Crystalline Silicate Grains Research Articles

Annealing of Silicate Dust by Nebular Shocks at 10 AU

Silicate dust grains in the interstellar medium are known to be mostly amorphous, yet crystalline silicate grains have been observed in many long-period comets and in protoplanetary disks. Annealing of amorphous silicate grains into crystalline grains requires temperatures greater than or approximately equal to 1000 K, but exposure of dust grains in comets to such high temperatures is apparently incompatible with the generally low temperatures experienced by comets. This has led to the proposal of models in which dust grains were thermally processed near the protoSun, then underwent considerable radial transport until they reached the gas giant planet region where the long-period comets originated. We hypothesize instead that silicate dust grains were annealed in situ, by shock waves triggered by gravitational instabilities. We assume a shock speed of 5 km/s, a plausible value for shocks driven by gravitational instabilities. We calculate the peak temperatures of pyroxene grains under conditions typical in protoplanetary disks at 5-10 AU. We show that in situ annealing of micron-sized dust grains can occur, obviating the need for large-scale radial transport.

On Ultrasmall Silicate Grains in the Diffuse Interstellar Medium

The abundance of both amorphous and crystalline silicates in very small grains is limited by the fact that the 10 micron silicate emission feature is not detected in the diffuse ISM. On the basis of the observed IR emission spectrum for the diffuse ISM, the observed ultraviolet extinction curve, and the 10 micron silicate absorption profile, we obtain upper limits on the abundances of ultrasmall (a < 15 Angstrom) amorphous and crystalline silicate grains. Contrary to previous work, as much as ~20% of interstellar Si could be in a < 15 Angstrom silicate grains without violating observational constraints. Not more than ~5% of the Si can be in crystalline silicates (of any size).

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.

Thermal emission from BOK globules

Models of the thermal emission from dust within Bok globules are developed, assuming the heat source is the interstellar radiation field (IRF) and accurately solving the radiative-transfer problem using the half-range moment method in spherical geometry. The models are characterized by the central optical depth, the dust density distribution, and the grain type. The FIR flux spectrum and surface-brightness profile are calculated. The primary interest is in the thermalization efficiency of globules. Only globules containing amorphous dust grains are able to convert the incident IRF into FIR emission with the high efficiency implied by the observations. However, this is incompatible with the observed FIR emission with the high efficiency implied by the observations. However, this is incompatible with the observed spectra, which indicate the presence of crystalline silicate grains. Perhaps these conclusions could be reconciled by the addition of an internal radiant heat source, such as a protostar. This would also explain why many globules appear to emit more photons than they can possibly absorb from the IRF. 32 references.