AbstractsAstronomy & Space Science

Massive stars form on different scales, ranging from large, dispersed OB associations to compact, dense starburst clusters. The question whether one single star formation mechanism is responsible for this wide range of properties is not answered yet. The complex structure of regions of massive star formation, and the involved short timescales provide a challenge for our understanding of their birth and early evolution. In this thesis, I study the formation process of the massive stellar content in two of the most massive and luminous star forming regions in our Galaxy: W49 and W51. I analyse near-infrared (NIR) observations obtained with state-of-the-art ground based telescopes. NIR spectroscopic observations provide reliable classification on the nature of massive stars still heavily embedded in Giant Molecular Clouds (GMCs). With the derived physical properties of the massive stars, we aim to investigate and determine the star formation history of the star forming regions. A very massive star (VMS) (M > 100 M⊙) is discovered in the central cluster of W49. It is classified as an O2-3.5If* star based on its K-band spectrum. By comparing with Geneva stellar evolutionary models, the initial mass of this star is estimated as between 100 M⊙ and 180 M⊙. With the complete spectroscopic observations of W49, thirteen O type stars as well as two Young Stellar Objects (YSOs) are identified. The age of the cluster is estimated as ~1.5 Myr, with star formation still ongoing in different parts of the region. The stellar content of W51 is also studied in this thesis. Evidence has been found that the star formation in W51 has started ~5 Myr ago and still active until now. The distinct environments and properties of sub-clusters are discussed. Despite the fact that the W51 and W49 GMCs have similar mass, we find very different massive stellar population and star formation history. W51 does not contain any VMS, while W49 has 3-4 stars more massive than 100 M⊙. This might be related to differences in the star formation process between these two regions. Advisors/Committee Members: Henning, Thomas (advisor).