Dust and Gas in the Cradles of Star Formation

I investigate how stars form from dense clouds of molecular gas and dust, especially focussing on how we can measure physical properties of these structures. First, I develop methods of Mid- and Far-Infrared extinction (MIREX/FIREX) mapping of infrared dark clouds (IRDCs), utilizing Spitzer and Herschel imaging data. These enable construction of the deepest ever extinction maps, probing cloud structures that may be initial conditions for massive star and star cluster formation. A byproduct of this work is a first study of dust opacity variation with infrared wavelength in these regions, testing models of grain composition and evolution. Second, I extend this dust opacity law investigation by analyzing Spitzer-IRS spectra (15-38micron) of IRDCs to develop a new spectroscopic infrared extinction (SIREX) mapping technique. I find evidence of grain growth and ice mantle formation in high density regions. Third, I develop methods of analyzing sub-mm thermal emission from IRDCs to derive their temperature and mass surface density structures. I compare these maps, including mass surface density probability distribution functions (PDFs), of a massive IRDC and surrounding giant molecular cloud (GMC) with those derived from extinction mapping. The two mass surface density-PDFs are consistent, being well fit by a single log-normal, with only a small mass fraction (~0.03-0.08) in a high-density power-law tail, even though gas kinematics indicate the IRDC and GMC are self-gravitating. Fourth, I extend these methods to a larger samples of clouds, including 10 IRDCs. I also analyze molecular line emission from these clouds, especially 13CO (1-0). I derive gas phase abundances of 13CO, including spatial variations and dependence on temperature and mass surface density. This constrains the process of CO freeze-out onto dust grain ice mantles, with important astrochemical implications. The 13CO spectra also yield radial velocity kinematics of the clouds, enabling study of their dynamical state. The internal velocity dispersion s of the IRDCs indicate they are gravitationally bound. These studies are foundational for more general investigations of the dynamics of forming clusters across the evolutionary sequence from starless clumps to embedded clusters to young, optically-revealed clusters.