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Abstract
There is occasionally an absorption feature around 11μm in the spectra of forming stars and probably ISM sightlines. The carrier is still debated to be possibly water ice, polycyclic aromatic hydrocarbons, silicon carbide, carbonates or crystalline silicates all of which have been detected in cosmic dust. In this thesis, we present over 60 new ground-based spectra from 8-13μm of more than 40 objects, including three evolved stars which is considered as dust factories, several embedded young stellar objects in which
dust involve in star formation and planet formation, and several objects whose sightlines intersects with many dense an/or diffuse clouds. Our observations were made with a variety of spectrographs on different telescopes, including Michelle on Gemini North, Michelle on UKIRT, TReCS on Gemini South, TIMMI2 on the ESO 3.6 m telescope and COMICS on Subaru. Most of the spectra are compared with the data obtained with the Infrared Space Observatory (ISO) on these and other targets; they show an inflection around 11μm. We aim to identify the carrier of this feature. By comparing the strength of that feature with those at other wavelengths, we are able to exclude all of the possible carriers above but crystalline silicate. The evidence includes the similarity in shape between the 11μm band profile and the emissivity of crystalline olivine, the correlation between this band and the amorphous silicate band at 9.7μm and the best model fit of the spectra, which determines the mass fraction of rystalline silicates in each object. We propose that crystalline silicates exist in the embedded YSO phase and even in the ISM, which was thought to have amorphous silicates only. This result leads to several implications on the life cycle of silicates in particular and cosmic dust in general. There are probably several possibilities, for instance, the destruction of the ISM silicates is not complete or the crystalline silicates can form in the ISM due to a low-temperature crystallisation process. We also investigate self-absorption in the spectra of an embedded young stellar objects (YSOs) and a post-AGB star using an optically thin models with absorption on the sightline and a modified version of the radiative transfer model by Towers et al. (2009). These models, each of which has its own advantages and disadvantages, not only can reproduce the shapes of the spectra, but also reveal the properties of the dust surrounding the objects such as the composition, temperature and thickness.