Stability of amorphous circumstellar silicate grains

Within the framework of classical nucleation theory we have shown that Mg₂SiO₄ grains (radii of nearly 0.1 μm) which are likely candidates as condensation products in circumstellar shells around late-type stars must condense at about 750 K to remain amorphous. If the condensation temperature is about 1000 K the newly formed grains must be rather small to rest in the amorphous state during cooling. Submicron sized MgSiO₃grains are entire stable against crystallization for T_cond ≤ 1000 K.     

Interstellar depletion anomalies and ionization potentials

Satellite observations indicate that (1) most elements are depleted from the gas phase when compared to cosmic abundances, (2) some elements are several orders of magnitude more depleted than others, and (3) these depletions vary from cloud to cloud. Since the most likely possibility is that the missing atoms are locked into grains, depletions occur either by accretion onto core particles in interstellar clouds or earlier, during the period of primary grain formation. If the latter mechanism is dominant, then the most important depletion parameter is the condensation temperature of the elements and their various compounds. However, this alone is not sufficient to explain all the observed anomalies. It is shown that electrostatic effects—under a wide variety of conditions—can enormously enhance the capture cross-section of the grain. It is suggested that this mechanism can also account for such anomalies as the apparent overabundance of the alkali metals in the gas phase.     

The formation of plum-pudding interstellar grains

A calculation is given of the growth of multicored (plum-pudding) interstellar grains as the result of simultaneous condensation of an icy matrix on small cores combined with coagulation of the resulting grains induced by radiation pressure. It is shown that starting with cores of radius 10^–6 cm in normal gas clouds generally gives rise to plum-pudding grains of radius 10^–5 cm after 10⁸–10⁹ yr.     

Column densities of interstellar matter in the direction of the starκ Cas (B1 Ia)

We have studied the interstellar column densities in the direction of the star Cas with the IUE high resolution SWP spectrum so as to see the relations of element depletions with condensation temperature and compare our results with the interstellar column densities in the directions of the other B-type stars obtained with the COPERNICUS satellite.Based on the observations made by the International Ultraviolet Explorer (IUEO, and collected at the Villafranca Satellite Tracking Station of European Space Agency.     

On coronal temperatures,temperature gradients and compositions

Average solar wind properties at 1 AU either alone or together with the electron density distribution are used to obtain or review some results that relate coronal temperatures, temperature gradients, and compositions. Measured values of the temperature (T) and the temperature gradient parameter ( = -d ln T/d ln r) are used to find compositions that satisfy the equations used to obtain the results. The total energy equation may be satisfied if the thermal conductivity is reduced by considerable depletions of H⁺ in the corona. The electron energy equation only gives information on coronal compositions that are coupled with d/d ln r. The hydrostatic approximation (momentum equation) for the electron density distribution also appears to require considerable depletions of H⁺ in the corona. Results from the integrated momentum equation for the solar wind support the hydrostatic results; together, they give some information on the minimum value of in the outer corona. Some changes in assumptions or values of parameters that may modify these interpretations are discussed.     

Refractory forsterite in primitive meteorites: Condensates from the solar nebula?

