 mu extinction feature as observed towards the galactic center. We use very irregularly shaped coated and non-coated porous Gaussian Random Field particles as well as a statistical approach to model shape effects. For the dust materials we use amorphous and crystalline silicates with various composition and SiC. The results of our analysis of the 10 mu feature are used …
We have studied the dust along the line-of-sight towards the Galactic Center using Short Wavelength Spectrometer (SWS) data obtained with the Infrared Space Observatory (ISO). We focussed on the wavelength region from 8-13 micron which is dominated by the strong silicate absorption feature. Using the absorption profiles observed towards Galactic Center Sources (GCS) 3 and 4, which are C-rich Wolf-Rayet Stars, as reference objects, we are able to disentangle the interstellar silicate absorption and the silicate emission intrinsic to the source, toward Sgr A* and derive a very accurate profile f…
We have analyzed the 9.7 and ``18'' micron interstellar silicate absorption features along the line of sight toward four heavily extincted galactic WC-type Wolf-Rayet (WR) stars. We construct two interstellar extinction curves from 1.25 to 25 micron using near-IR extinction measurements from the literature along with the silicate profiles of WR 98a (representing the local ISM) and GCS 3 (representing the Galactic Center). We have investigated the mineralogy of the interstellar silicates by comparing extinction profiles for amorphous silicates with olivine and pyroxene stochiometry to the 9.7 a…
An accurate knowledge of the mineralogy (chemical composition and crystal structure) of the silicate dust in the interstellar medium (ISM) is crucial for understanding its origin in evolved stars, the physical and chemical processing in the ISM, and its subsequent incorporation into protostellar nebulae, protoplanetary disks and cometary nuclei where it is subjected to further processing. While an appreciable fraction of silicate dust in evolved stars, in protoplanetary disks around pre-main sequence stars, in debris disks around main sequence stars, and in cometary nuclei is found to be in cr…
Experimental results on the formation of molecular hydrogen on amorphous silicate surfaces are presented and analyzed using a rate equation model. The energy barriers for the relevant diffusion and desorption processes are obtained. They turn out to be significantly higher than those obtained for polycrystalline silicates, demonstrating the importance of grain morphology. Using these barriers we evaluate the efficiency of molecular hydrogen formation on amorphous silicate grains under interstellar conditions. It is found that unlike polycrystalline silicates, amorphous silicate grains are effi…
Experimental results on the formation of molecular hydrogen on amorphous silicate surfaces are presented for the first time and analyzed using a rate equation model. The energy barriers for the relevant diffusion and desorption processes are obtained. They turn out to be significantly higher than those obtained earlier for polycrystalline silicates, demonstrating the importance of grain morphology. Using these barriers we evaluate the efficiency of molecular hydrogen formation on amorphous silicate grains under interstellar conditions. It is found that unlike polycrystalline silicates, amorpho…
The flattening of the 10mu silicate emission feature observed in the spectra of T Tauri and Herbig Ae/Be stars is usually interpreted as an indicator of grain growth. We show in this paper that a similar behaviour of the feature shape occurs when the porosity of composite grains varies. The fluffy aggregates, having inclusions of different sizes, were modeled by multi-layered spheres consisting of amorphous carbon, amorphous silicate and vacuum. It is also found that the inclusion of crystalline silicates in composite porous particles can lead to a shift of the known resonances and production …
Radiative torques, due to the absorption and scattering of starlight, are thought to play a major role in the alignment of grains with the interstellar magnetic field. The absorption of radiation also gives rise to recoil torques, associated with the photoelectric effect and photodesorption. The recoil torques are much more difficult to model and compute than the direct radiative torque. Here, we consider the relatively simple case of a spheroidal grain. Given our best estimates for the photoelectric yield and other relevant grain physical properties, we find that the recoil torques contribute…
Silicate dust grains in the interstellar medium are known to be mostly amorphous, yet crystalline silicate grains have been observed in many long-period comets and in protoplanetary disks. Annealing of amorphous silicate grains into crystalline grains requires temperatures > 1000 K, but exposure of dust grains in comets to such high temperatures is incompatible with the generally low temperatures experienced by comets. This has led to the proposal of models in which dust grains were thermally processed near the protoSun, then underwent considerable radial transport until they reached the gas g…
We report on laboratory measurements of molecular hydrogen formation and recombination on an olivine slab as a function of surface temperature under conditions relevant to those encountered in the interstellar medium. On the basis of our experimental evidence, we recognize that there are two main regimes of H coverage that are of astrophysical importance; for each of them we provide an expression giving the production rate of molecular hydrogen in interstellar clouds.
