| A combination of theoretical work and observational discoveries over the past three decades has led to significant advances in our understanding of the star and planet formation process. However, many important questions remain to be addressed, especially regarding the earliest phases of protostellar collapse and the transformation of circumstellar disks into planetary systems. In this thesis, I have undertaken a theoretical study of “Class 0” protostars and an observational investigation of the evolution of protoplanetary disks, diversity of planetary debris systems, and the kinship between dusty remnants and planets, using a new generation of infrared and sub-millimeter instruments.;I present radiative transfer calculations of infalling envelopes surrounding Class 0 sources, compare them to the observed spectral energy distributions and radial intensity profiles, and derive mass infall rates. The rapid infall, probably inevitable given their dense environments, and the relatively flat inferred density distribution, perhaps due to contributions from external cloud material, lead us to suggest that many Class 0 sources could be the protostars of dense regions.;It has been suggested that circumstellar disks evolve from massive, optically thick, actively accreting structures to low-mass, optically thin, passive remnants in about 10 Myr. That transition may mark the assembly of grains into planetesimals, or clearing of the disk by planets. I present mid infrared observation of the TW Hydrate Association, a recently identified nearby group of 10-Myr-old stars. The results suggest rapid evolution of inner disks as does our discovery of a spatially-resolved disk with a central cavity around the young A star HR 4796A. I also present the results of mid-infrared imaging of 11 other Vega-like stars, derive global properties of the dust disks, place constraints on their sizes, and discuss several interesting cases in detail. Finally, I report the detection of dust emission from a possible Kuiper Belt around 55 Cancri, a star with known planetary companion(s). |
