Time-domain statistical studies of mid-infrared and submm variability in nearby star-forming regions show that at least half of all protostars are variable. In this study, we present a statistical analysis of the mid-infrared variability of Young Stellar Objects (YSOs) in the distant, massive star-forming region W51, based on data from the Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE) All-Sky Survey. From a catalog of 81 protostars, 527 disk objects, and 37,687 other sources, including diskless pre-main sequence stars (Class III) and likely evolved contaminants such as AGB stars (collectively labeled as PMS+E), we identified significant variability in both the 3.4 μm(W1) and 4.6 μm(W2) bands. Due to the large distance (∼5.4 kpc) and high extinction of W51, our sample primarily includes intermediate- to high-mass YSOs (≥2 M⊙), in contrast to nearby star-forming regions such as Taurus or the Gould Belt, which are dominated by low-mass stars (≤1M⊙). This mass bias may affect the observed variability characteristics and their interpretation. Specifically, 11.1% of protostars, 7.6% of Disk objects, and 0.6% of PMS+E sources exhibited secular variability in the W2, while 8.6% of protostars, 2.3% of Disk objects, and 0.5% of PMS+E sources exhibited stochastic variability. Similar trends were observed in the W1 band. Both the fraction and amplitude of variability increase toward earlier evolutionary stages. Protostars exhibit predominantly stochastic variability with high amplitudes, likely driven by dynamic accretion, and extinction changes. In contrast, disk objects show more secular variability, including linear, curved, and periodic patterns, possibly caused by moderate accretion changes or geometric modulation in the inner disk. Analysis of brightness and color changes revealed that protostars typically become redder as they brighten, while disk objects show more complex behavior: they appear roughly balanced in W1 but more often become bluer in W2. These trends are consistent with enhanced dust emission or extinction in protostars, and reduced extinction or accretion-induced hotspot modulation in disk objects. These trends reflect the distinct physical mechanisms at play across evolutionary stages and demonstrate that mid-infrared variability provides useful insight into the accretion and disk evolution processes in young stars.

Introduction
Sample and Data
Variability in Mid-Infrared
    Variability at Different Stages
    Possible Contamination
Result and Discussion
    Characterization of Variability in Protostars and Disk Sources
    Color Variability in the CMD
    Comparison with Variables in Other Environmental Mass Star-Forming Regions
Summary
Acknowledgments
References

• Mi-Ryang Kim(Astronomy Program, Department of Physics and Astronomy, Seoul National University, Seoul 08826, Republic of Korea) Corresponding author
• Jeong-Eun Lee(Astronomy Program, Department of Physics and Astronomy, Seoul National University, Seoul 08826, Republic of Korea, SNU Astronomy Research Center, Seoul National University, Seoul 08826, Republic of Korea) Corresponding author
• Carlos Contreras Peña(Astronomy Program, Department of Physics and Astronomy, Seoul National University, Seoul 08826, Republic of Korea, Research Institute of Basic Sciences, Seoul National University, Seoul 08826, Republic of Korea)
• Gregory Herczeg(Kavli Institute for Astronomy and Astrophysics, Peking University, 100871 Beijing, People’s Republic of China, Department of Astronomy, Peking University, 100871 Beijing, People’s Republic of China)
• Doug Johnstone(NRC Herzberg Astronomy and Astrophysics, Victoria, BC V9E 2E7, Canada, Department of Physics and Astronomy, University of Victoria, Victoria, BC V8P 5C2, Canada)
• Miju Kang(Korea Astronomy and Space Science Institute, Daejeon 34055, Republic of Korea)