Mid-infrared spectra of 65 T Tauri stars (TTS) taken with the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope are modeled using dust at two temperatures to probe the radial variation in dust composition in the uppermost layers of protoplanetary disks. Most spectra indicating crystalline silicates require Mg-rich minerals and silica, but a few suggest otherwise. Spectra indicating abundant enstatite at higher temperatures also require crystalline silicates at temperatures lower than those required for spectra showing high abundance of other crystalline silicates. A few spectra …
We present 8-13 micron spectra of eight red giants in the globular cluster 47 Tucanae (NGC 104), obtained at the European Southern Observatory 3.6m telescope. These are the first mid-infrared spectra of metal-poor, low-mass stars. The spectrum of at least one of these, namely the extremely red, large-amplitude variable V1, shows direct evidence of circumstellar grains made of amorphous silicate.
Recent results indicate that the grain size and crystallinity inferred from observations of silicate features may be correlated with spectral type of the central star and/or disk geometry. In this paper, we show that grain size, as probed by the 10 um silicate feature peak-to-continuum and 11.3-to-9.8 um flux ratios, is inversely proportional to log L_star. These trends can be understood using a simple two-layer disk model for passive irradiated flaring disks, CGPLUS. We find that the radius, R_10, of the 10 um silicate emission zone in the disk goes as (L_star/L_sun)^0.56, with slight variati…
We examine the thermal infrared spectra of large dust grains of different chemical composition and mineralogy. Strong resonances in the optical properties result in detectable spectral structure even when the grain is much larger than the wavelength at which it radiates. We apply this to the thermal infrared spectra of compact amorphous and crystalline silicates. The weak resonances of amorphous silicates at 9.7 and 18 micron virtually disappear for grains larger than about 10 micron. In contrast, the strong resonances of crystalline silicates produce emission dips in the infrared spectra of l…
Infrared ~5--35 um spectra for 40 solar-mass T Tauri stars and 7 intermediate-mass Herbig Ae stars with circumstellar disks were obtained using the Spitzer Space Telescope as part of the c2d IRS survey. This work complements prior spectroscopic studies of silicate infrared emission from disks, which were focused on intermediate-mass stars, with observations of solar-mass stars limited primarily to the 10 um region. The observed 10 and 20 um silicate feature strengths/shapes are consistent with source-to-source variations in grain size. A large fraction of the features are weak and flat, consis…
We present 8--13 micron low resolution spectra (R~100) of 8 low-mass protostellar objects ("proplyds") in the Orion Nebula using the Long Wavelength Spectrometer (LWS) at the W. M. Keck Observatory. All but one of the sources in our sample show strong circumstellar silicate emission, with profiles that are qualitatively similar to those seen in some T Tauri and Herbig Ae/Be stars. The silicate profile in all cases is significantly flattened compared to the profile for typical interstellar dust, suggesting that the dominant emitting grains are significantly larger than those found in the inters…
Infrared (IR) emission spectra are calculated for dust composed of mixtures of amorphous silicate and graphitic grains, including varying amounts of polycyclic aromatic hydrocarbon (PAH) particles. The models are constrained to reproduce the average Milky Way extinction curve. The calculations include the effects of single-photon heating. Updated IR absorption properties for the PAHs are presented, that are consistent with observed emission spectra, including those newly obtained by Spitzer Space Telescope. We find a size distribution for the PAHs that results in emission band ratios consisten…
The 8-14 micron emission spectra of 12 T Tauri stars in the Taurus/Auriga dark clouds and in the TW Hydrae association obtained with the Infrared Spectrograph (IRS; The IRS is a collaborative venture between Cornell University and Ball Aerospace Corporation funded by NASA through the Jet Propulsion Laboratory and the Ames Research Center.) on board Spitzer are analyzed. Assuming the 10 micron features originate from silicate grains in the optically thin surface layers of T Tauri disks, the 8-14 micron dust emissivity for each object is derived from its Spitzer spectrum. The emissivities are fi…