Abstract— All groups of chondritic meteorites contain discrete grains of forsteritic olivine with FeO contents below 1 wt% and high concentrations of refractory elements such as Ca, Al, and Ti. Ten such grains (52 to 754 μg) with minor amounts of adhering matrix were separated from the Allende meteorite. After bulk chemical analysis by instrumental neutron activation analysis (INAA), some samples were analyzed with an electron microprobe and some with an ion microprobe. Matrix that accreted to the forsterite grains has a well‐defined unique composition, different from average Allende matrix in having higher Cr and lower Ni and Co contents, which implies limited mixing of Allende matrix. All samples have approximately chondritic relative abundances of refractory elements Ca, Al, Sc, and rare‐earth elements (REE), although some of these elements, such as Al, do not quantitatively reside in forsterite; whereas others (e.g., Ca) are intrinsic to forsterite. The chondritic refractory element ratios in bulk samples, the generally high abundance level of refractory elements, and the presence of Ca‐Al‐Ti‐rich glass inclusions suggest a genetic relationship of refractory condensates with forsteritic olivine. The Ca‐Al‐Ti‐rich glasses may have acted as nuclei for forsterite condensation. Arguments are presented that exclude an origin of refractory forsterite by crystallization from melts with compositions characteristic of Allende chondrules: (a) All forsterite grains have CaO contents between 0.5 and 0.7 wt% with no apparent zoning, requiring voluminous parental melts with 18 to 20 wt% CaO, far above the average CaO content of Allende chondrules. Similar arguments apply to Al contents. (b) The low FeO content of refractory forsterite of 0.2‐0.4 wt% imposes an upper limit of ～1 wt% of FeO on the parental melt, too low for ordinary and carbonaceous chondrule melts, (c) The Mn contents of refractory forsterites are between 30 to 40 ppm. This is at least one order of magnitude below the Mn content of chondrule olivines in all classes of meteorites. The observed Mn contents of refractory forsterite are much too low for equilibrium between olivine and melts of chondrule composition, (d) As shown earlier, refractory forsterites have O‐isotopic compositions different from chondrules (Weinbruch et al., 1993a). Refractory olivines in carbonaceous chondrites are found in matrix and in chondrules. The compositional similarity of both types was taken to indicate that all refractory forsterites formed inside chondrules (e.g., Jones, 1992). As refractory forsterite cannot have formed by crystallization from chondrule melts, we conclude that refractory forsterite from chondrules are relic grains that survived chondrule melting and probably formed in the same way as refractory forsterite enclosed in matrix. We favor an origin of refractory forsterite by condensation from an oxidized nebular gas.     

Grain destruction in interstellar shock waves

Interstellar shock waves can erode and destroy grains present in the shocked gas, primarily as the result of sputtering and grain-grain collisions. Uncertainties in current estimates of sputtering yields are reviewed. Results are presented for the simple case of sputtering of fast grains being stopped in cold gas. An upper limit is derived for sputtering of refractory grains in C-type MHD shocks: shock speedsv _s 50 km s^–1 are required for return of more than 30% of the silicate to the gas phase. Sputtering can also be important for removing molecular ice mantles from grains in two-fluid MHD shock waves in molecular gas. Recent estimates of refractory grain lifetimes against destruction in shock waves are summarized, and the implications of these short lifetimes are discussed.     

Structural clues to the origin of refractory metal alloys as condensates of the solar nebula

Abstract– Alloys of the refractory metals Re, Os, W, Ir, Ru, Mo, Pt, and Rh with small amounts of Fe and Ni are predicted to be one of the very first high‐temperature condensates in a cooling gas of solar composition. Recently, such alloy grains were found in acid‐resistant residues of the Murchison CM2 chondrite. We used focused ion beam (FIB) preparation to obtain electron‐transparent sections of 15 submicrometer‐sized refractory metal nuggets (RMNs) from the original Murchison residue. We studied their crystallography, microstructures, and internal compositional variations using transmission electron microscopy (TEM). Our results show that all RMNs studied have hexagonal close‐packed (hcp) crystal structures despite considerable variations of their bulk compositions. Crystallographic superstructures or signs of spinodal decomposition are absent and defect microstructures are scarce. Internally, RMNs are compositionally homogeneous, with no evidence for zoning patterns or heterogeneities due to exsolution. Many RMNs show well‐defined euhedral crystal shapes and all are nearly perfect single crystal. Our findings are consistent with a direct (near‐) equilibrium condensation of refractory metals into a single alloy at high temperature in the solar nebula as predicted by current condensation models. We suggest that this alloy is generally hcp structured due to an extended ε‐phase field in the relevant multicomponent alloy system. The high degree of structural perfection and compositional homogeneity is attributed to high defect energies, high formation temperatures, slow cooling rates, small grain sizes, and rapid internal diffusion.     

