Spitzer Space Telescope - General Observer Proposal #60014 Young Stellar Object Variability (YSOVAR): Mid Infrared Clues to Accretion Disk Physics and Protostar Rotational Evolution Principal Investigator: John Stauffer Institution: Spitzer Science Center Technical Contact: Luisa Rebull, Spitzer Science Center Co-Investigators: Rachel Akeson, NExScI Lori Allen, CfA/NOAO David Ardila, NHSC David Barrado, LAEFF Amelia Bayo, LAEFF Jerome Bouvier, Obs. de Grenoble Sean Carey, SSC Nuria Calvet, Univ. Michigan John Carpenter, Caltech David Ciardi, NExScI Kevin Covey, CfA Fabio Favata, ESA Kevin Flaherty, U. Arizona Jan Forbrich, CfA Sylvain Guieu, SSC Rob Gutermuth, Smith College Lee Hartmann, U. Michigan Lynne Hillenbrand, Caltech Joe Hora, CfA Mark McCaughrean, Exeter Tom Megeath, U. Toledo Maria Morales-Calderon, LAEFF James Muzerolle, U. Arizona Peter Plavchan, NExScI Luisa Rebull, SSC Howard Smith, CfA Mike Skrutskie, U. Virginia Inseok Song, U. Georgia Karl Stapelfeldt, JPL Hwankyung Sung, SSC Susan Terebey, CSULA Fred Vrba, USNO Mike Werner, JPL Barbara Whitney, Space Science Institute Elaine Winston, CfA Kenny Wood, University of St. Andrews Science Category: young stellar objects Observing Modes: IracPostCryoMap Hours Approved: 550.0 Abstract: Spitzer/IRAC in the warm mission is the only facility now existing or planned capable of carrying out an extensive, accurate time series photometric monitoring survey of star-forming regions in the thermal infrared. The demonstrated sensitivity and stability of IRAC allows measurement of the relative fluxes of YSO's down to the substellar mass limit to 1-2% accuracy in star-forming regions out to >500 pc. We propose a time series monitoring exploration science survey of the Orion Nebula Cluster and 11 very young, populous embedded star-forming cores which will provide >= 80 epochs of data for > 1500 YSO's. We will complement these observations with contemporaneous optical and near-IR monitoring data in order to allow comparison of the phase, amplitude and light-curve shape as a function of wavelength. These data will allow us to: (a) provide otherwise unobtainable constraints on the structure of the inner disks in Class I and II YSOs - and hence, perhaps, provide clues to the formation and migration of planets at young ages; (b) measure the short and long-term stability of hot spots on the surfaces of YSO's of all evolutionary stages; and (c) determine rotational periods for the largest sample to date of Class I YSO's and hence obtain the best measure of the initial angular momentum distribution of young stars.