Infrared spectroscopy of interstellar grains

Abstract

Spectra are presented of interstellar 1120 and CO ices in three molecular douds. The high signal—to—noise in many of these enabled high quality comparisons to be made with laboratory data, constraining mantle constituents and thermal history.
CGS 4 spectra of H 2 0 ices in the p Oph cloud were best fitted with two—component laboratory spectra of non—processed and thermally processed 1120 ices. Variations in the depth of the 3.4 jim excess absorption between lines of sight implied that heating weakens this feature. No features due to C11 30H ices were detected, but a feature at 3.47 pm was
observed towards two objects, recently attributed to diamond—like carbon.
Solid CO features in p Oph were similarly obtained and compared to laboratory features. They were found to be consistent with pure or nearly pure CO and generally possessed weak or undetectable long—wavelength shoulders implying rigorous segregation between 1120 and CO in the mantles. The CO features were not consistent with CO in 02, implying segregation between these two components, or a lack of 02. Gas phase CO was also observed in some lines of sight and shown not to be spatially coincident with solid CO.
Solid CO was observed in the Taurus cloud towards four background field stars and one embedded object. The feature towards Elias 16 was similar to those observed in p Oph, although other spectra were consistent with CO in CO 2 , but the quality of three of these was disputed. The effect of UV on the grains was considered, and assuming cloud uniformity, was found to have an effect only at the outer edges or close to YSOs.
Age estimates of the cloud using observational data and theoretical models were found to disagree. A brief study was made of CO ices in the Serpens cloud within which it was concluded that solid CO is a widespread component, although its host matrix may differ between lines of sight within the cloud, implying subtle differences in regional chemistry.
Infrared spectroscopy of interstellar grains has enabled many grain components to be identified and their importance in the ISM to be understood. An analysis of cold molecular cloud mantles has been presented in this thesis, and should provide a basis for future study, especially with the launch of ISO in the near future.