Intermediate to high-mass pre-main-sequence stars and their disks

This thesis came about against the backdrop of the query of massive star formation, with a special focus on young intermediate- to high-mass stars in their natal environment: massive star forming regions (mSFRs). The outcome of the (massive) star-formation process is studied by characterizing young (high-mass) stars that reveal their photosphere as well as signs remnant of formation in optical to near-infrared spectra. A sample of these relatively massive pre-main-sequence (PMS) stars in the giant H II region M17 is extensively studied in multiple wavelength regimes with the aim to characterize their circumstellar disks. The observational and theoretical results presented touch upon a broad range of topics ranging from the formation of high-mass close binaries to disk formation and evolution in the harsh and dense environments of giant H II regions. Following the introduction in Chapter 1, Chapter 2 presents a census of the young stellar content of the three giant H II regions M8, G333.6−0.2, and NGC6357, including the classification of H- and K-band spectra of about 200 stars in these regions. Chapter 3 presents an analysis of the CO ro-vibrational first and second overtone bandhead emission from the circumstellar disks of five intermediate- to high-mass PMS stars in the mSFR M17. Chapter 4 is a further study of four of those objects, using high resolution ALMA (sub-)mm continuum and CO rotational line observations, that provide constraints on disk size, and total gas and dust mass. Chapter 5 presents a theoretical exploration of the CO fundamental ro-vibrational transitions as a probe of the intermediate disk regions (1-10 au) between the innermost and extended disk areas investigated in the previous two chapters.