Birth of the presolar nebula: The sequence of condensation revealed in the Allende meteorite

This work applies the well-known supernova-trigger hypothesis for solar system formation to explain in detail many properties of the Allende meteorite. The Allende carbonaceous chondrite meteorite is an assemblage of millimetre- to centimetre-sized Ca-Al-rich inclusions (CAI's), fine-grained alkali-rich spinel aggregates, amoeboid olivine aggregates, olivine chondrules and sulfide chondrules set in an extremely fine-grained black matrix. Detailed isotopic, chemical and textural properties show that these components formed in the above order as independent cosmic grains. Some CAI's containmicron-sized metal nuggets in which the normally incompatible refractory (Mo, Re, W) and platinum group (Pt, Os, Ir, Ru) metals are alloyed together in approximately cosmic proportions, suggesting that these nuggets also condensed as cosmic grains.From the consistent pattern of enclosure of earlier components on the above list within later ones, it appears that in the environment where these materials formed, condensation moved inexorably in the direction of increasing olivine and decreasing refractory element and¹⁶O content (from 4% excess¹⁶O to normal terrestrial oxygen isotopic composition). Condensation sequences are all short and incomplete, from which it is concluded that condensing materials were soon separated from the condensing environment and isolated until all were brought together in a final snowstorm of fine-grained, olivine crystals constituting the meteorite matrix.These major properties can be accounted for in a model in which a supernova remnant (SNR) in the snowplow phase, whose oxygen was initially pure¹⁶O, pushes into a dark interstellar cloud. In the model, condensation of CAI's begins in the SNR shell when it has been diluted with 2500 times its mass of matter from the cloud, which also in part explains the rarity of observed isotopic anomalies in CAI's. The retardation of the SNR by the cloud propels condensed grains ahead toward the cloud under their own momentum. Continuing dilution by the cloud and continuing removal of the most refractory elements in grains can explain the evolving patterns of fractionation and depletion of refractory elements, including REE's, in successive condensates. Features such as rims on CAI's and concentric zonation of fine-grained aggregates can also be satisfied in the model. A presolar origin and a short ( 10 000 years) formation time for inclusions in carbonaceous chondrites are major implications of the model.Invited contribution to the Proceedings of a Workshop onThermodynamics and Kinetics of Dust Formation in the Space Medium held at the Lunar and Planetary Institute, Houston, 6–8 September, 1978.     

Processes affecting the composition and structure of silicate grains in cometary nuclei

Nucleation is a non-equilibrium process: the products of this process are seldom the most thermodynamically stable condensates but are instead those which form fastest. It should therefore not be surprising that grains formed in a circumstellar outflow will undergo some degree of metamorphism if they are annealed or are exposed to a chemically active reagent. Metamorphism of refractory particles continues in the interstellar medium (ISM) where the driving forces are sputtering by cosmic ray particles, annealing by high energy photons and grain destruction in supernova generated shocks. Studies of the depletion of the elements from the gas phase of the interstellar medium tell us that if grain destruction occurs with high efficiency in the ISM, then there must be some mechanism by which grains can be formed in the ISM. Various workers have shown that refractory mantles could form on refractory cores by radiation processing of organic ices. A similar process may operate to produce refractory inorganic mantles on grain cores which survived the supernova shocks. Most grains in a cloud which collapses to form a star will be destroyed; many of the surviving grains will be severely processed. Grains in the outermost regions of the nebula may survive relatively unchanged by thermal processing or hydration. It is these grains which we hope to find in comets. However, only those grains encased in ice at low temperature can be considered pristine since a considerable degree of hydrous alteration might occur in a cometary regolith if the comet enters the inner solar system. Some discussion of the physical, chemical and isotopic properties of a refractory grain at each stage of its life cycle will be attempted based on the limited laboratory data available to date. Suggestions will be made concerning types of experimental data which are needed in order to better understand the processing history of cosmic dust.     

The formation of condensed carbon particles in the atmospheres of white dwarfs

The existence of condensed carbon in the form of liquid droplets and graphite grains is found in white dwarf atmospheres with parametersg=10⁸ cm s^–2, H/He10^–3, andT _eff6000 K on the basis of model atmospheres techniques. It is shown that the condensation layers are dynamically stable and, consequently, that white dwarfs cannot supply the condensed particles to the interstellar medium. Possible observable effects are considered.     

