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 (LSP) analysis and linear fitting to the light curves, classifying them into distinct categories: {\it Secular} ({\it Linear}, {\it Curved}, and {\it Periodic}) and {\it Stochastic} ({\it Burst}, {\it Drop}, and {\it Irregular}). Our findings show that 5,467 (26.2$\pm$0.3\%) of the sources exhibit variability, with most (19.7$\pm$0.3\%) showing {\it Irregular} variations, followed by {\it Curved} and {\it Periodic} variations. In addition, 235 sources of {\it Bursts} and 122 {\it Drop} sources were identified. Variability is more pronounced in Class I sources with a higher fraction of variables (36.3$\pm$0.6\%) compared to Class II (22.1$\pm$0.4\%) and Class III (22.5$\pm$1.0\%) sources. The color (W1 $-$ W2) versus magnitude analysis (W2) using linear fitting shows that the trend redder-when-brighter" (RWB) is more prevalent (85.4$\pm$0.5\%) among YSOs. In contrast, the trendbluer-when-brighter" (BWB) is more common in younger sources compared to more evolved ones, having a BWB fraction of 29.0$\pm$1.1\% for Class I to 4.0$\pm$0.9\% for Class III.