A hidden reservoir of Fe/FeS in interstellar silicates?

view Abstract Citations (3568) References (88) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Optical Properties of Interstellar Graphite and Silicate Grains Draine, B. T. ; Lee, H. M. Abstract The dielectric functions for graphite and astronomical silicate material are computed using available laboratory and astronomical data. It is noted that the magnetic dipole contribution to absorption in the infrared wavelengths can be important for conducting particles such as graphite. Formulas are given for evaluating electric and magnetic dipole cross-sections for small particles. Absorption cross-sections are evaluated for graphite and silicate particles with sizes between 0.003 and 1.0 microns, and wavelengths from 300 A to 1000 microns. On the basis of polarization profiles computed for both prolate and oblate graphite-silicate spheroids, it is concluded that interstellar silicate grain are predominantly oblate. Extinction curves are calculated for Mathis-Rumpl-Nordsieck graphite-silicate grain mixtures and are compared to observations. The model is found to be in good agreement with available infrared observations. Publication: The Astrophysical Journal Pub Date: October 1984 DOI: 10.1086/162480 Bibcode: 1984ApJ...285...89D Keywords: Granular Materials; Graphite; Interstellar Matter; Optical Properties; Silicates; Absorption Cross Sections; Infrared Astronomy; Opacity; Particle Interactions; Scattering Cross Sections; Astrophysics full text sources ADS | data products SIMBAD (6) Related Materials (1) Erratum: 1987ApJ...318..485D

To explain how cometary silicates crystallize yet still preserve volatile interstellar ices in their parent comets, we experimentally demonstrate the possibility of chemical-reaction-driven crystallization, which is called nonthermal crystallization, using laboratory-synthesized amorphous Mg-bearing silicate grains. Analog silicate grains ~50-100 nm in diameter covered with a carbonaceous layer consisting of amorphous carbon, CH 4, and other organics to a thickness of ~10-30 nm were used as models. The analog silicate grains crystallized via the direct flow of surface reaction energy, which is produced by the graphitization of the carbonaceous layer due to oxidation at room temperature in air, into the silicates. The experimental results imply that amorphous silicates are transformed into crystalline silicates as the grains leave the comet's surface, rather than as the comet was accreted 4.5 billion years ago. Thus, primordial ices and amorphous silicate grains are predicted to reside in most comets until they approach the Sun.

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 µm peak of both types of silicate grains amounts to 3000 cm2 g 1 and the ratio of 20 to 10 µm peaks amounts to about 0.5. Finally, open questions in connection with laboratory experiments are mentioned and recommendations for future experiments are given.

view Abstract Citations (164) References (40) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Laboratory Results on Millimeter-Wave Absorption in Silicate Grain Materials at Cryogenic Temperatures Agladze, N. I. ; Sievers, A. J. ; Jones, S. A. ; Burlitch, J. M. ; Beckwith, S. V. W. Abstract Absorption spectra of crystalline enstatite and forsterite grains and amorphous silicate grains synthesized by sol-gel reaction (size 0.1-1 μm) are measured between 0.7 and 2.9 mm wavelength (3.5-15 cm-1) at temperatures between 1.2 and 30 K. Some of the amorphous powders are precursors to forsterite (Mg2SiO4) and enstatite (MgSiO3). For the amorphous substances MgO SiO2, 2MgO SiO2, and MgO 2SiO2 at 20 K, the millimeter-wave mass opacity coefficients are found to be up to factors of 0.9, 3.5, and 11 times the Draine & Lee values usually adopted for interstellar silicate grains. The measured coefficients are found to depend on the powder production technique. Enstatite (MgSiO3) is part of pyroxene [(Mg, Fe)SiO3] and forsterite (Mg2SiO4) is part of olivine [(Mg, Fe)2SiO4], both of which are thought to be principal constituents of interstellar dust particles. The frequency dependence of the absorption coefficient follows a power law with a temperature-dependent exponent for all three amorphous silicates. Depending on the precise temperature, the power-law exponent ranges between a minimum value of 1.5 and a maximum of 2.5 for 2MgO SiO2 and MgO SiO2. At 20 K the index value is about 2. For the strongest absorber MgO 2SiO2, the power-law index has nearly a constant value of 1.2 over the entire temperature range; this value is significantly smaller than 2, the value normally adopted for interstellar dust. The frequency-dependent absorption coefficients per unit mass for Mg2SiO4 and MgSiO3 are about 4 times larger for the amorphous precursor grains than for the crystalline ones. The millimeter-wave absorption coefficient for amorphous grains first decreases with increasing temperature until about 20 K and then increases at higher temperatures. This unusual temperature-dependent property forms a significant part of the overall absorption at long wavelengths: the relative change is as large as 50% at 1 mm wavelength for 2MgO SiO2, 35% for MgO SiO2, and 14% for MgO 2SiO2. A weaker temperature-dependent change is observed for the crystalline forsterite and enstatite powders. The observed temperature dependence of the far-IR absorption coefficient in the powders is well described by a two-level population effect previously found for the ubiquitous low-lying tunnelling states in bulk glasses. Publication: The Astrophysical Journal Pub Date: May 1996 DOI: 10.1086/177217 Bibcode: 1996ApJ...462.1026A Keywords: INFRARED: ISM: CONTINUUM; ISM: DUST; EXTINCTION; METHODS: LABORATORY full text sources ADS |

