Spitzer Space Telescope - Directors Discretionary Time Proposal #286 Protosteller Evolution in the Main Accretion Phase Principal Investigator: Melissa Enoch Institution: Caltech Science Category: Star Formation Observing Modes: IrsStare Hours Approved: 12.3 Abstract: I will focus my research on the evolution of the most deeply embedded protostellar objects in their main accretion phase. The projects outlined in my proposal are designed to study the lifetime of the main accretion phase, the innermost envelope structure, the timescale for disk formation, and the disk-to-envelope mass fraction in young protostars. My research will utilize archival Spitzer Legacy data, primarily from the Cores to Disks Legacy Project, as well as proposing new Spitzer IRS and CARMA millimeter interferometric observations of highly embedded protostars.