Spitzer Space Telescope - General Observer Proposal #30091 Circumstellar Disks in the Upper Scorpius OB Association Principal Investigator: John Carpenter Institution: California Institute of Technology Co-Investigators: Cathy Slesnick, California Institute of Technology Eric Mamajek, Smithsonian Astrophysical Observatory Michael Meyer, University of Arizona Lynne Hillenbrand, California Institute of Technology Science Category: circumstellar/debris disks Observing Modes: IrsStare Hours Approved: 42.1 Abstract: Most 1 Myr old stars are surrounded by circumstellar accretion disks that undoubtedly represent the formation sites of planetary systems. Based on Spitzer and ground-based surveys from near-infrared to submillimeter wavelengths, it is becoming increasingly clear that by an age of 10 Myr, the reservoir of primordial dust grains in disks has been drastically depleted over all orbital radii. If planet formation is a common outcome of the star formation process, evidently the time period between 1 and 10 Myr is a critical evolutionary stage when the raw disk material is converted into larger bodies. We are currently conducting a photometric survey of the 5 Myr old Upper Scorpius OB association using IRAC, MIPS, and the IRS 16um peak-up array that will probe the frequency of disks in this period of disk evolution. From the initial results of our survey we have identified 30 sources with masses ranging from brown dwarfs to late B-type stars that are surrounded by circumstellar disks. The proposed observations will enable us to 1) determine the diversity of disk architectures at a fixed age based on variations in the spectral energy distribution within our sample; 2) constrain the grain composition and the occurrence of grain growth from the shape of spectral features at 10um and 18um; and 3) establish the temporal evolution of disks as a function of mass by comparing our results with younger star-forming regions being studied by other Spitzer programs.