Core-mantle Grains Research Articles

Nucleation experiments on a titanium-carbon system imply nonclassical formation of presolar grains.

Just as the shapes of snowflakes provide us with information on the temperature and humidity of the upper atmosphere, the characteristics of presolar grains in meteorites place limits on their formation environment in a stellar outflow. However, even in the case of well-characterized presolar grains consisting of a titanium carbide core and a graphitic carbon mantle, it is not possible to delimit their formation environment. Here, we have demonstrated the formation of core-mantle grains in gravitational and microgravity environments and have found that core-mantle grains are formed by a nonclassical nucleation pathway involving the three steps: (i) primary nucleation of carbon at a substantially high supersaturation, (ii) heterogeneous condensation of titanium carbide on the carbon, and (iii) fusion of nuclei. We argue that the characteristics of not only core-mantle grains but also other presolar and solar grains might be accurately explained by considering a nonclassical nucleation pathway.

Collisions between ice-covered silica grains: An atomistic study

We use molecular dynamics simulation to study collisions of silica grains covered by an ice mantle. Such heterogeneous particles may belong to debris disks of planetary systems below the snow line. For hydroxylated silica spheres, our results of the bouncing velocity obtained for nm-sized grains extrapolate well to experimental results for μm-sized particles. However, with increasing thickness of the ice mantle, the bouncing velocity strongly increases while the coefficient of restitution is reduced, meaning that the inelastic losses increase. An analysis of the processes occurring during the collision of two core–mantle grains shows that the ice mantle in the collision zone is strongly heated during the collision-induced compression. Temperatures surpass the triple point of water such that the ice may melt and become soft enough to yield sideways during the collision, dissipating collision energy. The ice thus acts both as a ‘cushion’ to soften the collision dynamics and as a glue to bind the two grains together. As a consequence of the energy dissipation, the bouncing velocity increases with increasing mantle thickness. The consequences of our findings on collisions of larger ice-coated grains are discussed. We conclude that collision-induced heating constitutes an important process in the collision of ice-covered grains, since it changes the collision physics from a ‘dry’ to a ‘wet’ contact.

Optical and UV surface brightness of translucent dark nebulae

Context. Dark nebulae display a surface brightness because dust grains scatter light of the general interstellar radiation field (ISRF). High-galactic-latitudes dark nebulae are seen as bright nebulae when surrounded by transparent areas which have less scattered light from the general galactic dust layer. Aims. Photometry of the bright dark nebulae LDN 1780, LDN 1642, and LBN 406 shall be used to derive scattering properties of dust and to investigate the presence of UV fluorescence emission by molecular hydrogen and the extended red emission (ERE). Methods. We used multi-wavelength optical photometry and imaging at ground-based telescopes and archival imaging and spectroscopic UV data from the spaceborn GALEX and SPEAR/FIMS instruments. In the analysis we used Monte Carlo RT and both observational data and synthetic models for the ISRF in the solar neighbourhood. The line-of-sight extinctions through the clouds have been determined using near infrared excesses of background stars and the 200/250 μm far infrared emission by dust as measured using the ISO and Herschel space observatories. Results. The optical surface brightness of the three target clouds can be explained in terms of scattered light. The dust albedo ranges from ~0.58 at 3500 Å to ~0.72 at 7500 Å. The spectral energy distribution of LDN 1780 is explained in terms of optical depth and background scattered light effects instead of the original published suggestion in terms of ERE. The far-ultraviolet surface brightness of LDN 1780 cannot be explained by scattered light only. In LDN 1780, H2 fluorescent emission in the wavelength range 1400–1700 Å has been detected and analysed. Conclusions. Our albedo values are in good agreement with the predictions of the dust model of Weingartner and Draine and with the THEMIS CMM model for evolved core-mantle grains. The distribution of H2 fluorescent emission in LDN 1780 shows a pronounced dichotomy with a strong preference for its southern side where enhanced illumination is impinging from the Sco OB2 association and the O star ζ Oph. A good correlation is found between the H2 fluorescence and a previously mapped 21-cm excess emission. The H2 fluorescence emission in LDN 1780 has been modelled using a PDR code; the resulting values for H2 column density and the total gas density are consistent with the estimates derived from CO observations and optical extinction along the line of sight.

