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The frequency of large variations in the near-infrared fluxes of T Tauri stars

Variability is a characteristic feature of young stellar objects (YSOs) and could contribute to the large scatter observed in HR diagrams for star forming regions. For typical YSOs, however, the long-term effects of variability are poorly constrained. Here I use archived near-infrared photometry from 2MASS, UKIDSS, and DENIS to investigate the long-term variability of high-confidence members of the four star forming regions Rho-Oph, ONC, IC348, and NGC1333. The total sample comprises more than 600 objects, from which ~320 are considered to have a disk. The dataset covers timescales up to 8 yr. About half of the YSOs are variable on a 2sigma level, with median amplitudes of 5-20%. The fraction of highly variable objects with amplitudes >0.5 mag in at least two near-infrared bands is very low - 2% for the entire sample and 3% for objects with disks. These sources with strong variability are mostly objects with disks and are prime targets for follow-up studies. The variability amplitudes are largest in NGC1333, presumably because it is the youngest sample of YSOs. The frequency of highly variable objects also increases with the time window of the observations (from weeks to years). These results have three implications: 1) When deriving luminosities for YSOs from near-infrared magnitudes, the typical error introduced by variability is in the range of 5-20% percent and depends on disk fraction and possibly age. 2) Variability is a negligible contribution to the scatter in HR diagrams of star forming regions (except for a small number of extreme objects), if luminosities are derived from near-infrared magnitudes. 3) Accretion outbursts with an increase in mass accretion rate by several order of magnitudes, as required in scenarios for episodic accretion, occur with a duty cycle of >2000-2500 yr in the Class II phase. (abridged)