The Composition of Saturn's Rings

A composite spectrum between 0.30 and 4.05 μm of Saturn's rings is analyzed using the Shkuratov scattering theory (Shkuratov et al. 1999, Icarus137, 235-246). Several types of surface and composition are discussed. We demonstrate that both the strong reddening over the interval 0.3-0.7 μm and the water ice absorption features are well reproduced by an intimate (“salt-and-pepper”) mixture of four coarse particles of two different materials: 93% are grains (typical sizes of 10, 200, and 2000 μm) of water ice containing a few percent of refractory organic solid (tholin) impurities within their bulk, and 7% are coarse grains of a dark material (amorphous carbon). The cosmogenic implications of the inferred composition are discussed.     

Rate of H2 formation on amorphous grains

The rate of formation of molecular hydrogen from hydrogen atoms adsorbed on grains is analyzed, assuming that the grains are single crystals, polycrystalline or amorphous. On polycrystalline grains, and on graphite platelets, this rate could be orders of magnitude lower than on single crystal grains. The same is true for amorphous grains because there, at low temperatures, only atoms absorbed on neighboring sites can form molecules. Suitable formulae are derived and compared with the classical results for single crystal grains. Quantitative results are given for crystalline and amorphous ice, but with small changes these should also be valid for other solids. The rates for amorphous grains can approximate, within a factor of 10 or so, those for crystalline grains if the density of H atoms is high and the density of H₂ molecules is low and only when the temperature of the grains satisfies a relation which for ice and graphite leads to a value in the proximity of 15–17 K. This maximum rate occurs only a degree or so above the temperature at which the grains are totally covered by an H₂ layer and the reaction ceases. Furthermore, for a constant number density of grains, the rates on amorphous grains are second order while those on crystalline grains are first order. Both these circumstances predict amorphous grains to lead to H₂ clouds with irregular and sharply delineated features in contrast to more uniform clouds formed on crystalline grains.     

A refractory Ca-SiO-H2-O2 vapor condensation experiment with implications for calciosilica dust transforming to silicate and carbonate minerals

Condensates produced in a laboratory condensation experiment of a refractory Ca-SiO-H₂-O₂ vapor define four specific and predictable deep metastable eutectic calciosilica compositions. The condensed nanograins are amorphous solids, including those with the stoichiometric CaSiO₃ pyroxene composition. In evolving dust-condensing astronomical environments they will be highly suitable precursors for thermally supported, dust-aging reactions whereby the condensates form more complex refractory silicates, e.g., diopside and wollastonite, and calcite and dolomite carbonates. This kinetically controlled condensation experiment shows how the aging of amorphous refractory condensates could produce the same minerals that are thought to require high-temperature equilibrium condensation. We submit that evidence for this thermal annealing of dust will be the astronomical detection of silica (amorphous or crystalline) that is the common, predicted, by-product of most of these reactions.     

Suprathermal grains: On intergalactic magnetic fields

Charged dust grains of radiia3×10^–63×10^–5 cm may be driven out of the galaxy due to radiation pressure of starlight. Once clear of the main gas-dust layer, dust grains may then escape into intergalactic space. Such grains are virtually indestructible-being evaporated only during galaxy formation. The dust grains, once injected into the intergalactic medium, may acquire suprathermal energy, thus suprathermal grains in collision with magnetized cloud by the Fermi process. In order to attain relativistic energy, suprathermal grains have to move in and out (scattering) of the magnetic field of the medium. It is now well established that high energy cosmic rays are of the order 10²⁰ eV or more. We have speculated that these high energy (>-10¹⁸ eV) cosmic ray particles are charged dust grains, of intergalactic origin. This is possible only if there exists a magnetic field in the intergalactic medium.     