Modeling Galactic Extinction with dust and "real" PAHs

We elaborated an interstellar dust model assuming a distribution of core–mantle grains and a collection of single polyciclic aromatic hydrocarbons. Exploiting this model, we are able to reproduce a very large sample of galactic extinction profiles with very different flavours, proving that a polyciclic aromatic hydrocarbons population can reproduce extinction curve features in the ultraviolet range. Dust grains are composed by an hollow silicatic core and a carbonaceous mantle (this description is mutuated by the cycle of carbon in the interstellar medium); molecular population is represented by 54 molecules in four charged states and, despite the large number of free parameters (we have 9 parameters to represent grain distribution and 54 × 4 column densities to reproduce the molecular contribution to the extiction), we are able to determine some global properties for molecular ensemble and we found that these properties are indipendent by specific species which we use in our model.

MODELING GALACTIC EXTINCTION WITH DUST AND “REAL” POLYCYCLIC AROMATIC HYDROCARBONS

We investigate the remarkable apparent variety of galactic extinction curves by modeling extinction profiles with core-mantle grains and a collection of single polycyclic aromatic hydrocarbons. Our aim is to translate a synthetic description of dust into physically well-grounded building blocks through the analysis of a statistically relevant sample of different extinction curves. All different flavors of observed extinction curves, ranging from the average galactic extinction curve to virtually 'bumpless' profiles, can be described by the present model. We prove that a mixture of a relatively small number (54 species in 4 charge states each) of polycyclic aromatic hydrocarbons can reproduce the features of the extinction curve in the ultraviolet, dismissing an old objection to the contribution of polycyclic aromatic hydrocarbons to the interstellar extinction curve. Despite the large number of free parameters (at most the 54 Multiplication-Sign 4 column densities of each species in each ionization state included in the molecular ensemble plus the 9 parameters defining the physical properties of classical particles), we can strongly constrain some physically relevant properties such as the total number of C atoms in all species and the mean charge of the mixture. Such properties are found to be largely independent of the adoptedmore » dust model whose variation provides effects that are orthogonal to those brought about by the molecular component. Finally, the fitting procedure, together with some physical sense, suggests (but does not require) the presence of an additional component of chemically different very small carbonaceous grains.« less

Modeling galactic extinction

We present a model for interstellar extinction dust, in which we assume a bimodal distribution of extinction carriers, a dispersion of core–mantle grains, supplemented by a collection of PAHs in free molecular form. We use state–of–the–art methods to calculate the extinction due to macroscopic dust particles, and the absorption cross–sections of PAHs in four different charge states. While successfull for most of observed Galactic extinction curves, in few cases the model cannot provide reliable results. Paradoxically, these failures appear to be very promising, suggesting that the whole body of dust extinction features might be described within the cycle of carbon in the interstellar medium.

Modelling peculiar extinction curves

We investigate the remarkable variety of IR-through-UV extinction curves by modelling extinction profiles with core-mantle grains and a collection of single and stacked polycyclic aromatic hydrocarbons. Such a model can closely reproduce the observed curves, by simply assuming a different ratio of the molecular component to classical dust, while leaving relatively unmodified the underlying global picture. In particular, we find that for extinction curve morphologies differing significantly from the average Galactic extinction, dust grain sizes exhibit a low-end cut-off at approximately 100 nm. An additional component of very small grains of sizes around 10 nm is required to model lines of sight whose extinctions are similar to the average Galactic curve. We conclude that to be accurately described, the so-called peculiar extinction curves do not need dust grains with exotic properties.

