Elemental abundances in the interstellar medium

Abstract

One method to investigate the chemical composition of the interstellar medium (ISM) and interstellar dust grains is to conduct interstellar elemental depletion studies, especially of highly abundant species. The role refractory element, silicon (Si) in extinction is not clearly understood and the distribution and evolution of moderately volatile sulfur (S) in the ISM is still an open problem. The key motivation of the work is to investigate the chemical composition of ISM of our Galaxy, and the formation, processing and distribution of interstellar dust in its different environments, mainly focusing on silicon and sulfur abundances, both in gas and dust. In the work outlined in this thesis, I will be describing the gas and dust phase abundances of Si and S in the interstellar medium using archival observations, and their probable role in the observed extinction. In this work, we also have measured the column density of S II along 9 Galactic sight lines using archival high-resolution observations from the Space Telescope Imaging Spectrograph and determined the abundances of S in both gas and dust phases. Using Archival spectral data towards 131 target stars in the Galaxy, interstellar Si abundances and depletion along those lines of sight has been surveyed. Oscillator strength correction has been performed to account for its improvements, using most recent values. This is an extensive survey done using a much larger data sample compared to previous investigations, but it substantiate the majority of the findings, which show that Si depletion is linked to both the average hydrogen density (n (H)) and the fraction of molecular hydrogen (f(H2)) along the lines of sight. Using this data, the distribution of Si and the variation of dust attributes with Si abundances also has been investigated and found that the linear component of the extinction curve is unrelated to depletion of silicon.