Circumstellar dust and related phenomena at phase 0.8 in Miras

SeveralM-type Mira variables (o Cet, RT Cyg) exhibit a sudden polarization increase near phaseP=0.8 of visual light period, whereas the position angle of the linearly polarized component remains almost unchanged. The degree of polarization is observed to be high (6%) in the violet and blue region and to decrease slowly toward the infrared. In connection with appearance of the 10 m band this indicates the presence of circumstellar silicate grains of diameters varying between 100 nm and 1 m.In dirty silicate particles, i.e. SiO₂ + metal ion admixture, the colour centres can be activated by absorbing of energetic photons, which results in increasing imaginary part of the refractive index. As showed by Svat of (1980), this mechanism can explain the polarization rise of light transmitted through a layer of aligned nonspherical dirty silicate glassy grains. From the appearance of the Balmer emission lines in the lower atmospheric layer, SiO maser activity disruption and the other phenomena occuring atP0.8 it is concluded, that the source of the activating short wavelength radiation can lie in the ionization front ahead of the expanding shock wave (assumed in the model of Miras atmosphere by Hinkle and Barnes, 1979) and in other eruptive phenomena in Miras. The same mechanism can be effective in Nova outbursts, RS CVn flares and T Tau sudden luminosity variations. The role of magnetic field in the Miras atmosphere is discussed.     

Small grains in nebulae: The case of IC 418

We present near infrared maps of the planetary nebula IC 418 taken at 5 arc sec spatial resolution. These show an extremely compact source structure at 1.65 m, and very much broader and more elliptical emission distributions at 2.2 and 3.6 m. The possibility that the near infrared continuum derives from small grains with low thermal capacity is discussed, and synthetic spectra are derived. These indicate a very much slower variation of flux with wavelength than would be deduced for grains at unitary temperature, and imply a near infrared continuum which is significantly cooler than maximum grain temperatures. Fits to the spectrum of IC 418 suggest that peak grain temperatures may approachT _p010³ K (for ^–2), for instance, compared to the observed colour temperature of 650 K.Finally, the relevant grains are required to be refractory and, given the large C/O ratio in this nebula, probably consist of graphite. Their mass is also required to be small, and is probably of order 2×10^–7 M . This compares with the probable total mass for large dust grains of 10^–4 M . Although they are both small and warm, such grains would be expected to persist over the lifetime of the nebula, and may also be responsible for various NIR band emission features.     

Experimental investigations of astronomically important interstellar silicates

In this paper methods and results of laboratory experiments for the investigation of the silicate component of interstellar dust are reviewed. In Section 2 basic properties expected for astronomically important interstellar silicates (AIIS) are discussed. Chemical constraints coming from the abundance of elements, from the depletion in the interstellar gas and from theoretical calculations of the condensation processes point to magnesium silicates. Some basic structural properties of interstellar silicates, the expected high degree of lattice disorder and spectral features expected for interstellar silicate grains are discussed. In Section 3 a review on laboratory investigations of AIIS is given. Physical and chemical methods for producing amorphous silicates are summarized. Important measurements of optical data for AIIS are listed. Spectral characteristics of amorphous silicates produced in order to simulate the interstellar dust silicates are discussed. From the comparison of the observed MIR silicate bands with those of the experimentally produced silicates it is concluded that at least two types of dust silicates exist in interstellar space: molecular-cloud silicate (suggested to be of pyroxene-type) and late-type star silicate (suggested to be of olivine-type). The mass absorption coefficient at the 10 m peak of both types of silicate grains amounts to 3000 cm² g^–1 and the ratio of 20 to 10 m peaks amounts to about 0.5. Finally, open questions in connection with laboratory experiments are mentioned and recommendations for future experiments are given.Paper presented at a Workshop on The Role of Dust in Dense Regions of Interstellar Matter, held at Georgenthal, G.D.R., in March 1986.     

Formation of H2 on amorphous ice grains and their importance for planetary atmospheres

The mechanism and the rate of formation of H₂ molecules from adsorbed H atoms on interstellar ice grains (or on ice coated non-icy grains) are investigated assuming that the ice is not crystalline but amorphous. Using the available theory and experimental data it is concluded that, in contrast to crystalline grains, the mobility of the adsorbed atoms on amorphous grains at temperatures of 10–20 K is exceedingly low so that the controlling factor is the probability that two H atoms are accidentally adsorbed within a site or two of each other. The rate of H₂ formation on ice grains per unit volume is much lower than previously estimated and is very sensitive to temperature. This conclusion applies not only to pure amorphous ice investigated here, but also to impure ice and to other grains (carbon or silicates) which would not be crystalline, such as graphite, but may be highly imperfect or actually amorphous aggregates of atoms or molecules.It is further shown that the presence of amorphous ice and clathrate grains in the early solar system would play a significant role in our understanding of the compositional anomalies in the Earth's atmosphere. The lifetime of these grains would be strongly affected by the absence of crystallinity.Invited contribution to the Proceedings of a Workshop onThermodynamics and Kinetics of Dust Formation in the Space Medium held at the Lunar and Planetary Institute, Houston, 6–8 September, 1978.     