Nonthermal crystallization of amorphous silicates in comets

Recent observations show the ubiquity of crystalline silicate in various objects, among which comets provide the best opportunity to study possible processing of pristine matter during their formation and evolution. While thermal processing of bare amorphous silicates in the hot inner solar nebula has been invoked, its drawback is a difficulty in explaining the interstellar composition of cometary ices. Here we apply a model of core-mantle interstellar grains to propose nonthermal crystallization of the amorphous silicate core due to the energy released by chemical reactions in the organic refractory mantle when moderately heated by solar radiation. By formulating the nonthermal crystallization, we find that the degree of crystallinity is determined by a single parameter that is proportional to the amount of the released energy. We show that the present mechanism of crystallization is capable of reproducing the strengths of crystalline silicate features observed in comets. Our model can reconcile crystalline silicate in the comae of comets with the interstellar composition of ices in their nuclei.

Luminosities and mass-loss rates of SMC and LMC AGB stars and red supergiants

(Abridged) Dust radiative transfer models are presented for 101 carbon stars and 86 oxygen-rich evolved stars in the Magellanic Clouds for which 5-35 \mum\ {\it Spitzer} IRS spectra are available. The spectra are complemented with available optical and infrared photometry to construct the spectral energy distribution. A minimisation procedure is used to fit luminosity, mass-loss rate and dust temperature at the inner radius. Different effective temperatures and dust content are also considered. Periods from the literature and from new OGLE-III data are compiled and derived. The O-rich stars are classified in foreground objects, AGB stars and Red Super Giants. For the O-rich stars silicates based on laboratory optical constants are compared to "astronomical silicates". Overall, the grain type by Volk & Kwok (1988) fit the data best. However, the fit based on laboratory optical constants for the grains can be improved by abandoning the small-particle limit. The influence of grain size, core-mantle grains and porosity are explored. Relations between mass-loss rates and luminosity and pulsation period are presented and compared to the predictions of evolutionary models, those by Vassiliadis & Wood (1993) and their adopted mass-loss recipe, and those based on a Reimers mass-loss law with a scaling of a factor of five. The Vassiliadis & Wood models describe the data better, although there are also some deficiencies, in particular to the maximum adopted mass-loss rate. The OGLE-III data reveal an O-rich star in the SMC with a period of 1749 days. Its absolute magnitude of $M_{\rm bol}= -8.0$ makes it a good candidate for a super-AGB star.

Condensation of MgS in outflows from carbon stars

The basic mechanism responsible for the widespread condensation of MgS in the outflows from carbon rich stars on the tip of the AGB is discussed with the aim of developing a condensation model that can be applied in model calculations of dust formation in stellar winds. The different possibilities how MgS may be formed in the chemical environment of outflows from carbon stars are explored by some thermochemical calculations and by a detailed analysis of the growth kinetics of grains in stellar winds. The optical properties of core-mantle grains with a MgS mantle are calculated to demonstrate that such grains reproduce the structure of the observed 30 $\mu$m feature. These considerations are complemented by model calculations of circumstellar dust shells around carbon stars. It is argued that MgS is formed via precipitation on silicon carbide grains. This formation mechanism explains some of the basic observed features of MgS condensation in dust shells around carbon stars. A weak secondary peak at about 33 ... 36 $\mu$m is shown to exist in certain cases if MgS forms a coating on SiC.

Porous irregular aggregates of sub-micron sized grains to reproduce cometary dust light scattering observations

The present study intends to interpret some of the characteristic features of the light scattered by cometary dust, such as phase angle and wavelength dependence of its polarization, through simulations using Ballistic Cluster-Cluster Aggregation (BCCA) or Ballistic Particle-Cluster Aggregation (BPCA) aggregates of up to 128 sub-micron sized grains (spherical and spheroidal with a possible size distribution) of various composition (silicates, organics, silicates core with organics mantle). The dependence of the linear polarization with the size parameter is shown to depend highly on the size and composition of the constitutive grains. Internal interactions induced by shape or orientation averaging of the grains may lessen this dependence, leading to results comparable to those observed on cometary dust for fluffy aggregates of grains with a size parameter in the 1.3–1.8 range. A size distribution of realistically shaped particles (aggregates of spheroids and larger spheroids) following a power law size distribution with a power index of - 3 , the smallest grains radius by 0.03– 0.2 μ m and the largest spheroids effective radius by 20 μ m , gives a very good fit to the Hale-Bopp observed phase curves. The best silicates–organics ratio ranges from about 50–75% organics and 25–50% silicates in volume considering the eventual presence of core-mantle grains.

