| The goal of this Thesis is to provide observational constraints for models (1) of massive star formation and (2) of the evolution of interstellar material on galactic scales. Interstellar material in three star-forming regions, rho Oph, Cep OB2, and Cep OB3, is probed through a study of absorption lines against the continua of background stars. High-resolution spectra (at ∼1.3--1.7 km s-1) of interstellar CN, CH, CH+, Ca I, Ca II, and K I absorption lines toward 29 lines of sight are presented. The velocity component structure of each species, as well as equivalent width and line width for each component, are obtained by analyzing the spectra. Values of equivalent width are converted into column densities using curves of growth.; A dozen possible correlations between b-values and column densities among species for individual velocity components, not total columns along a line of sight as in the past, are examined. The study shows various levels of correlations which are interpreted in terms of cloud structure. The analysis reveals that there are two kinds of CH in diffuse molecular gas: CN-like CH, which is associated with CN, and CH+-like CH, which is related to CH+. Disentangling the amount of CH in each appears possible when utilizing the relation between columns of CH+-like CH and CH+. The distribution of species and the depletion of Ca and K were also examined. The results show that different species are mainly distributed in environments with different gas density, with CN absorption arising from the densest gas, CH and K I absorption from gas with high- to moderately high-density, CH+ and Ca I absorption from intermediate density gas, and Ca II absorption from gas with intermediate and low density. Both Ca and K are depleted onto grains in high density gas, but with different dependences on local gas density.; Gas densities for components where CN was detected were inferred from a chemical model. Analysis of cloud structure indicates that our data are generally consistent with the large-scale structure suggested by maps of CO radio emission. On small scales, the gas density is seen to vary by factors greater than 5.0 over scales of ∼ 10,000 AU.; Global aspects of the effects on star formation could be discerned as well. Comparison of results for individual star-forming regions shows that cloud systems associated with rho Oph and Cep OB2 share some similarities, such as b-values for species, and molecular mass of each star-forming cloudlet. On the other hand, clouds associated with Cep OB3 seem quite different because they have lower mean density, lower molecular mass, and possibly lower star-forming efficiency. We suggest these differences may arise from different star formation histories. |
