Large Silicate Grains Research Articles

Detrital remanent magnetization of single-crystal silicates with magnetic inclusions: constraints from deposition experiments

SUMMARY Quasi-linear field-dependence of remanence provides the foundation for sedimentary relative palaeointensity studies that have been widely used to understand past geomagnetic field behaviour and to date sedimentary sequences. Flocculation models are often called upon to explain this field dependence and the lower palaeomagnetic recording efficiency of sediments. Several recent studies have demonstrated that magnetic-mineral inclusions embedded within larger non-magnetic host silicates are abundant in sedimentary records, and that they can potentially provide another simple explanation for the quasi-linear field dependence. In order to understand how magnetic inclusion-rich detrital particles acquire sedimentary remanence, we carried out depositional remanent magnetization (DRM) experiments on controlled magnetic inclusion-bearing silicate particles (10–50 μm in size) prepared from gabbro and mid-ocean ridge basalt samples. Deposition experiments confirm that the studied large silicate host particles with magnetic mineral inclusions can acquire a DRM with accurate recording of declination. We observe a silicate size-dependent inclination shallowing, whereby larger silicate grains exhibit less inclination shallowing. The studied sized silicate samples do not have distinct populations of spherical or platy particles, so the observed size-dependence inclination shallowing could be explained by a ‘rolling ball’ model whereby larger silicate particles rotate less after depositional settling. We also observe non-linear field-dependent DRM acquisition in Earth-like magnetic fields with DRM behaviour depending strongly on silicate particle size, which could be explained by variable magnetic moments and silicate sizes. Our results provide direct evidence for a potentially widespread mechanism that could contribute to the observed variable recording efficiency and inclination shallowing of sedimentary remanences.

Silicon and iron dust in gamma-ray burst host galaxy absorbers

ABSTRACT Depletion studies provide a way to understand the chemical composition of interstellar dust grains. We here examine 23 gamma-ray bursts (GRBs) optical afterglow spectra (spanning 0.6 ≤ z ≤ 5.0) and compare their silicon and iron dust-phase column densities with different extinction curve parameters to study the composition of the interstellar dust grains in these high-redshift GRB host galaxies. The majority of our sample (87 per cent) show featureless extinction curves and only vary in shape. We observe strong correlations (with $\gt 96{{\ \rm per\ cent}}$ significance) between the total-to-selective extinction, RV, and the dust-phase column densities of Si and Fe. Since a large fraction of interstellar iron is locked in silicate grains, this indicates that high Si and Fe depletion leads to an increase in the fraction of large silicate grains and vice versa. This suggests that silicates play a vital role to induce the entire extinction at any wavelength. On the other hand, the far-ultraviolet (UV) extinction is usually attributed to the presence of small silicates. However, we find no trend between the far-UV parameter of the extinction curve, c4, and the abundance of Si and Fe in the dust phase. We, therefore, propose that the far-UV extinction could be a combined effect of small (probably nanoparticles) dust grains from various species.

Are the Formation and Abundances of Metal-poor Stars the Result of Dust Dynamics?

Abstract Large dust grains can fluctuate dramatically in their local density, relative to the gas, in neutral turbulent disks. Small, high-redshift galaxies (before reionization) represent ideal environments for this process. We show via simple arguments and simulations that order-of-magnitude fluctuations are expected in local abundances of large grains (>100 Å) under these conditions. This can have important consequences for star formation and stellar metal abundances in extremely metal-poor stars. Low-mass stars can form in dust-enhanced regions almost immediately after some dust forms even if the galaxy-average metallicity is too low for fragmentation to occur. We argue that the metal abundances of these “promoted” stars may contain interesting signatures as the CNO abundances (concentrated in large carbonaceous grains and ices) and Mg and Si (in large silicate grains) can be enhanced and/or fluctuate almost independently. Remarkably, the otherwise puzzling abundance patterns of some metal-poor stars can be well fit by standard IMF-averaged core-collapse SNe yields if we allow for fluctuating local dust-to-gas ratios. We also show that the observed log-normal distribution of enhancements in these species agrees with our simulations. Moreover, we confirm that Mg and Si are correlated in these stars; the abundance ratios are similar to those in local silicate grains. Meanwhile [Mg/Ca], predicted to be nearly invariant from pure SNe yields, shows very large enhancements and variations up to factors of ≳100 as expected in the dust-promoted model, preferentially in the [C/Fe]-enhanced metal-poor stars. Together, this suggests that (1) dust exists in second-generation star formation, (2) local dust-to-gas ratio fluctuations occur in protogalaxies and can be important for star formation, and (3) the light element abundances of these stars may be affected by the local chemistry of dust where they formed, rather than directly tracing nucleosynthesis from earlier populations.