Are TiC Grains a Carrier of the 21 Micron Emission Band Observed around Post–Asymptotic Giant Branch Objects?

The carrier of the 21 μm band observed in post-asymptotic giant branch (post-AGB) stars is examined. We analyze the infrared spectra of the TiC clusters measured by von Helden et al. in 2000 and determine the absorption efficiency Q in the 21 μm band. Using Q, we estimate the Ti/Si abundance ratios needed to realize the flux ratios of the 21 and 11 μm emission observed in the infrared spectra of the post-AGB stars exhibiting both 21 and 11 μm emission. In view of the nature of the TiC condensation by which TiC grains are quickly mantled by graphite, we calculate the emission spectra of the graphite-coated TiC grains and other possible types of core-mantle grains and compare with the observed spectra. Both the abundance and condensation considerations strongly suggest that TiC is an implausible carrier of the observed infrared 21 μm feature around carbon-rich post-AGB stars.

Composition, Structure, and Size Distribution of Dust in the Local Interstellar Cloud

The average properties of interstellar dust have previously been inferred from remote astronomical observations of interstellar extinction and from gas depletion measurements. In addition to recent high-resolution observations of gas absorption spectra, in situ measurements of dust in the solar system provide an alternative approach to deducing the properties of interstellar dust, in particular, those of dust in the Local Interstellar Cloud (LIC), in which the Sun resides. We constrain the composition and structure of dust in the LIC by the dust-phase elemental abundances derived from gas absorption measurements and by the dynamical behavior inferred from dust impact measurements. The elemental abundances of the LIC dust are consistent with core-mantle grains consisting of Mg-rich pyroxene and Mg-rich olivine with inclusions of troilite, Fe-rich kamacite, and corundum in the core and organic refractory compounds of C, N, and O in the mantle. The mass of the organic refractory mantle is comparable to the mass of the silicate core that is abundant in pyroxene compared to olivine. Taking into account these results, the dynamical behavior of the LIC dust in the solar system indicates that bare silicates and bare carbonaceous materials may be present as grains smaller than 10-17 kg. The LIC grains with mass exceeding 10-17 kg are most likely aggregates of submicron-sized silicate core, organic mantle grains. The mass distribution of dust in the LIC can be well explained by coagulation growth of core-mantle grains but is scarcely explained by severe destruction of grains in interstellar shocks.

Mid‐Infrared Spectropolarimetric Constraints on the Core‐Mantle Interstellar Dust Model

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 μm C–H and 9.7 μm 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 μm region and in the 9.7 μm region would be of great value to test the core-mantle dust model. The fact that the 3.4 μm feature is not polarized along the line of sight toward the Galactic center source IRS 7 is not yet sufficient to reject the core-mantle model because of the lack of spectropolarimetric observation of this region in the 9.7 μm region.

On the Light-Absorbing Surface Layer of Cometary Nuclei: I. Radiative Transfer

A small but increasing volume of observations of cometary nuclei has accumulated during the past two decades. This development is accelerating with upcoming space missions such as Stardust, Contour, and Rosetta. In response to the growing need for a theoretical understanding of optical properties of cometary nuclei, we have calculated synthetic reflectance spectra in the wavelength region 0.2–2.0 μm, photometric colors in the Johnson–Kron–Cousins UBVRI system, and visual geometric albedos for a large number of porous ice-dust mixtures with differing composition, regolith grain sizes, and grain morphologies, such as core-mantle grains, dense clusters of such grains, and large irregular particles with internal scatterers. The calculations are based on Mie theory, the discrete dipole approximation, Hapke theory, and a numerical solution to the equation of radiative transfer in particulate media. In addition, wavelength-integrated directional-hemispherical albedos and flux attenuation profiles in the regolith as functions of depth have been calculated in order to improve the energy budget and treatment of energy boundary conditions in thermal models of cometary nuclei. Our results are compared with spectra and colors of observed cometary nuclei. Our main conclusions are that only regolith consisting of relatively large core-mantle grains, or clusters of smaller core-mantle grains, is capable of reproducing the red colors seen in comets; that ice-dust mixtures actually can be darker than ice-free regolith in certain circumstances; and that solar radiation sometimes penetrates to a depth that is comparable to the region in which diurnal temperature variations occur.