Silicates are an important component of interstellar dust and the structure of these grains -- amorphous versus crystalline -- is sensitive to the local physical conditions. We have studied the infrared spectra of a sample of ultra-luminous infrared galaxies. Here, we report the discovery of weak, narrow absorption features at 11, 16, 19, 23, and 28 microns, characteristic of crystalline silicates, superimposed on the broad absorption bands at 10 and 18 microns due to amorphous silicates in a subset of this sample. These features betray the presence of forsterite (Mg_2SiO_4), the magnesium-ric…
Several mechanisms have been proposed to explain the alignment of grains with the interstellar magnetic field, including paramagnetic dissipation, radiative torques, and supersonic gas-grain streaming. These must compete with disaligning processes, including randomly directed torques arising from collisions with gas atoms. I describe a novel disalignment mechanism for grains that have a time-varying electric dipole moment and that drift across the magnetic field. Depending on the drift speed, this mechanism may yield a much shorter disalignment timescale than that associated with random gas at…
The {\it assumption of additive absorptivity} by different components in compound particles is a widely used method applied in the literature to the analysis of the chemical and structural properties of astronomical (circumstellar, interstellar, protostellar, and cometary) silicates as well as other materials. The errors intrinsic in this additivity assumption, which, in application to astronomical spectra, amounts in some case to {\it spectrum subtraction} have not always been adequately considered in previous works on silicate mineralogy. The failings in the ``{\it spectrum subtraction metho…
We analyse the optical and IR spectra, as well as the spectral energy distribution (UV to mm) of the candidate Herbig Ae star HD100453. This star is particular, as it shows an energy distribution similar to that of other isolated Herbig Ae/Be stars (HAEBEs), but unlike most of them, it does not have a silicate emission feature at 10 um, as is shown in Meeus (2001). We confirm the HAEBE nature of HD100453 through an analysis of its optical spectrum and derived location in the H-R diagram. The IR spectrum of HD100453 is modelled by an optically thin radiative transfer code, from which we derive …
We report 3 - 13 micron spectroscopy of 4 comets observed between August 2002 and February 2003: C/2002 O4 (Honig) on August 1, 2002, C/2002 V1 (NEAT) on Jan. 9 and 10, 2003, C/2002 X5 (Kudo-Fujikawa) on Jan. 9 and 10, 2003, and C/2002 Y1 (Juels-Holvorcem) on Feb. 20, 2003. In addition, we include data obtained much earlier on 69P/Taylor (February 9, 1998) but not previously published. For Comets Taylor, Honig, NEAT, and Kudo-Fujikawa, the silicate emission band was detected, being approximately 23%, 12%, 15%, and 10%, respectively, above the continuum. The data for Comet Juels-Holvorcem were …
We present the results of N-band spectro-interferometric observations of the silicate carbon star IRAS08002-3803 with the MID-infrared Interferometric instrument (MIDI) at the Very Large Telescope Interferometer (VLTI) of the European Southern Observatory (ESO). The observations were carried out using two unit telescopes (UT2 and UT3) with projected baseline lengths ranging from 39 to 47 m. Our observations of IRAS08002-3803 have spatially resolved the dusty environment of a silicate carbon star for the first time and revealed an unexpected wavelength dependence of the angular size in the N ba…
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$μm$ 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 fi…
Observationally, both the 3.4micron aliphatic hydrocarbon C--H stretching absorption feature and the 9.7micron amorphous silicate Si--O stretching absorption feature show considerable variations from the local diffuse interstellar medium (ISM) to Galactic center (GC): both the ratio of the visual extinction (A_V) to the 9.7micron Si--O optical depth (\tausil) and the ratio of A_V to the 3.4micron C--H optical depth (\tauahc) of the solar neighborhood local diffuse ISM are about twice as much as that of the GC. In this work, we try to explain these variations in terms of a porous dust model con…
We announce the discovery of SST-Lup3-1, a very low mass star close to the brown dwarf boundary in Lupus III with a circum(sub)stellar disk, discovered by the `Cores to Disks' Spitzer Legacy Program from mid-, near-infrared and optical data, with very conspicuous crystalline silicate features in its spectrum. It is the first of such objects with a full 5 to 35 micron spectrum taken with the IRS and it shows strong 10 and 20 micron silicate features with high feature to continuum ratios and clear crystalline features out to 33 micron. The dust in the disk upper layer has a crystalline silicate …