Glass with embedded metal and sulfides (GEMS), the major components of chondritic-porous interplanetary dust particles (CP-IDPs), is one of the most primitive materials in the Solar System and may be analogous to the amorphous silicate dust observed in various astronomical environments. Mineralogical characteristics of GEMS should reflect their formation process and condition. In this study, synthetic experiments in the sulfur-bearing system of Fe–Mg–Si–O–S were performed with a systematic change in redox conditions using thermal plasma systems to reproduce the mineralogy and textures of GEMS. The resulting condensates were composed of amorphous silicates with Fe-bearing nano-inclusions. The Fe content and texture in the amorphous silicates as well as the mineral phases of the nanoparticles correlate with redox conditions. Fe dissolved in the amorphous silicate as FeO in oxidizing conditions formed Fe-metal nanoparticles in intermediate redox conditions, and gupeiite (Fe3 Si) nanoparticles in reducing conditions. In intermediate to reducing redox conditions, Fe-poor amorphous silicate formed a biphasic texture with Mg- and Si-rich regions, indicating liquid immiscibility during the melt phase. Most Fe-metal particles were surrounded by FeS and formed on the surface of amorphous silicate grains. Condensates produced in intermediate to slightly reducing redox conditions resemble GEMS in that they have similar mineral assemblages and chemical compositions to amorphous silicate, except that the Fe-metal grains are absent from the interior of the amorphous silicate grains. This textural difference can be explained by the sulfidation at high temperatures in this study, in contrast to sulfidation occurring at low temperatures in the presence of H2 in natural GEMS formation. Based on the two-liquid structures observed in the experimental products and in GEMS, also recognized in infrared spectra, we propose that GEMS condensed as silicate melt under limited redox conditions followed by incorporation of multiple metal grains into the silicate melt or by aggregation of coreshell structured grains before sulfidation of the metallic iron. Condensates produced in oxidizing conditions are similar to GEMS-like material in the matrices of primitive carbonaceous chondrite meteorites, indicating the possibility that they form by direct condensation from nebula gas in relatively oxidizing conditions compared to GEMS.

Reports on the detection of carbonates in planetary nebulae (PNe) and protostars have suggested the existence of a mechanism that produces these compounds in stellar winds and outflows. A subsequent laboratory study has reported a possible mechanism by presenting the non-thermodynamic-equilibrium (TE), gas-phase condensation of amorphous silicate grains with amorphous calcium carbonate inclusions. The authors concluded that water vapor was necessary for the formation of the carbonates. We present a laboratory study with pulsed laser ablation of a MgSi target in O2 and CO2 gases and report, in the absence of water vapor, the non-TE, gas-phase condensation of amorphous carbonated magnesium silicate dust. It consists of amorphous silicate grains with the formula MgSiO3, which comprise carbonate groups homogeneously dispersed in their structure. The IR spectra of the grains show the characteristic bands of amorphous silicates and two bands at ∼6.3 and ∼7.0 μm, which we assign to the carbonate groups. The silicate bands are not significantly affected at an estimated Si:C ratio of 9:1–9:2. Such grains could form in winds and outflows of evolved stars and PNe if C atoms are present during silicate condensation. Additionally, we find that Lyα radiation dissociates the carbonate groups at the surface of the carbonated silicate grains and we estimate the corresponding photodissociation cross section of (0.04 ± 0.02) ×10−16 cm2. Therefore, photodissociation would limit the formation of carbonate groups on grains in winds and outflows of stars emitting vacuum ultraviolet photons, and the carbonates observed in protostars have not formed by gas-phase condensation.

