Samples of Stars Beyond the Solar System: Silicate Grains in Interplanetary Dust (original) (raw)

The shape and composition of interstellar silicate grains

Astronomy and Astrophysics, 2008

We investigate the composition and shape distribution of silicate dust grains in the interstellar medium. The effects of the amount of magnesium and iron in the silicate lattice are studied in detail. We fit the spectral shape of the interstellar 10 µm extinction feature as observed towards the galactic center using various particle shapes and dust materials. 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 as well as silicon carbide (SiC). The results of our analysis of the 10 µm feature are used to compute the shape of the 20 µm silicate feature and to compare this with observations of this feature towards the galactic center. By using realistic particle shapes to fit the interstellar extinction spectrum 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 (O/Si ≈ 3.5). This composition is not consistent with that of the glassy material found in GEMS in interplanetary dust particles indicating that the amorphous silicates found in the Solar system are, in general, not unprocessed remnants from the interstellar medium. Also, we find that a significant fraction of silicon carbide (∼3%) is present in the interstellar dust grains. 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.

A mineralogy of extrasolar silicate dust from 10- m spectra

Monthly Notices of the Royal Astronomical Society, 2002

The 10-and 18-mm spectroscopy of a variety of galactic environments reveals smooth bands which have been associated with (respectively) Si-O stretching and bending modes in amorphous silicates, since the spectra of crystalline silicates are narrow and highly structured. The standard approach to the interpretation of astronomical spectra is to assume that the silicates are amorphous and then to add in crystalline components (usually a single olivine followed by a pyroxene) to match fine structures in the data. Conversely, in this analysis we match the gross properties of the astronomical profiles-the full width at half-maximum (FWHM) and the wavelength of the peak (l c)-with a mixture of crystalline silicates from different structural (and hence different spectral) classes and add a component of amorphous silicate only if there is too much structure in the simulation. We find that the narrow bands of crystalline grains could blend to form the broad 10-mm bands observed. For all the environments included herein, if crystalline silicates are included in the mixture, x 2 /n of the fits improves significantly (by factors of 1:3-4:4Þ and the number of silicon atoms required to model the spectra decreases by 30-50 per cent. Upper limits to the mass fraction of crystalline pyroxene increases with the FWHM of the profile from , 50 per cent in the sampled circumstellar environments to , 80 per cent in the Taurus molecular cloud (TMC) and its embedded young stellar objects (YSOs). Fine structures common to both the averaged spectra of laboratory silicates and astronomical profiles suggest that & 10 per cent by mass of the silicates in circumstellar and star-forming environments could be partially crystalline hydrous (i.e. OH 2 containing) silicates similar to clays like talc and montmorillonite, but that these grains are absent from the ultraviolet-rich diffuse medium towards Cyg OB2 no. 12. In contrast, the relative abundance of submicrometre-sized crystalline olivine is insufficient (&25 per cent) in these circumstellar, diffuse-medium, molecular-cloud and YSO spectra to produce an 11.2-mm emission or absorption feature. Using this method of spectral analysis, the mass fraction of amorphous silicate in these spectra could be as low as 17 per cent in the TMC and 0 per cent in some circumstellar environments.

Comment on “The shape and composition of interstellar silicate grains”

Astronomy & Astrophysics, 2008

In the paper entitled "The shape and composition of interstellar silicate grains" (A&A, 462, 667-676 (2007)), Min et al. explore non-spherical grain shape and composition in modeling the interstellar 10 and 20 μm extinction features. This progression towards more physically realistic models is vitally important to enabling valid comparisons between dust observations and laboratory measurements. Min et al. proceed to compare their model results with GEMS (glass with embedded metal and sulfides) from interplanetary dust particles (IDPs) and to discuss the nature and origin of GEMS. Specifically, they evaluate the hypothesis of Bradley (1994) that GEMS are remnant interstellar (IS) amorphous silicates. From a comparison of the mineralogy, chemical compositions, and infrared (IR) spectral properties of GEMS with their modeling results, Min et al. conclude that "the composition of interstellar medium (ISM) silicates is not consistent with that of GEMS" and that "GEMS are, in general, not unprocessed leftovers from the diffuse ISM". These original conclusions were based, however, on erroneous GEMS data.

