Browsing by Subject "infrared-absorption spectra"

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c2d Spitzer-IRS Spectra Of Disks Around T Tauri Stars IV. Crystalline Silicates
(2009-11) Olofsson, J.; Augereau, J. C.; van Dishoeck, E. F.; Merin, B.; Lahuis, F.; Kessler-Silacci, J.; Dullemond, C. P.; Oliveira, I.; Blake, G. A.; Boogert, A. C. A.; Brown, J. M.; Evans, Neal J.; Geers, V.; Knez, C.; Monin, J. L.; Pontoppidan, K.; Kessler-Silacci, J.; Evans, N. J.
Aims. Dust grains in the planet-forming regions around young stars are expected to be heavily processed due to coagulation, fragmentation, and crystallization. This paper focuses on the crystalline silicate dust grains in protoplanetary disks for a statistically significant number of TTauri stars (96). Methods. As part of the cores to disks (c2d) legacy program, we obtained more than a hundred Spitzer/IRS spectra of TTauri stars, over a spectral range of 5-35 mu m where many silicate amorphous and crystalline solid-state features are present. At these wavelengths, observations probe the upper layers of accretion disks up to distances of a dozen AU from the central object. Results. More than 3/4 of our objects show at least one crystalline silicate emission feature that can be essentially attributed to Mg-rich silicates. The Fe-rich crystalline silicates are largely absent in the c2d IRS spectra. The strength and detection frequency of the crystalline features seen at lambda > 20 mu m correlate with each other, while they are largely uncorrelated with the observational properties of the amorphous silicate 10 mu m feature. This supports the idea that the IRS spectra essentially probe two independent disk regions: a warm zone (<= 1 AU) emitting at mu similar to 10 mu m and a much colder region emitting at lambda > 20 mu m (<= 10 AU). We identify a crystallinity paradox, as the long-wavelength (lambda > 20 mu m) crystalline silicate features are detected 3.5 times more frequently (similar to 55% vs. similar to 15%) than the crystalline features arising from much warmer disk regions (lambda similar to 10 mu m). This suggests that the disk has an inhomogeneous dust composition within similar to 10 AU. The analysis of the shape and strength of both the amorphous 10 mu m feature and the crystalline feature around 23 mu m provides evidence for the prevalence of mu m-sized (amorphous and crystalline) grains in upper layers of disks. Conclusions. The abundant crystalline silicates found far from their presumed formation regions suggest efficient outward radial transport mechanisms in the disks around TTauri stars. The presence of mu m-sized grains in disk atmospheres, despite the short timescales for settling to the midplane, suggests efficient (turbulent) vertical diffusion, probably accompanied by grain-grain fragmentation to balance the expected efficient growth. In this scenario, the depletion of submicron-sized grains in the upper layers of the disks points toward removal mechanisms such as stellar winds or radiation pressure.
c2d Spitzer-IRS Spectra Of Disks Around T Tauri Stars IV. Crystalline Silicates
(2009-11) Olofsson, J.; Augereau, J. C.; van Dishoeck, E. F.; Merin, B.; Lahuis, F.; Kessler-Silacci, J.; Dullemond, C. P.; Oliveira, I.; Blake, G. A.; Boogert, A. C. A.; Brown, J. M.; Evans, Neal J.; Geers, V.; Knez, C.; Monin, J. L.; Pontoppidan, K.; Kessler-Silacci, J.; Evans, N. J.