Infrared emission and the destruction of dust in HII regions

The generation of infrared (IR) radiation and the observed IR intensity distribution at wavelengths of 8, 24, and 100 micron in the ionized hydrogen region around a young, massive star is investigated. The evolution of the HII region is treated using a self-consistent chemical-dynamical model in which three dust populations are included -- large silicate grains, small graphite grains, and polycyclic, aromatic hydrocarbons (PAHs). A radiative transfer model taking into account stochastic heating of small grains and macromolecules is used to model the IR spectral energy distribution. The computational results are compared with Spitzer and Herschel observations of the RCW 120 nebula. The contributions of collisions with gas particles and the radiation field of the star to stochastic heating of small grains are investigated. It is shown that a model with a homogeneous PAH content cannot reproduce the ring-like IR-intensity distribution at 8 micron. A model in which PAHs are destroyed in the ionized region provides a means to explain this intensity distribution. This model is in agreement with observations for realistic characteristic destruction times for the PAHs.

Replacement of olivine by serpentine in the carbonaceous chondrite Nogoya (CM2)

Coarse (chondrule and isolated) olivine in some CM chondrites is replaced by serpentine in both centripetal and meshwork replacement textures. Locally preserved textures formed by partial replacement of coarse olivine by serpentine in the carbonaceous chondrite Nogoya (CM2) establish unique associations between each individual mass of serpentine and the specific olivine from which that serpentine formed. Electron probe microanalyses show that the composition of serpentine replacing coarse olivine is uniform throughout all analyzed volumes of Nogoya, and is independent of the composition of the olivine being replaced. If, as previously proposed, late-stage alteration fluids were Mg-rich because Fe-source minerals were depleted in earlier stages, then the uniform Mg-rich composition of the serpentine replacing large silicate grains during advanced stages of alteration may indicate diffusional homogenization of the aqueous solutions over progressively larger spatial scales, enabled by long timescales and previously proposed stagnant or slow-moving fluids.The range of olivine compositions replaced in Nogoya is even larger than previously reported from ALH 81002 (CM2). This militates against hypotheses of strong primary-mineral control on the compositions of alteration products, at least at advanced stages of alteration. The serpentine formed by olivine replacement in Nogoya is more magnesian than the counterpart serpentine replacing all anhydrous primary silicates in ALH 81002. This intermeteorite heterogeneity of replacement-serpentine composition between ALH 81002 and Nogoya indicates that the aqueous solutions in which the olivine-serpentine replacement reactions occurred were of different compositions in the two different CM parent-body volumes sampled by ALH 81002 and Nogoya. The more magnesian character of serpentines in Nogoya than in ALH 81002 indicates that the Nogoya aqueous-alteration environment was even more highly evolved toward Mg-rich solutions than the environment indicated by the composition of the serpentine in ALH 81002.Persistence of primary-silicate remnants within centripetal and meshwork serpentine indicates that either the aqueous alteration episodes in the parent-body volumes represented by individual meteorites were too short to allow complete replacement of olivine by serpentine, or one or more reactants (most likely water) were completely consumed before the coarse primary silicate was completely replaced. Seemingly incompatible arguments for and against primary-mineral control of serpentine composition during CM chondrite alteration may be reconciled by considering the different grain sizes and reaction timescales that likely existed in different textural settings.

The inner rim structures of protoplanetary discs

The inner boundary of protoplanetary discs is structured by the dramatic opacity changes at the transition from the dust-containing to a dust-free zone. This paper explores the variety and limits of inner rim structures in passively heated dusty discs. For this study, we implemented detailed sublimation physics in a fast Monte Carlo radiative transfer code. We show that the inner rim in dusty discs is not an infinitely sharp wall but a diffuse region which may be narrow or wide. Furthermore, high surface densities and large silicate grains as well as iron and corundum grains decrease the rim radius, from a 2.2AU radius for small silicates around a 47 Solar luminosity Herbig Ae star typically to 0.4AU and as close as 0.2AU. A passive disc with grain growth and a diverse dust composition must thus have a small inner rim radius. Finally, an analytical expression is presented for the rim location as a function of dust, disc and stellar properties.

