A mineralogy of extrasolar silicate dust from 10-µm spectra

Abstract

The 10- and 18-μm spectroscopy of a variety of galactic environments reveals smooth bands which have been associated with (respectively) Si-O stretching and bending modes in amorphous silicates, since the spectra of crystalline silicates are narrow and highly structured. The standard approach to the interpretation of astronomical spectra is to assume that the silicates are amorphous and then to add in crystalline components (usually a single olivine followed by a pyroxene) to match fine structures in the data. Conversely, in this analysis we match the gross properties of the astronomical profiles - the full width at half-maximum (FWHM) and the wavelength of the peak (λ c ) - with a mixture of crystalline silicates from different structural (and hence different spectral) classes and add a component of amorphous silicate only if there is too much structure in the simulation. We find that the narrow bands of crystalline grains could blend to form the broad 10-μm bands observed. For all the environments included herein, if crystalline silicates are included in the mixture, Χ 2 /n of the fits improves significantly (by factors of 1.3-4.4) and the number of silicon atoms required to model the spectra decreases by 30-50 per cent. Upper limits to the mass fraction of crystalline pyroxene increases with the FWHM of the profile from ∼50 per cent in the sampled circumstellar environments to ∼ 80 per cent in the Taurus molecular cloud (TMC) and its embedded young stellar objects (YSOs). Fine structures common to both the averaged spectra of laboratory silicates and astronomical profiles suggest that ≤ 10 per cent by mass of the silicates in circumstellar and star-forming environments could be partially crystalline hydrous (i.e. OFF - containing) silicates similar to clays like talc and montmorillonite, but that these grains are absent from the ultraviolet-rich diffuse medium towards Cyg OB2 no. 12. In contrast, the relative abundance of submicrometre-sized crystalline olivine is insufficient (≤ 25 per cent) in these circumstellar, diffuse-medium, molecular-cloud and YSO spectra to produce an 11.2-μm emission or absorption feature. Using this method of spectral analysis, the mass fraction of amorphous silicate in these spectra could be as low as 17 per cent in the TMC and 0 per cent in some circumstellar environments.