Amorphization of crystalline olivine to glass with a pyroxene composition is well known from high-energy irradiation experiments. This report is on the first natural occurrence of this process preserved in a chondritic aggregate interplanetary dust particle. The Fe-rich olivine grain textures and compositions and the glass grain compositions delineate this transformation that yielded glass with Fe-rich pyroxene compositions. The average glass composition, (Mg, Fe)3Si2O7, is a serpentine-dehydroxylate with O/Si = 3.56 ± 0.25, (Mg+Fe)/Si = 1.53 ± 0.24, and Mg/(Mg+Fe) = 0.74 ± 0.1. These measured atomic ratios match the ratios that have been proposed for amorphous interstellar silicate grains very well, albeit the measured Mg/(Mg+Fe) ratio is lower than was proposed for amorphous interstellar silicate grains, Mg/(Mg+Fe) > 0.9.

Context. Interstellar silicate grains are thought to be amorphized by interaction with high- and low-energy particle interactions in astrophysical environments. In addition, low energy (a few keV) particles will implant atoms within the grains.Aims. In this paper we experimentally investigate the consequence of the implantation of H+ at low irradiation energies into analogues of interstellar silicate grains, and look for the formation of hydroxyl radicals within the silicate matrix.Methods. Thin amorphous silicate films (~100 nm) were sequentially irradiated with H+ ions at low energies (3.5, 2.5 and then 1.5 keV) ensuring an implantation of the ions through the full depth of the films. The fluences used, 3 × 1016 , 1017 and 3 × 1017 H+ /cm2 , are compatible with those expected in shocks in the interstellar medium. We used infrared spectroscopy to monitor and quantify the OH band evolution after irradiation. In order to distinguish the newly formed OH groups from those originating from unavoidable atmospheric contamination, the D/H depth ratios were measured with a NanoSIMS ion microprobe.Results. An increase in the OH band strength in the infrared spectra after irradiation reveals the formation of OH bonds within the irradiated silicate thin films. NanoSIMS measurements of the D/H signature in the region of ion implantation show that the newly-formed OH groups make up about 40% of the observed OH band in the IR, the rest are due to an atmospheric hydroxylation of the sample. Only about 2% of the incident ions lead to OH bond formation and, at most, the irradiated silicates retain about 3% of the incident protons as OH groups within their structure. Conclusions. Our laboratory experimental simulations show a possible production and storage of hydroxyl radicals in amorphous laboratory silicates. In the astrophysical context, such OH radicals, strongly bonded to pre-accretion material, could constitute a non negligible reservoir of -OH, thus water. These experimental results allow us to revisit and reinstate the hypothesis of a possible “wet” accretion of the telluric planets early in the history of the formation of the Solar System.

The composition of silicate dust in the diffuse interstellar medium and in protoplanetary disks around young stars informs our understanding of the processing and evolution of the dust grains leading up to planet formation. An analysis of the well-known 9.7 μm feature indicates that small amorphous silicate grains represent a significant fraction of interstellar dust and are also major components of protoplanetary disks. However, this feature is typically modeled assuming amorphous silicate dust of olivine and pyroxene stoichiometries. Here, we analyze interstellar dust with models of silicate dust that include non-stoichiometric amorphous silicate grains. Modeling the optical depth along lines of sight toward the extinguished objects Cyg OB2 No. 12 and ζ Ophiuchi, we find evidence for interstellar amorphous silicate dust with stoichiometry intermediate between olivine and pyroxene, which we simply refer to as “polivene.” Finally, we compare these results to models of silicate emission from the Trapezium and protoplanetary disks in Taurus.

Stardust is newly-formed in the ejected shells of gas that surround stars towards the end of their lives. Observations of the thermal emission from this dust, which is at relatively low temperatures ( T = 50–200 K), in the circumstellar shells around these stars indicate that the dust consists of both amorphous and crystalline materials. The observed solid phases include: almost pure crystalline Mg-rich silicates (forsterite and clinoenstatite), amorphous silicates, diopside, spinel, oxides (corundum and Fe 0.9 Mg 0.1 O), and also carbon-rich solids such as: (hydrogenated) amorphous carbons, aromatic hydrocarbons and silicon carbide. Crystalline grains with isotopic signatures that indicate that they formed around evolved stars, and that therefore pre-date the formation of the solar system ( e.g. , the pre-solar silicate, nanodiamond, silicon carbide, graphite, corundum, spinel, hibonite, titanium carbide and silicon nitride grains), have now been extracted from primitive meteorites. Pre-solar forsterite and amorphous silicate grains have also been extracted from interplanetary dust particles. The dust formed around evolved stars is ejected into the surrounding interstellar medium by the relatively benign effects of stellar winds, where it is subject to stochastic and violent processing in fast supernova-generated shock waves. In this medium between the stars all the silicate dust appears to be completely amorphous and it is generally thought that dust processing, via ion irradiation/implantation in shocks and/or by cosmic rays, leads to the amorphisation of the crystalline silicate grains that were formed around the evolved stars. The formation of the dust, in circumstellar environments and subsequently ejected into the interstellar medium, is thus balanced by destructive processes that erode and eventually destroy it. This paper introduces the subject of interstellar, circumstellar and pre-solar dust composition (Astromineralogy), discusses where the dust comes from, how it evolves and what its eventual fate might be.

