The Absence of Crystalline Silicates in the Diffuse Interstellar Medium

By F. Kemper, W. Vriend, A. Tielens. 2004

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Infrared spectroscopy provides a direct handle on the composition and structure of interstellar dust. We have studied the dust along the line of sight toward the Galactic center using Short Wavelength Spectrometer data obtained with the Infrared Space Observatory (ISO). We focused on the wavelength region from 8 to 13 μm, which is dominated by the strong silicate absorption feature. Using the absorption profiles observed toward Galactic center sources 3 and 4, which are C-rich Wolf-Rayet Stars, as reference objects, we are able to disentangle the interstellar silicate absorption and the silicate emission intrinsic to the source, toward Sgr A* and derive a very accurate profile for the intrinsic 9.7 μm band. The interstellar absorption band is smooth and featureless and is well reproduced using a mixture of 15.1% amorphous pyroxene and 84.9% of amorphous olivine by mass, all in spherical submicron-sized grains. There is no direct evidence for substructure due to interstellar crystalline silicates. By minimizing χ2 of spectral fits to the absorption feature, we are able to determine an upper limit to the degree of crystallinity of silicates in the diffuse interstellar medium (ISM) and conclude that the crystalline fraction of the interstellar silicates is 0.2% ± 0.2% by mass. This is much lower than the degree of crystallinity observed in silicates in the circumstellar environment of evolved stars, the main contributors of dust to the ISM. There are two possible explanations for this discrepancy. First, an amorphization process occurs in the ISM on a timescale significantly shorter than the destruction timescale, possibly caused by particle bombardment by heavyweight ions. Second, we consider the possibility that the crystalline silicates in stellar ejecta are diluted by an additional source of amorphous silicates, in particular supernovae. We also compare our results with a study on silicate presolar grains found in interplanetary dust particles.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/0945f2e3417032340833125743738d1c4b52b56a

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Citations in the corpus

Probing dust grain evolution in IM Lupi's circumstellar disc. Multi-wavelength observations and mo (2008, background): “Overall, these features indicate a significant level of processing of the silicate grains in the disc compared to those observed in the interstellar medium, which instead are found to be submicron-sized and largely amorphous (<1% of crystalline silicates, Kemper et al. 2004).”
The dusty torus in the Circinus galaxy: a dense disk and the torus funnel (2013, background): “The template absorption profile τ(λ) is the same as in Tristram et al. (2007), i.e. it is derived from the extinction curves of Schartmann et al. (2005) and Kemper et al. (2004).”
The primordial nucleus of comet 67P/Churyumov-Gerasimenko (2015, background): “These crystalline grains are too large to be annealed interstellar grains (these are amorphous ≤0.1 µm particles, Kemper et al. 2004).”
Evolution of interstellar dust and stardust in the solar neighbourhood (2007, background): “Also, crystalline Mg-silicates are not considered since they are not found in the inter-stellar medium (Kemper et al. 2004), possibly because they are rapidly amorphized in the ISM after their ejection by interaction with energetic electrons and ions (cf. Demyk et al. 2004; Jäger et al. 2003).”
Chondrules and the Protoplanetary Disk (2011, background): “%) amorphous (Bradley 2003, Kemper et al. 2004, Brownlee et al. 2006, Wooden et al. 2006).”
Resolving the obscuring torus in NGC 1068 with the power of infrared interferometry: revealing the inner funnel of dust (2009, methodology): “We show that in many aspects the nucleus of NGC 1068 is irregular: the orientation of the dust is tilted with respect to the jet; the PA of the visible ionization cone does not match the PA of the inner dust funnel; and the chemical composition of the dust in the torus is different than dust observe”
A 10 μm spectroscopic survey of Herbig Ae star disks: Grain growth and crystallization (2005, methodology): “The shape of the emission bands with a low 11.3/9.8 ratio is similar to that of the ISM silicate absorption feature.”
The c2d Spitzer Spectroscopic Survey of Ices around Low-Mass Young Stellar Objects. I. H2O and the 5-8 μm Bands (2007, background): “4) and subtraction of a standard interstellar silicate spectrum (GCS 3; Kemper et al. 2004) shows a broad feature centered on ∼12.3 µm that is attributed to the H2O libration mode.”
The Mid-Infrared Spectrum of Star-forming Galaxies: Global Properties of Polycyclic Aromatic Hydrocarbon Emission (2006, methodology): “The 9.7µm component of PSi(λ) is taken from the Galactic center profile of Kemper et al. (2004) between 8.0µm and 12.5µm, with PSi(8µm) = 0.06.”
LINE DERIVED INFRARED EXTINCTION TOWARD THE GALACTIC CENTER (2011, methodology): “At longer wavelengths however the interpolation is less certain, due to the fact that there are fewer lines, larger extinction errors and silicate emission in the GC region (Kemper et al. 2004).”
Far-infrared to millimeter astrophysical dust emission. I. A model based on physical properties of a (2007, background): “Observations of the “silicate” absorption feature near 10 µm indicate that their mass is dominated by amorphous silicates (see Kemper et al. 2004).”
Astrophysics with Extraterrestrial Materials (2016, background): “Many studies indicate that the timescale for dust destruction in the ISM is much shorter than the timescale of injection of fresh stardust, and this is commonly taken to mean that much of the observed dust forms directly in the ISM (Draine 1990, 2009).”
Pixie Dust: The Silicate Features in the Diffuse Interstellar Medium (2005, background): “In the ISM, crystalline silicates are not detected, and recently a stringent limit of <1% of the total silicate population by mass has been placed on it (Kemper et al. 2004).”