Aims. Dust grains in the planet-forming regions around young stars are expected to be heavily processed due to coagulation, fragmentation, and crystallization. This paper focuses on the crystalline silicate dust grains in protoplanetary disks for a statistically significant number of TTauri stars (96). Methods. As part of the cores to disks (c2d) legacy program, we obtained more than a hundred Spitzer/IRS spectra of TTauri stars, over a spectral range of 5-35 mu m where many silicate amorphous and crystalline solid-state features are present. At these wavelengths, observations probe the upper layers of accretion disks up to distances of a dozen AU from the central object. Results. More than 3/4 of our objects show at least one crystalline silicate emission feature that can be essentially attributed to Mg-rich silicates. The Fe-rich crystalline silicates are largely absent in the c2d IRS spectra. The strength and detection frequency of the crystalline features seen at lambda > 20 mu m correlate with each other, while they are largely uncorrelated with the observational properties of the amorphous silicate 10 mu m feature. This supports the idea that the IRS spectra essentially probe two independent disk regions: a warm zone (<= 1 AU) emitting at mu similar to 10 mu m and a much colder region emitting at lambda > 20 mu m (<= 10 AU). We identify a crystallinity paradox, as the long-wavelength (lambda > 20 mu m) crystalline silicate features are detected 3.5 times more frequently (similar to 55% vs. similar to 15%) than the crystalline features arising from much warmer disk regions (lambda similar to 10 mu m). This suggests that the disk has an inhomogeneous dust composition within similar to 10 AU. The analysis of the shape and strength of both the amorphous 10 mu m feature and the crystalline feature around 23 mu m provides evidence for the prevalence of mu m-sized (amorphous and crystalline) grains in upper layers of disks. Conclusions. The abundant crystalline silicates found far from their presumed formation regions suggest efficient outward radial transport mechanisms in the disks around TTauri stars. The presence of mu m-sized grains in disk atmospheres, despite the short timescales for settling to the midplane, suggests efficient (turbulent) vertical diffusion, probably accompanied by grain-grain fragmentation to balance the expected efficient growth. In this scenario, the depletion of submicron-sized grains in the upper layers of the disks points toward removal mechanisms such as stellar winds or radiation pressure.
The Dust Cloud Around The White Dwarf G 29-38. II. Spectrum From 5 To 40 Mu M And Mid-Infrared Photometric Variability
(2009-03) Reach, William T.; Lisse, Carey; von Hippel, Ted; Mullally, Fergal; Mullally, Fergal
We model the mineralogy and distribution of dust around the white dwarf G29-39 using the infrared spectrum from 1 to 35 mu m. The spectral model for G29-38 dust combines a wide range of materials based on spectral studies of comets and debris disks. In order of their contribution to the mid-infrared emission, the most abundant minerals around G29-38 are amorphous carbon (lambda < 8 mu m), amorphous and crystalline silicates (5-40 mu m), water ice (10-15 and 23-35 mu m), and metal sulfides (18-28 mu m). The amorphous C can be equivalently replaced by other materials (like metallic Fe) with featureless infrared spectra. The best-fitting crystalline silicate is Fe-rich pyroxene. In order to absorb enough starlight to power the observed emission, the disk must either be much thinner than the stellar radius (so that it can be heated from above and below) or it must have an opening angle wider than 2 degrees. A "moderately optically thick" torus model fits well if the dust extends inward to 50 times the white dwarf radius, all grains hotter than 1100 K are vaporized, the optical depth from the star through the disk is tau(parallel to) = 5, and the radial density profile alpha r(-2.7); the total mass of this model disk is 2 x 1019 g. A physically thin (less than the white dwarf radius) and optically thick disk can contribute to the near-infrared continuum only; such a disk cannot explain the longer-wavelength continuum or strong emission features. The combination of a physically thin, optically thick inner disk and an outer, physically thick and moderately optically thin cloud or disk produces a reasonably good fit to the spectrum and requires only silicates in the outer cloud. We discuss the mineralogical results in comparison to planetary materials. The silicate composition contains minerals found from cometary spectra and meteorites, but Fe-rich pyroxene is more abundant than enstatite (Mg-rich pyroxene) or forsterite (Mg-rich olivine) in G29-38 dust, in contrast to what is found in most comet or meteorite mineralogies. Enstatite meteorites may be the most similar solar system materials to G29-38 dust. Finally, we suggest the surviving core of a "hot Jupiter" as an alternative (neither cometary nor asteroidal) origin for the debris, though further theoretical work is needed to determine if this hypothesis is viable.