Illuminating Youth: Decades of Mid-infrared Variability and Color Evolution of Young Stellar Objects

Abstract The variability of young stellar objects (YSOs) is a crucial tool for understanding the mechanisms driving flux changes. In this study, we present an infrared variability analysis of a large sample of over 20,000 candidate YSOs, using data from the ALLWISE and NEOWISE surveys, which span around a decade with a 6 month cadence. We applied Lomb–Scargle periodogram analysis and linear fitting to the light curves, classifying them into distinct categories: secular (linear, curved, and periodic) and stochastic (burst, drop, and irregular). Our findings show that 5467 (26.2% ± 0.3%) of the sources exhibit variability, with most (19.7% ± 0.3%) showing irregular variations, followed by curved and periodic variations. In addition, 235 burst sources and 122 drop sources were identified. Variability is more pronounced in Class I sources with a higher fraction of variables (36.3% ± 0.6%) compared to Class II (22.1% ± 0.4%) and Class III (22.5% ± 1.0%) sources. The color ( W 1 − W 2) versus magnitude analysis ( W 2) using linear fitting shows that the “redder-when-brighter” trend is more prevalent (85.4% ± 0.5%) among YSOs. In contrast, the “bluer-when-brighter” (BWB) trend is more common in younger sources compared to more evolved ones, having a BWB fraction of 29.0% ± 1.1% for Class I to 4.0% ± 0.9% for Class III.