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 to compute the shape of the 20 mu silicate feature and to compare this with observations. By using realistic particle shapes we are, for the first time, able to derive the magnesium fraction in interstellar silicates. We find that the interstellar silicates are highly magnesium rich (Mg/(Fe+Mg)>0.9) and that the stoichiometry lies between pyroxene and olivine type silicates. This composition is not consistent with that of the glassy material found in GEMS in interplanetary dust particles indicating that these are, in general, not unprocessed remnants from the interstellar medium. Also, we find a significant fraction of SiC (~3%). We discuss the implications of our results for the formation and evolutionary history of cometary and circumstellar dust. We argue that the fact that crystalline silicates in cometary and circumstellar grains are almost purely magnesium silicates is a natural consequence of our findings that the amorphous silicates from which they were formed were already magnesium rich.

We construct a theoretical model for low-temperature crystallization of amorphous silicate grains induced by exothermic chemical reactions. As a first step, the model is applied to the annealing experiments, in which the samples are (1) amorphous silicate grains and (2) amorphous silicate grains covered with an amorphous carbon layer. We derive the activation energies of crystallization for amorphous silicate and amorphous carbon from the analysis of the experiments. Furthermore, we apply the model to the experiment of low-temperature crystallization of an amorphous silicate core covered with an amorphous carbon layer containing reactive molecules. We clarify the conditions of low-temperature crystallization due to exothermic chemical reactions. Next, we formulate the crystallization conditions so as to be applicable to astrophysical environments. We show that the present crystallization mechanism is characterized by two quantities: the stored energy density Q in a grain and the duration of the chemical reactions τ. The crystallization conditions are given by Q>Qmin and τ < τcool regardless of details of the reactions and grain structure, where τcool is the cooling timescale of the grains heated by exothermic reactions, and Qmin is minimum stored energy density determined by the activation energy of crystallization. Our results suggest that silicate crystallization occurs in wider astrophysical conditions than hitherto considered.

Amorphous Mg-bearing silicate grains, which were produced by the coalescence between MgO and SiOx smoke particles, were crystallized to forsterite (Mg2SiO4) by electron-beam irradiation in a transmission electron microscope at room temperature. The crystallization induced by electron beams was accelerated by the presence of CH4 adsorbed on the surface and incorporated interior of the grains. This experimental result implies the possibility of low-temperature crystallization in a silicate carbon star. In the case of binary stars, since the materials that flow from the stars stationarily exist around the star, the formed amorphous silicate grains will be irradiated by electrons from the star for a long duration. As a result, a significant amount of crystalline silicates can be produced.

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, amorphous silicate grains are efficient catalysts of H2 formation within a temperature range that is relevant to diffuse interstellar clouds. The results also indicate that the hydrogen molecules are thermalized with the surface and desorb with low kinetic energy. Thus, they are unlikely to occupy highly excited states.

We investigate the composition of interstellar grains along the line of sight toward Zeta Ophiuchi, a well-studied environment near the diffuse-dense cloud transition. A spectral decomposition analysis of the solid-state absorbers is performed using archival spectroscopic observations from the Spitzer Space Telescope and Infrared Space Observatory. We find strong evidence for the presence of sub-micron-sized amorphous silicate grains, principally comprised of olivine-like composition, with no convincing evidence of H2O ice mantles. However, tentative evidence for thick H2O ice mantles on large (a ~ 2.8 microns) grains is presented. Solid-state abundances of elemental Mg, Si, Fe, and O are inferred from our analysis and compared to standard reference abundances. We find that nearly all of elemental Mg and Si along the line of sight are present in amorphous silicate grains, while a substantial fraction of elemental Fe resides in compounds other than silicates. Moreover, we find that the total abundance of elemental O is largely inconsistent with the adopted reference abundances, indicating that as much as ~156 ppm of interstellar O is missing along the line of sight. After taking into account additional limits on the abundance of elemental O in other O-bearing solids, we conclude that any missing reservoir of elemental O must reside on large grains that are nearly opaque to infrared radiation.