Molecular line observations of Infrared Dark Clouds in the Galaxy
Finn, Susanna C.
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Although massive stars play many important roles in the universe, their formation is poorly understood. Recently, a class of interstellar clouds known as Infrared Dark Clouds (IRDCs) has been identified as likely progenitors of massive stars and clusters. These clouds are dense (nH2 > 10^5 cm-3), cold (T < 20 K), have very high column densities (N ~ 10^23 10^25 cm-2), and contain dense clumps and cores. In this dissertation, I present radio observations of a large sample of IRDCs in order to examine their properties and explore the hypothesis that high-mass stars and clusters form in these dense, cold molecular clouds. I determine kinematic distances to a large sample of IRDCs in the inner Galaxy based on CS (2-1) radial velocities. IRDCs are concentrated at specific Galactocentric radii and their distribution appears to trace Milky 'vVay spiral structure. To identify IRDC clumps and determine properties such as mass, size, and chemical evolution, I map a sample of IRDCs in various high density-tracing molecular transitions. The size and mass estimates show that IRDC clumps are comparable in size to more evolved regions of massive star formation. I compare the integrated intensities and linewidths of the molecular emission with a proposed evolutionary sequence of the clumps. The ratio of N2H+ with HNC, HCN, and HCQ+ is a function of evolutionary stage. The linewidths and virial parameters of the clumps show no clear trend with the evolutionary sequence. Finally, I explore the filamentary shape of IRDCs. The "sausage instability," which describes clumps forming in a gas cylinder, is explored as a mechanism for star-forming clumps to collapse in filaments. First, I compare observations of the "Nessie Nebula," an extreme case of a filamentary IRDC, with predictions from the theory of the fluid instability and then expand the sample to other filamentary IRDCs. The observations are consistent with theoretical predictions of clump spacing, clump masses, and linear mass density. Fragmentation of filaments due to the sausage instability might be the dominant mode of star formation in the Universe.
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