In the framework of the silicate core-carbonaceous organic mantle interstellar dust model, the bulk of the visual/near-IR extinction and the entire polarization are from nonspherical and aligned core-mantle grains. The 3.4 micron C-H and 9.7 micron Si-O absorption features, respectively arising from the hydrocarbon mantle and the amorphous silicate core, are expected to be polarized to a modestly different degree. Spectropolarimetric observations toward the same lines of sight both in the 3.4 micron region and in the 9.7 micron region would be of great value to test the core-mantle dust model.…
The profile of the silicate 10 $μ$m IR band contains important information about the evolutional stage of dust in circumstellar environments and the possible ongoing process of planetesimal formation. In order to extract this information, the observed band profiles are compared with calculated or laboratory-measured absorption cross sections of amorphous and crystalline grains with different sizes and compositions. We present in this study the first laboratory measurements of the 10 $μ$m band profiles of nonembedded, i.e. free-flying, particles of amorphous and crystalline Mg$_2$SiO$_4$ (with …
Observations of interstellar extinction and polarization indicate that the interstellar medium consists of aligned non-spherical dust grains which show variation in the interstellar extinction curve for wavelengths ranging from NIR to UV. To model the extinction and polarization, one cannot use the Mie theory which assumes the grains as solid spheres. We have used a T-matrix based method for computing the extinction efficiencies of spheroidal silicate and graphite grains of different shapes (axial ratios) and sizes and used these efficiencies to evaluate the interstellar extinction curve in th…
Aliphatic hydrocarbons exhibit an absorption feature at 3.4 micron observed toward sources that sample diffuse regions of the interstellar medium. The absorbers responsible for this feature are assumed to reside in some component of interstellar dust, but the physical nature of the particles (size, shape, structure, etc.) is uncertain. Observations of interstellar polarization provide discrimination. Since the grains that carry the silicate absorption feature are known to be aligned, polarization across the 3.4 micron hydrocarbon feature can be used to test the silicate core-organic refractory…
We investigate the IR spectral features of a sample of D-type symbiotic stars. Analyzing unexploited ISO-SWS data, deriving the basic observational parameters of dust bands and comparing them with respect to those observed in other astronomical sources, we try to highlight the effect of environment on grain chemistry and physic. We find strong amorphous silicate emission bands at 10 micron and 18 micron in a large fraction of the sample. The analysis of the 10 micron band, along with a direct comparison with several astronomical sources, reveals that silicate dust in symbiotic stars shows feat…
We present spatial distribution of polycyclic aromatic hydrocarbons and ionized gas within the Galactic giant HII region NGC3603. Using the IRS instrument on board the Spitzer Space Telescope, we study in particular the PAH emission features at ~5.7, 6.2, 7.7, 8.6, and 11.3um, and the [ArII] 6.99um, [NeII] 12.81um, [ArIII] 8.99um, and [SIV] 10.51um forbidden emission lines. The observations probe both ionized regions and photodissociation regions. Silicate emission is detected close to the central cluster while silicate absorption is seen further away. We find no significant variation of the P…
Polarized far-infrared and submillimeter emission is calculated for models of nonspherical dust grains that are constrained to reproduce the observed wavelength-dependent extinction and polarization of starlight. For emission from regions where the magnetic field is perpendicular to the line-of-sight, the far-infrared emission is expected to have substantial linear polarization at wavelengths longer than 100 microns, but the degree of linear polarization, and its variation with wavelength, is model-dependent. Models in which the starlight polarization is produced by both amorphous silicate and…
We discuss the composition and size distribution of the dust in the coma of comet Hale-Bopp. We do this by fitting simultaneously the infrared emission spectrum measured by the infrared space observatory (ISO) and the measured degree of linear polarization of scattered light at various phase angles and 12 different wavelengths. The effects of particle shape on the modeled optical properties of the dust grains are taken into account. We constrain our fit by forcing the abundances of the major rock forming chemical elements to be solar. The infrared spectrum at long wavelengths reveals that larg…