Dust Processing in Disks around T Tauri Stars

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 fit with the opacities of laboratory analogs of cosmic dust. The fits include small nonspherical grains of amorphous silicates (pyroxene and olivine), crystalline silicates (forsterite and pyroxene), and quartz, together with large fluffy amorphous silicate grains. A wide range in the fraction of crystalline silicate grains as well as large silicate grains among these stars are found. The dust in the transitional-disk objects CoKu Tau/4, GM Aur, and DM Tau has the simplest form of silicates, with almost no hint of crystalline components and modest amounts of large grains. This indicates that the dust grains in these objects have been modified little from their origin in the interstellar medium. Other stars show various amounts of crystalline silicates, similar to the wide dispersion of the degree of crystallinity reported for Herbig Ae/Be stars of mass <2.5 solar masses. Late spectral type, low-mass stars can have significant fractions of crystalline silicate grains. Higher quartz mass fractions often accompany low amorphous olivine-to-amorphous pyroxene ratios. It is also found that lower contrast of the 10 micron feature accompanies greater crystallinity.

High angular resolutionN-band observation of the silicate carbon star IRAS08002-3803 with the VLTI/MIDI instrument

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 band: the uniform-disk diameter is found to be constant and ∼36 mas (72 R� ) between 8 and 10 µm, while it steeply increases longward of 10 µm to reach ∼53 mas (106 R� )a t 13µm. Model calculations with our Monte Carlo radiative transfer code show that neither spherical shell models nor axisymmetric disk models consisting of silicate grains alone can simultaneously explain the observed wavelength dependence of the visibility and the spectral energy distribution (SED). We propose that the circumstellar environment of IRAS08002-3803 may consist of two grain species coexisting in the disk: silicate and a second grain species, for which we consider amorphous carbon, large silicate grains, and metallic iron grains. Comparison of the observed visibilities and SED with our models shows that such disk models can fairly – though not entirely satisfactorily – reproduce the observed SED and N-band visibilities. Our MIDI observations and the radiative transfer calculations lend support to the picture where oxygen-rich material around IRAS08002-3803 is stored in a circumbinary disk surrounding the carbon-rich primary star and its putative low-luminosity companion.

Ultraviolet extinction to 10.8 inverse microns

UV extinction curves that represent observed variations i diffuse clouds (Zeta Oph, Xi Per, and Sigma Sco) have been derived by combining IUE and Voyager data and have been modeled by a multicomponent size distribution of silicate and carbonaceous giants. We have used theoretical model atmospheres as our reddening-free standards, and, in the case of Zeta Oph, also compare with the use of a stellar standard. An attempt was made to classify the UV spectra via theoretical spectral indices, but still missing line opacity appears to limit the use of this method at the present. The modeling includes five possible grain ingrediants, largge (0.015-0.25 micrometers) and small (0.005 micrometers) silicate grains, similarly large and small carbonaceous grains, and polycyclic aromatic hydrocarbons (PAHs). The observed extinction curves are fitted by the model via nonlinear Chi-square-minimization, varying the fractional amount of cosmic Si and C that are locked up in each igredient. The observed variation from 'steep' to 'flat' UV extinction curves is found to correspond to a modest model variation in the lower limit of the power-law size distribution of the large grains. Graphite is found to be necessary grain ingredient that cannot be replaced by the laboratory amorphous carbon materials. Carbonaceous mantles on the larger silicate grains provide no good fits to the observed curves. Using absorption properties of neutral laboratory PAHs limits PAHs to a few percent of the cosmic C abundance for acceptable fits that also give values for R(sub V) close to the observed values.

Interstellar polarization at 3.6 microns

view Abstract Citations (11) References (18) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Interstellar Polarization at 3.6 Microns Jones, Terry Jay Abstract Broadband polarimetry at 3.6 microns is presented for six stars reddened by the interstellar medium. The observed polarization is higher than expected when compared to the predictions of grain models incorporating traditional, large, astronomical silicates. An extra absorbing component, probably broadband in nature, must be present on the large silicate grains. This absorption component must show a significant increase from 2 to 4 microns. Publication: The Astronomical Journal Pub Date: June 1990 DOI: 10.1086/115468 Bibcode: 1990AJ.....99.1894J Keywords: Infrared Astronomy; Interstellar Extinction; Interstellar Matter; Optical Polarization; Absorption Spectra; Polarimetry; Silicates; Stellar Magnitude; Astrophysics; INTERSTELLAR: MATTER full text sources ADS | data products SIMBAD (9)

