Interpreting the 10um astronomical silicate feature

Abstract

10pm spectra of silicate dust in the diffuse medium towards Cyg 0B2 no. 12 and towards field and embedded objects in the Taurus Molecular Cloud (TMC) were obtained with CGS3 at the United Kingdom Infrared Telescope (UKIRT). Cold molecular-cloud silicates are sampled in quiescent lines of sight towards the field stars Taurus-Elias 16 and Elias 13, whilst observations of the embedded young stellar objects HL Tau, Taurus-Elias 7 (Haro6-10) and Elias 18 also include emission from heated dust.
To obtain the foreground silicate absorption profiles, featureless continua are estimated using smoothed astronomical and laboratory silicate emissivities. TMC field stars and Cyg 0B2 no. 12 are modelled as photospheres reddened by foreground
continuum and silicate extinction. Dust emissidn in the non-photospheric continua of ilL Tau and Elias 7 (ilaro6-10) is distinguished from foreground silicate absorption using a 10pm disk model, based on the IR-submm model of T Tauri stars by Adams, Lada & Shu (1988), with terms added to represent the foreground continuum and silicate extinction. The absorption profiles of HL Tau and Elias 7 are similar to that of the field star Elias 16. Fitted temperature indices of 0.43 (ilL Tau) and 0.33 (Elias 7) agree with Boss' (1996) theoretical models of the 200-300K region, but are lower than those of IR-submm disks (0.5-0.61; Mannings& Emerson 1994); the modelled 10pm emission of HL Tau is optically thin; that of Elias 7 is optically thick.
A preliminary arcsecond-resolution determination of the 10pm emissivity near 91 Ori D in the Trapezium region of Orion and a range of emission temperatures (225- 310K) are derived from observations by T. L. Hayward; this Ney-Allen emissivity is 0.61im narrower than the Trapezium emissivity obtained by Forrest et al. (1975) with a large aperture.
Published interstellar grain models, elemental abundances and laboratory studies of Solar System silicates (IDPs, GEMS and meteorites), the 10pm spectra of comets, interstellar silicates, synthetic silicates and terrestrial minerals, and the effects of
laboratory processing on the 10pm spectra of crystalline and amorphous silicates are reviewed to provide insight into the mineralogy of interstellar silicate dust. The wavelengths of the peaks of the 10pm silicate profiles decrease between circumstellar, diffuse medium and molecular-cloud environments, indicating (after Giirtler & Henning 1986) that the amorphous pyroxene content of initially olivine-rich interstellar dust increases with time. This is accompanied by an increase in the FWHM of the features which indicates an increase in grain size and/or an increasing fraction of chemically-varied crystalline pyroxene. Fine structure in the Cyg 0132 no. 12, Elias 16, Elias 7, HL Tau profiles indicate that hydrated layer silicates similar to terrestrial serpentines, clays and talc may be a ubiquitous component of interstellar dust. At 10pm the narrow bands of mixed crystalline pyroxenes blend, making their identification difficult. Since no fine structure is observed near 11.2pm, the fraction of crystalline olivine is small.
In geology direct olivine-plus-Si0 2 to pyroxene reactions occur only at high pressure within the terrestrial mantle. Therefore the fraction of amorphous pyroxene is probably increased by the hydration of Mg-rich olivine to form a serpentine-like hydrated silicate, which is subsequently annealed to form a mixture of amorphous pyroxene and olivine. Terrestrial and laboratory olivine samples are readily converted to serpentine in the presence of water, and (after extended annealing) the
first crystalline band to appear is the 11.2pm olivine feature frequently observed in cometary spectra.