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

Abstract

We present an analysis of J, H, and K_s time-series photometry obtained with the southern 2MASS telescope over a 0.°84 × 6° region centered near the Trapezium region of the Orion Nebula cluster. These data are used to establish the near-infrared variability properties of pre–main-sequence stars in Orion on timescales of ~1–36 days, ~2 months, and ~2 years. A total of 1235 near-infrared variable stars are identified, ~93% of which are likely associated with the Orion A molecular cloud. The variable stars exhibit a diversity of photometric behavior with time, including cyclic fluctuations with periods up to 15 days, aperiodic day-to-day fluctuations, eclipses, slow drifts in brightness over 1 month or longer, colorless variability (within the noise limits of the data), stars that become redder as they fade, and stars that become bluer as they fade. The mean peak-to-peak amplitudes of the photometric fluctuations are ~0.2 mag in each band, and 77% of the variable stars have color variations less than 0.05 mag. The more extreme stars in our sample have amplitudes as large as ~2 mag and change in color by as much as ~1 mag. The typical timescale of the photometric fluctuations is less than a few days, indicating that near-infrared variability results primarily 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 circumstellar disk as possible physical origins of the near-infrared variability. Cool spots alone can explain the observed variability characteristics in ~56%–77% of the stars, while the properties of the photometric fluctuations are more consistent with hot spots or extinction changes in at least 23% of the stars, and with variations in the disk mass accretion rate or inner disk radius in ~1% of our sample. However, differences between the details of the observations and the details of variability predicted 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 phenomena. Analysis of the star-count data indicates that the Orion Nebula cluster is part of a larger area of enhanced stellar surface density that extends over a 0.°4 × 2.°4 (3.4 pc × 20 pc) region containing ~2700 stars brighter than Ks = 14 mag.

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