A multicomponent model of the infrared emission from Comet Halley

We have attempted to incorporate the compositional information available from interplanetary dust particles (IDPs) and the recent Halley flybys into a coherent model to account for cometary infrared emission. We show that the entire infrared emission spectrum of Comet Halley is consistent with the presence of three general dust particle types. These dust types are (i) amorphous carbon grains, (ii) crystalline silicate grains, and (iii) crystalline silicate grains with thin carbonaceous coatings. In analogy with IDPs, the thin carbonaceous coatings on the silicate minerals may be responsible for the dark appearance of most comets. Good fits to the observational data are found for particle populations having particle size distributions similar to those inferred for Comet Halley, and silicate to amorphous carbon mass ratios of 8 to 40, values consistent with IDP and flyby data. Composite particles in which all the carbon resides in thick mantles on silicate cores fail to simultaneously match both the spectral and compositional constraints. Finally, the apparent strength of the 10-μm silicate feature is found to be particle size dependent. Consequently, as has been noted before, the absence of an obvious 10-μm feature in other comets does not imply the absence of silicates, but may instead imply the dominance of large silicate grains.

CLAST‐LADEN MELT‐ROCK FRAGMENT IN THE ADAMS COUNTY, COLORADO, H5 CHONDRITE

The Adams County, Colorado, H5 chondrite contains a lithic fragment, 1 cm in size, that is texturally and mineralogically quite different from the chondritic host. It is composed of: a groundmass of fine‐grained euhedral to subhedral olivine (3–15 μm) and interstitial glass enclosing larger olivine and pyroxene grains (0.15‐0.5 mm; about 15 vol %); an assemblage of enstatite grains (subfragment within) and an assemblage of olivine plus orthopyroxene (a second subfragment); and about 11 vol % grains of mixed troilite and nickel‐iron metal. Analyses yielded these results: (i) olivine grains of the fragment groundmass have a compositional range (Fa12–45) and most grains contain substantial CaO and Cr2O3 (∼ 0.20 and 0.30 avg. wt%, respectively); interstitial glass has ∼ 55 wt% SiO2; (ii) larger olivine grains of the fragment are similarly high in CaO and Cr2O3 and also have a wide FeO/MgO range; one unusual pyroxene is an Mg‐rich pigeonite; (iii) the metal is martensite in composition (11–14 wt% Ni); and (iv) major and trace element analyses by INAA indicate an H‐group bulk composition for the entire 1 cm lithic fragment.On the basis of its texture and bulk and mineral compositions, the fragment is interpreted to represent unequilibrated H‐group material that was partly melted by impact. The Ca‐ and Cr‐enriched groundmass olivine and interstitial glass resulted from rapid crystallization of the chondritic melt. The Ca‐ and Cr‐enriched larger silicate grains, including the enstatite sub‐fragment and the pigeonite grain, are residual, unmelted clasts from the target material (this is supported by the presence of similar material in actual H3 chondrites). Further impact brecciation of the clast‐laden melt material, and resultant impact‐splashing accounts for the presence of the fragment in the H‐group Adams County host and documents the coexistence of unequilibrated and equilibrated H‐group material as surface regolith on one parent body.

Two-component dust models of near-infrared emission from compact H II regions

view Abstract Citations (17) References (59) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Two-component dust models of near-infrared emission from compact H II regions. Zeilik, M., II Abstract Two-component dust models are developed to match the observed characteristics of the nearinfrared and radio emission from selected compact H ii regions. The dust, assumed to be bare and well-mixed with the ionized gas, consists of large silicate grains and small graphite grains. The La radiation field predominantly heats the silicate grains, which produce the emission longward of 8 m. The stellar radiation field heats the graphite grains, which generate most of the emission from 3.5 to 5 m. Both grain types in the ionized volume become too hot to provide the observed peak emission at far-infrared wavelengths from H ii regions. Subject headings: infrared: general - interstellar: matter - nebulae: general Publication: The Astrophysical Journal Pub Date: April 1977 DOI: 10.1086/155128 Bibcode: 1977ApJ...213...58Z Keywords: Astronomical Models; Cosmic Dust; H Ii Regions; Interstellar Matter; Interstellar Radiation; Near Infrared Radiation; Backscattering; Hydrogen Ions; Interstellar Gas; Ionized Gases; Nebulae; Radio Sources (Astronomy); Silicates; Astrophysics full text sources ADS | Related Materials (1) Erratum: 1977ApJ...216..972Z