2025MNRAS.537.2763M

The mass accretion process controls pre-main-sequence evolution, although its intrinsic instability has yet to be fully understood, especially towards the protostellar stage. In this work, we have undertaken a thorough examination of the mid-infrared (MIR) variability of Spitzer-selected young stellar objects (YSOs) in the Cygnus-X star-forming region over the last decade, using the Near-Earth Object Wide-field Infrared Survey Explorer time series. This work compares two groups of young stars: embedded Class I objects, and the more evolved flat-spectrum/Class II sources. We report on 48 candidate eruptive variables within these groups, including 14 with characteristics that resemble the photometric behaviour of FUors. We also include an additional 20 YSOs, which are of a less certain categorization. We find the candidate FUors to be an order of magnitude more common among the younger Class I systems than more evolved objects. A large number of the identified short-duration eruptive YSOs display MIR colour behaviour that is redder-when-brighter, which contrasts with optically bright outbursts seen in YSOs. Finally, we note the unusual long-term rising behaviours of four Class I YSOs, with rise time-scales longer than 5 yr, which is far slower than $\sim$6-12 month time-scale for the majority of optically discovered FUors. Additionally, our broader investigation of MIR variability for embedded Class I YSOs shows that there is a higher incidence of high-amplitude variability for these stars, than is seen in Class II sources. This holds true for all variable Class I YSOs, not just the eruptive sources.