Steps toward interstellar silicate mineralogy. I. Laboratory results of a silicate glass of mean cosmic composition

Astronomy and Astrophysics

Crystalline olivines are an important component of silicate dust particles in space. ISO observations revealed the presence of crystalline silicates in comets, protoplanetary accretion disks, and outflows from evolved stars. For the interpretation of astronomical spectra, the relevant material data at a variety of temperatures and over a broad wavelength range, are urgently needed. In contrast to this need, optical properties of the astronomically interesting olivines are scarcely available at present. In order to close this gap, we studied the optical properties of three minerals of the olivine group by reflection spectroscopy on single crystals in the infrared spectral range. We measured the iron endmember (fayalite, Fe2SiO4), an Mg-rich olivine (Mg1.9Fe0.1SiO4), and the magnesium endmember (forsterite, Mg2SiO4) of the (Mg, Fe)2SiO4 series. For a direct comparison with astronomical observations, we present calculated mass absorption coefficients in the Rayleigh limit for different shapes and varying iron content of the dust particles. The laboratory data together with a set of ISO data for envelopes around evolved stars (Molster 2000) are used to constrain the properties of circumstellar silicates. We find that essentially all band positions are shifted to larger wavelengths with increasing iron content. The particle shape influences very significantly the strong bands such as the B1u:ν3 mode that appears as the "11.4 µm" band of forsterite, whereas e.g. the two FIR modes longward of 40 µm remain practically unaffected by the particle shape but shift due to increasing iron content. The comparison with the band positions in ISO spectra points to the presence of olivine crystals strongly elongated along the crystallographic c-axis. In addition, we apply the calculated mass absorption coefficients to evaluate transmission measurements of particles embedded in a matrix-a technique which is frequently used in laboratory astrophysics. All data shown in this paper will be made available in digital form via the electronic database http://www.astro.uni-jena.de.

Formation of Silicate Grains in Circumstellar Environments: Experiment, Theory and Observations

Amongst chemical reactions(1) in the molecular universe(2), condensation reaction is probably the most poorly understood. The condensation of a solid from its components in the gas phase occurs in many parts of our galaxy such as stellar mass outflows, the 'terrestrial' region of protoplanetary disks and in primordial solar nebula(3). But how does the transition occur from molecules to intermediate clusters to macroscopic grains? The major focus of the present work is the identification of chemical condensation reaction pathways that lead to the formation of stoichiometry, composition and crystallinity of cosmic silicates from vapor phase species.

Steps toward interstellar silicate mineralogy

Astronomy & Astrophysics, 2001

The Chemical Imprint of Silicate Dust on the Most Metal-Poor Stars

The Astrophysical Journal, 2014

We investigate the impact of dust-induced gas fragmentation on the formation of the first lowmass, metal-poor stars (< 1M) in the early universe. Previous work has shown the existence of a critical dust-togas ratio, below which dust thermal cooling cannot cause gas fragmentation. Assuming the first dust is silicon-based, we compute critical dust-togas ratios and associated critical silicon abundances ([Si/H] crit). At the density and temperature associated with protostellar disks, we find that a standard Milky Way grain size distribution gives [Si/H] crit = −4.5 ± 0.1, while smaller grain sizes created in a supernova reverse shock give [Si/H] crit = −5.3 ± 0.1. Other environments are not dense enough to be influenced by dust cooling. We test the silicate dust cooling theory by comparing to silicon abundances observed in the most iron-poor stars ([Fe/H] < −4.0). Several stars have silicon abundances low enough to rule out dust-induced gas fragmentation with a standard grain size distribution. Moreover, two of these stars have such low silicon abundances that even dust with a shocked grain size distribution cannot explain their formation. Adding small amounts of carbon dust does not significantly change these conclusions. Additionally, we find that these stars exhibit either high carbon with low silicon abundances or the reverse. A silicate dust scenario thus suggests that the earliest low-mass star formation in the most metal-poor regime may have proceeded through two distinct cooling pathways: fine structure line cooling and dust cooling. This naturally explains both the carbon-rich and carbon-normal stars at extremely low [Fe/H].

Interstellar Matter in Meteorites and Interplanetary Dust

Symposium - International Astronomical Union, 2000

Meteorites and interplanetary dust particles (IDPs) are primitive solar system materials which contain preserved nebular condensates, circumstellar dust grains and partially preserved molecular cloud matter. The circumstellar dust grains found in meteorites are direct samples of a variety of stars, and provide detailed constraints on models of stellar nucleosynthesis. Hydrogen and nitrogen isotopic anomalies in organic matter in meteorites and IDPs are thought to originate from chemical processes in a presolar molecular cloud. This material is better preserved, but less well characterized among IDPs.

Samples of Stars Beyond the Solar System: Silicate Grains in Interplanetary Dust (original) (raw)

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