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We present a survey of the Orion A and B molecular clouds undertaken with theIRAC and MIPS instruments onboard Spitzer. In total, five distinct fields weremapped covering 9 sq. degrees in five mid-IR bands spanning 3-24 microns. Thesurvey includes the Orion Nebula Cluster, the Lynds 1641, 1630 and 1622 darkclouds, and the NGC 2023, 2024, 2068 and 2071 nebulae. These data are mergedwith the 2MASS point source catalog to generate a catalog of eight bandphotometry. We identify 3479 dusty young stellar objects (YSOs) in the Orionmolecular clouds by searching for point sources with mid-IR colors indicativeof reprocessed light from dusty disks or infalling envelopes. The YSOs aresubsequently classified on the basis of their mid-IR colors and their spatialdistributions are presented. We classify 2991 of the YSOs as pre-main sequencestars with disks and 488 as likely protostars. Most of the sources wereobserved with IRAC in 2-3 epochs over 6 months; we search for variabilitybetween the epochs by looking for correlated variability in the 3.6 and 4.5micron bands. We find that 50% of the dusty YSOs show variability. Thevariations are typically small (0.2 mag.) with the protostars showing a higherincidence of variability and larger variations. The observed correlationsbetween the 3.6, 4.5, 5.8 and 8 micron variability suggests that we areobserving variations in the heating of the inner disk due to changes in theaccretion luminosity or rotating accretion hot spots.

the astronomical journal

The Spitzer Space Telescope Survey of the Orion A & B Molecular Clouds -  Part I: A Census of Dusty Young Stellar Objects and a Study of their Mid-IR  Variability