Near-Infrared Photometric Variability of Stars toward the Orion A Molecular Cloud

We present an analysis of J, H, and K time series photometry obtained withthe southern 2MASS telescope over a 0.84 x 6 deg^2 region centered near theOrion Nebula Cluster. These data are used to establish the near-infraredvariability properties of pre-main-sequence stars in Orion on time scales of1-36 days, 2 months, and 2 years. A total of 1235 variable stars areidentified, ~93% of which are associated with the Orion A molecular cloud. Thevariable stars exhibit a diversity of photometric behavior with time, includingcyclic fluctuations, aperiodic day-to-day fluctuations, eclipses, slow driftsin brightness over one month, colorless variability, stars that become redderas they fade, and stars that become bluer as they fade. We examine rotationalmodulation of cool and hot star spots, variable obscuration from an innercircumstellar disk, and changes in the mass accretion rate and other propertiesin a circumstellar disk as possible origins of the variability. Cool spots canexplain the variability characteristics in 56-77% of the stars, while theproperties of the photometric fluctuations are more consistent with hot spotsor extinction changes in at least 23% of the stars, and with variations in thedisk mass accretion rate or inner disk radius in 1% of our sample. However,differences between the details of the observations and the details ofvariability predicted these models suggest either that another variabilitymechanism not considered here may be operative, or that the observedvariability represents the net results of several of these phenomena. Analysisof the star count data indicates that the ONC is part of a larger area ofenhanced stellar surface density which extends over a 0.4 x 2.4 deg^2 (3.4 x 20pc^2) region containing 2700 stars brighter than K=14. (abridged version)