Quantifying Variability of Young Stellar Objects in the Mid-infrared Over 6 Years with the Near-Earth Object Wide-field Infrared Survey Explorer

Variability in young stellar objects (YSOs) can be caused by various time-dependent phenomena associated with star formation, including accretion rates, geometric changes in the circumstellar disks, stochastic hydromagnetic interactions between stellar surfaces and inner-disk edges, reconnections within the stellar magnetosphere, and hot/cold spots on stellar surfaces. We uncover and characterize ~1700 variables from a sample of ~5400 YSOs in nearby low-mass star-forming regions using mid-IR light curves obtained from the 6.5 yr Near-Earth Object Wide-field Infrared Survey Explorer All Sky Survey. The mid-IR variability traces a wide range of dynamical, physical, and geometrical phenomenon. We classify six types of YSO mid-IR variability based on their light curves: secular variability (linear, curved, and periodic) and stochastic variability (burst, drop, and irregular). YSOs in earlier evolutionary stages have higher fractions of variables and higher amplitudes for the variability, with the recurrence timescale of FUor-type outbursts (defined here as ΔW1 or ΔW2 > 1 mag followed by inspection of candidates) of ~1000 yr in the early embedded protostellar phase. Known eruptive young stars and subluminous objects show fractions of variables similar to the fraction (~55%) found in typical protostars, suggesting that these two distinct types are not distinct in variability over the 6.5 yr timescale. Along with brightness variability, we also find a diverse range of secular color variations, which can be attributed to a competitive interplay between the variable accretion luminosity of the central source and the variable extinction by material associated with the accretion process.