Modeling of C stars with core/mantle grains: Amorphous carbon + SiC

A set of 45 dust envelopes of carbon stars has been modeled. Among them, 34 were selected according to their dust envelope class (as suggested by Sloan, Little-Marenin & Price, 1998) and 11 are extreme carbon stars. The models were performed using a code that describes the radiative transfer in dust envelopes considering core/mantle grains composed by an alpha-SiC core and an amorphous carbon (A.C.) mantle. In addition, we have also computed models with a code that considers two kinds of grains - alpha-SiC and A.C. - simultaneously. Core-mantle grains seem to fit dust envelopes of evolved carbon stars, while two homogeneous grains are more able to reproduce thinner dust envelopes. Our results suggest that there exists an evolution of dust grains in the carbon star sequence. In the beginning of the sequence, grains are mainly composed of SiC and amorphous carbon; with dust envelope evolution, carbon grains are coated in SiC. This phenomena could perhaps explain the small quantity of SiC grains observed in the interstellar medium. However, in this work we consider only alpha-SiC grains, and the inclusion of beta-SiC grains can perhaps change some of there results.

Dust Properties in the Local Interstellar Medium

Ulysses measurements yield reliable in-situdetection of large dust particles which stem from the interstellar medium (ISM) and which are not observed in interstellar extinction data. Both current models of large grains in the ISM: core-mantle grains as well as composite grains, are in agreement with dust properties implied by the Ulysses results. However, the size of particles detected by Ulysses still exceeds the size of the large grains that are predicted for the ISM.

On the Model of Dust in the Small Magellanic Cloud

We present here dust models for the Small Magellanic Cloud bar calculated for the first time with the regularization approach. A simple mixture of the core-mantle and/or composite grains (mostly made from silicates and organic refractory), together with silicon nanoparticles, is consistent with the following: (1) the observed extinction toward AzV 398 (a typical SMC bar sight line), (2) the elemental abundances, and (3) the strength of the interstellar radiation field. We predict the expected albedo and the asymmetry parameter of the models, which can be tested in future observations. The proposed dust models can also be tested by looking for the expected extended red emission.

The size distribution of dust grains in single clouds — II. The analysis of extinction using inhomogeneous grains

A B STR ACT The extinction curves of single clouds, seen towards the stars HD 147165, 179406 and 202904, have been modelled using various mixtures containing both the bare and inhomogeneous (composite and/or multilayer) grains. It has been shown that the models composed of the bare graphite and silicate grains together with the multilayer grains containing silicates, organic refractory and water ice, are more useful in explaining extinction under the reduced cosmic abundances. The models based on Mathis’ composite grains or on Greenberg & Li’s core-mantle grains can also provide quite good fits of the extinction and the measured scattering parameters, but still require an excessive amount of carbon which results in too large a C/O ratio. The inhomogeneous grains essentially contribute to the extinction in practically the whole wavelength range of our extinction curves. As a rule, such grains have quite wide size distributions, centred at around 100 nm, although graphite grains are mainly of small sizes.

The size distribution of dust grains in single clouds - II. The analysis of extinction using inhomogeneous grains

The extinction curves of single clouds, seen towards the stars HD 147165, 179406 and 202904, have been modelled using various mixtures containing both the bare and inhomogeneous (composite and/or multilayer) grains. It has been shown that the models composed of the bare graphite and silicate grains together with the multilayer grains containing silicates, organic refractory and water ice, are more useful in explaining extinction under the reduced cosmic abundances. The models based on Mathis' composite grains or on Greenberg & Li's core-mantle grains can also provide quite good fits of the extinction and the measured scattering parameters, but still require an excessive amount of carbon which results in too large a C/O ratio. The inhomogeneous grains essentially contribute to the extinction in practically the whole wavelength range of our extinction curves. As a rule, such grains have quite wide size distributions, centred at around 100 nm, although graphite grains are mainly of small sizes.