The Disks and Accretion Behaviorof Young Stellar ObjectsMin FangMax-Planck-Institut fur¨ AstronomieHeidelberg 2010Dissertation in Astronomysubmitted to theCombined Faculties of the Natural Sciences and Mathematicsof the Ruperto-Carola-University of Heidelberg, Germany,for the degree ofDoctor of Natural SciencesPut forward byMin Fangborn in Jiangsu, ChinaOral examination: 02.02.11The Disks and Accretion Behaviorof Young Stellar ObjectsReferees: Prof. Dr. Thomas HenningProf. Dr. Andreas QuirrenbachTo my familyAbstractCircumstellar disks play an essential role in the star formation process and are thought to be the siteswhere new planetary systems come into being. Knowledge of the evolution of these objects is pivotalto our understanding of star and planet formation. In this thesis, I present three studies aimed at charac-terizing the properties of newly formed stars and their circumstellar accretion disks in three qualitativelydifferent environments: the sparse stellar associationǫ Cha, the L1630 and L1641 regions of clustered anddistributed star formation in Orion, and the massive cluster Pismis 24. I combined infrared observations,taken from the ground and with the Spitzer Space Telescope, with optical imaging and spectroscopy ob-tained with various ground-based facilities.