To examine a recently proposed hypothesis that silicon nanoparticles are the source of extended red emission (ERE) in the interstellar medium, we performed a detailed modeling of the mean Galactic extinction in the presence of silicon nanoparticles. For this goal we used the appropriate optical constants of nanosized Si, essentially different from those of bulk Si due to quantum confinement. It was found that a dust mixture of silicon nanoparticles, bare graphite grains, silicate core-organic refractory mantle grains and three-layer silicate-water ice-organic refractory grains works well in ex…
Identification by the Ulysses spacecraft of interstellar grains inside the planetary system provides a new window for the study of diffuse interstellar matter. Dust particles observed by Ulysses and confirmed by Galileo are more massive ($\geq 10^{-13} {\rm g}$) than the 'classical' interstellar grains. Even bigger grains ($\approx 10^{-7} {\rm g}$) were observed in form of interstellar meteors. We analyze the consequences of the plentiful existence of massive grains in the diffuse interstellar medium. Astronomically observed 'classical' interstellar grains can be described by a size distribut…
Our current understanding of interstellar dust is summarized at an introductory level. Submicron-sized interstellar dust grains absorb and scatter light, and reradiate the absorbed energy in the infrared. The grain population spans a range of sizes, from molecules containing only tens of atoms, to particles containing 10^{10} atoms. Most of the grain mass appears to be due to two types of solid, in approximately equal amounts: amorphous silicate mineral, and carbonaceous material. Approximately 2/3 of the interstellar carbon in diffuse clouds is in solid form. Interstellar grains are important…
In this article we present the results from mid-infrared spectroscopy of a sample of 14 T Tauri stars with silicate emission. The qualitative analysis of the spectra reveals a correlation between the strength of the silicate feature and its shape similar to the one which was found recently for the more massive Herbig Ae/Be stars by van Boekel et al. (2003). The comparison with theoretical spectra of amorphous olivine with different grain sizes suggests that this correlation is indicating grain growth in the disks of T Tauri stars. Similar mechanisms of grain processing appear to be effective i…
The dust in the Small Magellanic Cloud (SMC), an ideal analog of primordial galaxies at high redshifts, differs markedly from that in the Milky Way by exhibiting a steeply rising far-ultraviolet extinction curve, an absence of the 2175 Angstrom extinction feature, and a local minimum at ~12 micron in its infrared emission spectrum, suggesting the lack of ultrasmall carbonaceous grains (i.e. polycyclic aromatic hydrocarbon molecules) which are ubiquitously seen in the Milky Way. While current models for the SMC dust all rely heavily on silicates, recent observations of the SMC sightline toward …
This review surveys the observed properties of interstellar dust grains: the wavelength-dependent extinction of starlight, including absorption features, from UV to IR; optical luminescence; IR emission; microwave emission; optical, UV, and X-ray scattering by dust; and polarization of starlight and of infrared emission. The relationship between presolar grains in meteorites and the interstellar grain population is discussed. Candidate grain materials and abundance constraints are considered. A dust model consisting of amorphous silicate grains, graphite grains, and PAHs is compared with obser…
We present laboratory studies of the micro-structural evolution of an amorphous ferro-magnesian silicate, of olivine composition, following thermal annealing under vacuum. Annealing under vacuum was performed at temperatures ranging from 870 to 1020 K. After annealing spheroidal metallic nano-particles (2-50 nm) are found within the silicate films. We interpret this microstructure in terms of a reduction of the initial amorphous silicate FeO component, because of the carbon-rich partial pressure in the furnace due to pumping mechanism. Annealing in a controlled oxygen-rich atmosphere confirms …
This review summarizes the observational characteristics of those interstellar grains which are prevented from entering the solar system by interactions with the heliopause. Such grains are typically less tha 100nm in radius, and they reveal their presence by interstellar absorption at ultraviolet wavelengths and by non-equilibrium emissions in the red and near-infrared portions of the spectrum.
