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Star formation processes originating from dense molecular clouds leave us a molecular universe. How molecules probe the physical conditions at different star-forming stages and how the physical environments control the formation of the chemical inventory becomes a key question to pursue. In the past, the understanding of this problem is impeded by instrumentallimitations. With instruments advanced in sensitivity and spatial/spectral resolution, this thesis investigates the molecular environment of different star-forming regions.

Half of this thesis (Chapter 2 and Appendix A) focuses on mapping cold dense molecular gas in an external galaxy, IC 342, at 3 Mpc. The distribution of molecular gas was efficiently mapped with a set of density-sensitive tracers with Argus. Argus is the first array receiver functioning at 3 mm on the 100 m Green Bank Telescope (GBT) and provides a resolution of 6′′–10′′. As this study was conducted in the early era of Argus’ deployment, valuable information on the instrument’s behavior is learned. The resolved molecular maps characterize the fundamental physical properties of the clouds including the volume density and the excitation conditions. Comparisons with results from radiative transfer modeling with RADEX help to decrypt this information. The high spatial resolution of Argus also provides an opportunity in inspecting a scale-scatter breakdown of the gas density-star formation correlation in nearby galaxies and in investigating the influence of a finer spatial resolution on the correlation.

The other half of the thesis (Chapters 3 and 4) studies the hot core, an embedded phase during massive star formation, of a proto-binary system W3 IRS 5 at 2.2 kpc. Rovibrational transitions of gaseous H2O, CO, and isotopologues of CO were detected with mid-IR absorption spectroscopy. The high spectral resolution (R ∼50,000–80,000) not only separates each transition individually but also decomposes different kinematic components residing in the system with a velocity resolution of a few km/s . Physical substructures such as the foreground cloud, high-speed “bullet”, and hot clumps in the disk surface are identified. Characterization of the physical substructures is conducted via the rotation diagram analysis and curve-of-growth analyses. The curve-of-growth analyses, under either a foreground slab model or a disk model, take account of the optical depth effects and correct the derived column densities by up to two orders of magnitude. The disk model specifically suggests a disk scenario with vertically-decreasing temperature from mid-plane, which is intrinsically different from externally illuminated disks in the low-mass protostellar systems that have hot surfaces. Connections between physicalsubstructures and chemical substructures were also established. Investigations on chemical abundances along the line of sight reveal the elemental carbon and oxygen depletion problem.