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Multispectral observations of the diffuse interstellar medium
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Multispectral observations of the diffuse interstellar medium
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Dissertation
Multispectral observations of the diffuse interstellar medium
1991
Overview
This work describes observations of the diffuse interstellar medium in three wavelength bands; UV (1000-1300 A covered by the Copernicus satellite), far-UV (500-1800 A from the Voyager 1 and 2 spacecrafts) and optical (3800-4300 A obtained from ground-based observations). Chapter 2 deals with abundances and depletions of elements in the ISM. Although Copernicus was limited to brighter stars we obtained abundances and elemental depletions for the lines of sight to $\\sigma$ Sco and $\\theta$ Car. We find that, for these and other lines of sight, the element-to-element depletion pattern is remarkably stable. Chapter 3 discusses extinction along the lines of sight to several O and B stars which were observed by Voyagers 1 and 2. By observing stars which were generally unreddened and comparing them to reddened stars, the interstellar extinction curve has been extended to below 1000 A. The results show that, for all of the cases we have studied, the selective extinction continues to rise toward shorter wavelengths. Chapter 4 deals with ground-based observations of CH$\\sp+$. The overabundance of CH$\\sp+$ in the ISM is a problem in that the radiative reaction rate of C$\\sp+$ and H is much too slow to give the abundances seen, and also because CH$\\sp+$ is easily destroyed. An endothermic reaction of C$\\sp+$ and H$\\sb2$ may create enough CH$\\sp+$, but it requires shock waves that provide the 0.4 eV of energy needed for the reaction to proceed. In many previous studies, the amount of CH$\\sp+$ seems to increase as E(B-V) increases, which is not what we would expect if the shock formation model is correct. However, these studies deal primarily with brighter stars with E(B-V) less than 0.5. In the work described in this chapter, we have looked at stars with E(B-V) up to +1.13, and have found that the abundance of CH$\\sp+$ does not appear to increase for more reddened stars.