Radiative torques on irregular dust grains, in addition to producing superthermal rotation, play a direct dynamical role in the alignment of interstellar dust with the local magnetic field. The equations governing the orientation of spinning, precessing grains are derived; H_2 formation torques and paramagnetic dissipation are included in the dynamics. Stationary solutions (constant alignment angle and spin rate) are found; these solutions may be stable ("attractors") or unstable ("repellors"). The equations of motion are numerically integrated for three exemplary irregular grain geometries, e…
The year of 2005 marks the 75th anniversary since Trumpler (1930) provided the first definitive proof of interstellar grains by demonstrating the existence of general absorption and reddening of starlight in the galactic plane. This article reviews our progressive understanding of the nature of interstellar dust.
The in-situ detection of interstellar dust grains in the Solar System by the dust instruments on-board the Ulysses and Galileo spacecraft as well as the recent measurements of hyperbolic radar meteors give information on the properties of the interstellar solid particle population in the solar vicinity. Especially the distribution of grain masses is indicative of growth and destruction mechanisms that govern the grain evolution in the interstellar medium. The mass of an impacting dust grain is derived from its impact velocity and the amount of plasma generated by the impact. Because the initia…
We present a quantitative model for the infrared emission from dust in the diffuse interstellar medium. The model consists of a mixture of amorphous silicate grains and carbonaceous grains, each with a wide size distribution ranging from molecules containing tens of atoms to large grains > 1 um in diameter. We assume that the carbonaceous grains have polycyclic aromatic hydrocarbon (PAH)-like properties at very small sizes, and graphitic properties for radii a > 50 A. On the basis of recent laboratory studies and guided by astronomical observations, we propose "astronomical" absorption cross s…
Interstellar medium is turbulent and this induces relative motions of dust grains. We calculate relative velocities of charged grains in a partially ionized magnetized gas. We account for anisotropy of magnetohydrodynamic (MHD) turbulence, grain coupling with magnetic field, and the turbulence cutoff arising from the ambipolar drag. We obtain grain velocities for turbulence with parameters consistent with those in HI and dark clouds. Those velocities are smaller than those in earlier papers, where MHD effects were disregarded. Finally, we compare grain velocities arising from photoelectric emi…
A model of spheroidal particles is used to calculate the steady-state temperature of dust grains immersed in the interstellar radiation field. It is found that the temperature of non-spherical grains with the aspect ratios a/b <= 2 deviates from that of spheres less than 10%. More elongated or flattened particles are usually cooler than spheres of the same mass and in some cases the temperatures may differ by even about a factor of 2. The shape effects increase with the infrared absorptivity of the grain material and seem to be more important in dark interstellar clouds.
In interstellar dust grains, internal processes dissipate rotational kinetic energy. The dissipation is accompanied by thermal fluctuations, which transfer energy from the vibrational modes to rotation. Together, these processes are known as internal relaxation. For the past several years, internal relaxation has been thought to give rise to thermal flipping, with profound consequences for grain alignment theory. I show that thermal flipping is not possible in the limit that the inertia tensor does not vary with time.
