Figure 3. Comparison of ISO SWSand LWS spectra on 7 October 1996 (thick line) with dust models. This solid line: size distribution power law a = 3.5; dashed line: a = 3.7 [3],

are extremely porous aggregates of such grains). Larger cometary particles will remain in bound orbits and may be swept up by the Earth. The chondritic aggregate IDPs are of likely cometary origin because some of them have high atmospheric entry speeds [69]. These IDPs are heterogeneous aggregates of sub-micron sized silicates and other minerals in a carbonaceous matrix. The submicron grain size, high Mg/Fe abundance ratio, mix of crystalline and glassy olivine and pyroxenes, and high carbon content have no counterpart in any other known meteoritic material. These properties are a very good match with the inferred nature of cometary dust.

There are strong indications that at least one silicate component in the chondritic aggregate IDPs is of interstellar origin. Bradley [70] argued that the common 0.1-0.5 urn glassy silicate grains, or GEMS (Glass with Embedded Metal and Sulfides) are interstellar, because of their high radiation dosage, relict microcrystals, and similarity to observed properties of interstellar silicates. FeNi inclusions are sufficient for magnetic alignment of interstellar grains [71], The 10 spectra of GEMS show a broad maximum that varies between 9.3 and 10.0 pm from sample to sample, as one would expect for a varying mix of glassy pyroxene and olivine [72], Strong D/H and 15N/I4N anomalies, approaching cold molecular cloud values, have been measured in the carbonaceous material in which the GEMS are embedded [73,74], It seems plausible that the GEMS acquired these coatings in cold interstellar clouds (e.g. [75]).

The origin (or origins) of the crystalline silicates in comets is less clear. Heating in the coma or on the nucleus is not sufficient for annealing of glassy or amorphous grains, and the 11.2 jxm peak in Hale-Bopp was just as prominent at 4.6 AU preperihelion [76] as at 0.92 AU. Crystalline grains can form by direct condensation from a hot gas at 7=1200-1400 K. A few enstatite whiskers, ribbons, and platelets in probable cometary IDPs have growth patterns indicating such direct condensation [77], Grain condensation could have occurred in the hot inner solar nebula or in presolar environments. If the crystalline silicates condensed in the inner solar nebula, then their presence in comets indicates large-scale mixing of high-temperature material to the cold outer regions where the comets formed.

Spectroscopy with ISO has given us a better picture of the distribution of crystalline silicates in astronomical sources. Mg-olivine was detected in the oxygen* rich outflows of some evolved stars [78] but is absent in spectra of the diffuse interstellar medium or molecular clouds such as the Trapezium. Nor are crystalline silicates detected in the circumstellar dust around most young stellar objects. Yet, crystalline olivine peaks are present in certain late-stage Herbig Ae/Be stars. The spectrum of HD100546 is very similar to that of Hale-Bopp [79]. These systems appear to have a population of sun-grazing comets, in order to explain transient gaseous emission spectral lines (e.g., [80]). The 11.2 nm peak is also present in the debris disk around (3 Pictoris [81]. Because the dynamical lifetime of the dust is shorter than the age of (3 Pictoris, comets are thought to be the replenishing source of the dust.

Non-solar isotopic ratios can be a signature of interstellar grains. The ability to analyze submicrometer sized volumes with nano SIMS ion microprobes is now possible. First results have identified oxygen isotopic anomalies in 0.1-1 pm silicate grains within IDPs, indicating a pre-solar origin for these grains [82], Wider application of this technique in the future should lead to a better understanding of the prevalence of interstellar crystalline silicates.

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