Observational studies of the structure, content and environment of intermediate and high-mass star-forming regions

This thesis describes observational studies of star-forming regions and their influence on the interstellar medium. First, in an effort to understand the factors that govern the transition from low- to high-mass star formation, a sample of intermediate-mass star-forming regions (IM SFRs) is identified for the first time. IM SFRs constitute embedded clusters where stars up to---but not exceeding--- ∼ 8 M⊙ are being produced. They are at an early evolutionary stage akin to compact H II regions, but they lack the massive ionizing central star(s). IRAS colors, Spitzer Space Telescope mid-IR images, millimeter continuum and 13CO maps were used to compile a sample of 50 IM SFRs in the inner Galaxy. The photodissociation regions that demarcate IM SFRs have typical diameters of ∼ 1 pc and luminosities of ∼ 104 L⊙ , making them an order of magnitude less luminous than (ultra)compact H II regions. IM SFRs coincide with molecular clumps of mass ∼ 103 M⊙ which, in turn, lie within larger molecular clouds spanning the lower end of the giant molecular cloud mass range, 104--10 5 M⊙ . The IR luminosity and associated molecular mass of IM SFRs are correlated, consistent with the known luminosity--mass relationship of compact H II regions. Peak mass column densities within IM SFRs are ∼ 0.1--0.5 g cm-2, a factor of several lower than ultra-compact H II regions, supporting the proposition that there is a threshold for massive star formation at ∼ 1 g cm -2.;Second, an investigation into the enormous H II region CTB 102 was carried out for the first time. Through a combination of new radio recombination line observations and available archival data, analysis shows that the filamentary structure surrounding the central region is physically associated with the central region. The first ever distance estimate for this H II region is provided, 4.3 kpc. The overall morphology and size of CTB 102 indicates that it is likely a large H II region combined with a wind-blown interstellar bubble/chimney structure, powerful enough to disrupt the interstellar medium and clear out a ∼ 100--130 pc region. The size and structure of CTB 102 makes it comparable to the W4 superbubble in the Perseus arm, one of the largest H II regions in the Galaxy.;Third, three halo H II regions, KR 7, KR 81 and KR 120, have been investigated using SCUBA 850 mum observations and molecular line observations as possible sites of triggered star formation. They were found to each have one dominant 850 mum source, corresponding to cold, dense material, located in the interface region between the H II region and the surrounding molecular material. They were also found to be less populated with sub-mm sources than the previously studied H II region of similar size, KR 140. The peak mass column density toward the sub-mm sources were found to be 0.1--0.3 g cm-2, comparable to IM SFRs. The masses found, 51, 27 and 21 M⊙ fall within the range of masses previously found for KR 140. The same is true for the sizes found, 0.6, 0.4 and 0.2 pc. A possible embedded cluster of young stellar objects (YSOs) was found within the dominant sub-mm source of KR 120, consistent with earlier studies. This is likely a site for ongoing star formation. Candidates for embedded YSOs within the sub-mm sources were also found for KR 7, while sources toward KR 81 can be explained as either foreground or background objects.