Vapor‐condensed phase processes in the early solar system

Abstract– Equilibrium thermodynamic calculations of the sequence of condensation of phases from a cooling gas of solar composition at total pressures thought to have prevailed in the inner part of the solar nebula successfully predict the primary mineral assemblages of refractory inclusions in CM2 and CV3 chondrites. Many refractory inclusions in CM2 chondrites contain a relatively SiO₂‐poor assemblage (spinel, hibonite, grossite, perovskite, corundum) that represents a high‐temperature stage of condensation, and some may be pristine condensates that escaped later melting. Compact Type A and Type B refractory inclusions, consisting of spinel, melilite, perovskite, Ca‐rich clinopyroxene ± anorthite, in CV3 chondrites are more SiO₂‐rich and equilibrated with the solar nebular gas at a slightly lower temperature. Textures of many of these objects indicate that they underwent melting after condensation, crystallizing into the same phase assemblage as their precursors. The Ti³⁺/Ti⁴⁺ ratio of their pyroxene indicates that this process occurred in a gas whose oxygen fugacity () was approximately 8.5 log units below that of the iron‐wüstite buffer, making them the only objects in chondrites known to have formed in a system whose composition was close to that of the sun. Relative to CI chondrites, these inclusions are uniformly enriched in a group of elements (e.g., Ca, REE, Zr, Ta, Ir) that are chemically diverse except for their high condensation temperatures in a system of solar composition. The enrichment factor, 17.5, can be interpreted to mean that these objects represent either the first 5.7 wt% of the condensable matter to condense during nebular cooling or the residue after vaporization of 94.3% of a CI chondrite precursor. The Mg and Si isotopic compositions of Types A and B inclusions are mass‐fractionated by up to 10 and 4 ‰/amu, respectively. When interpreted in terms of Rayleigh fractionation during evaporation of Mg and Si from the inclusions while they were molten, the isotopic compositions imply that up to 60% of the Mg and up to 25% of the Si were evaporated, and that approximately 80% of the enrichment in refractory (CaO+Al₂O₃) relative to more volatile (MgO+SiO₂) in the average inclusion is due to initial condensation and approximately 20% due to subsequent evaporation. The mineralogical composition, including the Ti³⁺/Ti⁴⁺ ratio of the pyroxene, in Inti, a particle sampled from Comet Wild 2 by the Stardust spacecraft, is nearly identical to that of a Type B inclusion, indicating that comets contain not only the lowest‐temperature condensates in the form of ices but the highest‐temperature condensates as well. The FeO/(FeO+MgO) ratios of olivine and pyroxene in the matrix and chondrules of carbonaceous and ordinary chondrites are too high to be made in a system of solar composition, requiring s only 1 or 2 log units below iron‐wüstite, more than 10⁵ times higher than that of a solar gas. Various ways have been devised to generate cosmic gases sufficiently oxidizing to stabilize significant FeO in olivine at temperatures above those where Fe‐Mg interdiffusion in olivine ceases. One is by vertical settling of dust toward the nebular midplane, enriching a region in dust relative to gas. Because dust is enriched in oxygen compared to carbon and hydrogen relative to solar composition, a higher results from total vaporization of the region, but the factor by which theoretical models have so far enriched the dust is 10 times too low. Another is by transporting icy bodies from the outer part of the nebula into the hot, inner part where vaporization of water ice occurs. Not only does this method fail to make the needed by a factor of 30–1000 but it also ignores simultaneous evaporation of carbon‐bearing ices that would make the even lower.