Exploring the Environment of Young Stars: Disks, Companions and Clusters
This thesis aims to further our understanding in four important, strongly related topics of star formation: Disk evolution, stellar multiplicity, FU Ori phenomenon and the formation of massive stars. The main results are the following: 1. By applying the novel method of polarimetric differential imaging and the NACO/VLT instrument we probe one of the closest circumstellar disks in the planet-forming age at scales of our inner Solar System. 2. Our adaptive optics survey discovers six companion candidates to young stars. The most interesting one is close to the well-known outburst star FU Ori and is a good candidate for being the outburst trigger. 3. We identify the second-closest cluster of young stars around the intermediate-mass star HD 104237, with at least three of them showing signs of disks. 4. The first global picture on the prototypical massive star-forming region IRAS 09002-4732 is provided and its stellar content, reflection nebula, compact H II region and globular dust filaments are analysed. 5. The first multi-epoch radial velocity survey of very young, embedded massive stars is presented. Two massive close binary stars are identified. The radial velocity measurements show large differences explainable only if substantial fraction of the young massive stars are close binaries. This high intrinsic binarity rate is naturally explained by the coalescence-type formation of massive stars.
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