Near-infrared photometric variability of stars toward the Orion A molecular cloud

We present an analysis of J, H, and K-s time-series photometry obtained wit h the southern 2MASS telescope over a 0.degrees 84 x 6 degrees region cente red near the Trapezium region of the Orion Nebula cluster. These data are u sed to establish the near-infrared variability properties of pre-main-seque nce stars in Orion on timescales of similar to1-36 days, similar to2 months , and similar to2 years. A total of 1235 near-infrared variable stars are i dentified, similar to 93% of which are likely associated with the Orion A m olecular cloud. The variable stars exhibit a diversity of photometric behav ior with time, including cyclic fluctuations with periods up to 15 days, ap eriodic day-to-day fluctuations, eclipses, slow drifts in brightness over 1 month or longer, colorless variability (within the noise limits of the dat a), stars that become redder as they fade, and stars that become bluer as t hey fade. The mean peak-to-peak amplitudes of the photometric fluctuations are similar to0.2 mag in each band, and 77% of the variable stars have colo r variations less than 0.05 mag. The more extreme stars in our sample have amplitudes as large as similar to2 mag and change in color by as much as si milar to1 mag. The typical timescale of the photometric fluctuations is les s than a few days, indicating that near-infrared variability results primar ily from short-term processes. We examine rotational modulation of cool and hot starspots, variable obscuration from an inner circumstellar disk, and changes in the mass accretion rate and other physical properties in a circu mstellar disk as possible physical origins of the near-infrared variability . Cool spots alone can explain the observed variability characteristics in similar to 56%-77% of the stars, while the properties of the photometric fl uctuations are more consistent with hot spots or extinction changes in at l east 23% of the stars, and with variations in the disk mass accretion rate or inner disk radius in similar to1% of our sample. However, differences be tween the details of the observations and the details of variability predic ted by hot-spot, extinction, and accretion disk models suggest either that another variability mechanism not considered here may be operative or that the observed variability represents the net results of several of these phe nomena. Analysis of the star-count data indicates that the Orion Nebula clu ster is part of a larger area of enhanced stellar surface density that exte nds over a 0.degrees4 x 2.degrees4(3.4 pc x 20 pc) region containing simila r to 2700 stars brighter than K-s =14 mag.