We present mid-IR spectroscopy of three T Tauri stars in the young Chamealeon I dark cloud obtained with TIMMI2 on the ESO 3.6m telescope. In these three stars, the silicate emission band at 9.7 micron is prominent. We model it with a mixture of amorphous olivine grains of different size, crystalline silicates and silica. The fractional mass of these various components change widely from star to star. While the spectrum of CR Cha is dominated by small amorphous silicates, in VW Cha (and in a lesser degree in Glass I), there is clear evidence of a large amount of processed dust in the form of c…
In the interstellar medium of the Milky Way, certain elements -- e.g., Mg, Si, Al, Ca, Ti, Fe -- reside predominantly in interstellar dust grains. These grains absorb, scatter, and emit electromagnetic radiation, heat the interstellar medium by photoelectric emission, play a role in the ionization balance of the gas, and catalyze the formation of molecules, particularly H2. I review the state of our knowledge of the composition and sizes of interstellar grains, including what we can learn from spectral features, luminescence, scattering, infrared emission, and observed gas-phase depletions. Th…
Our current understanding of the absorption and emission properties of interstellar grains are reviewed. The constraints placed by the Kramers-Kronig relation on the wavelength-dependence and the maximum allowable quantity of the dust absorption are discussed. Comparisons of the opacities (mass absorption coefficients) derived from interstellar dust models with those directly estimated from observations are presented.
A photon of wavelength lambda ~1 micron interacting with a dust grain of radius a_p ~ 1 mm (a "pebble") undergoes scattering in the forward direction, largely within a small characteristic diffraction angle theta_s ~ lambda/a_p ~ 100". Though mm-size dust grains contribute negligibly to the interstellar medium's visual extinction, the signal they produce in scattered light may be detectable, especially for variable sources. Observations of light scattered at small angles allows for the direct measurement of the large grain population; variable sources can also yield tomographic information of …
Recent observations with dust detectors on board the interplanetary spacecraft Ulysses and Galileo have recorded a substantial flux of large interstellar grains with radii between 0.25 and 2.0 mu entering the solar system from the local interstellar cloud. The most commonly used interstellar grain size distribution is characterized by a a^-3.5 power law in grain radii a, and extends to a maximum grain radius of 0.25 mu. The extension of the interstellar grain size distribution to such large radii will have a major effect on the median grain size, and on the amount of mass needed to be tied up …
In this paper we discuss the propagation of dust through the interstellar medium (ISM), and describe the destructive effects of stellar winds, jets, and supernova shock waves on interstellar dust. We review the probability that grains formed in stellar outflows or supernovae survive processing in and propagation through the ISM, and incorporate themselves relatively unprocessed into meteoritic bodies in the solar system. We show that very large (radii >= 5 micron) and very small grains (radii <= 100 Angstrom) with sizes similar to the pre-solar SiC and diamond grains extracted from meteorites,…
Irregular dust grains are subject to radiative torques when irradiated by interstellar starlight. It is shown how these radiative torques may be calculated using the discrete dipole approximation. Calculations are carried out for one irregular grain geometry, and three different grain sizes. It is shown that radiative torques can play an important dynamical role in spinup of interstellar dust grains, resulting in rotation rates which may exceed even those expected from H_2 formation on the grain surface. Because the radiative torque on an interstellar grain is determined by the overall grain g…
The temperatures of interstellar dust grains are analyzed using stochastic simulations, taking into account the grain composition and size and the discreteness of the photon flux. [...] The distribution of grain temperatures is calculated for a broad range of grain sizes and for different intensities of the interstellar radiation field, relevant to diffuse clouds and to PDRs. The dependence of the average grain temperature on its size is shown for different irradiation intensities. It is found that the average temperatures of grains with radii smaller than about 0.02 $μ$m are reduced due to th…
Detailed models are presented for the late epoch mid infrared (MIR) emission from collisionally heated grains in the shocked circumstellar gas around SN 1987A. Thermal dust emission from a region of moderate density interior to the thick inner ring seen with the Hubble Space Telescope (HST) is found to be a natural explanation for the MIR spectral energy distribution measured by ISOCAM. The MIR-spectrum can be reproduced by a mixture of silicate-iron or silicate-graphite grains or by a composition of pure graphite grains. A composition of pure iron grains on the other hand can be excluded and …
