Search

Large amplitude (> 1 magnitude), mid-infrared variability in young stellar objects (YSOs) provides a window into time dependence of mass accretion as well as moving structures surrounding young stars and protostars. Studies of the amplitudes and timescales of such variations are needed to assess the role of episodic accretion in both the formation of stars and planets, as well as refine our understanding of the types of variability and the mechanisms that drive large amplitude variations. We searched for large amplitude variations among 2983 pre-main-sequence stars with disks and 429 protostars in the Orion molecular clouds using multi-epoch data from the IRAC instrument on Spitzer and the WISE satellite. Initially, we compared 3.6 and 4.5 μm photometry from the Spitzer Orion Survey in 2004 with new photometry from the new Spitzer OrionTFE (the final epoch) survey in 2016/2017. We find four protostars that increased by > 2 magnitude outbursts. One of them is the known FU Ori object V2775 Ori/HOPS 223 and the other is the Class 0 protostar HOPS 383. The other two are previously unknown bursts from Class 0 protostars. These data show that roughly 3/4 of the > 2 magnitude outburst are Class 0 objects, indicating that episodic accretion is important during the earliest phase of protostellar evolution. Augmenting these data with observations from the Spitzer YSOVAR program (2009- 2010), (NEO)WISE archive (2009-2018) and data obtained during the Spitzer Beyond phase (2019), we construct light curves for the variable YSOs to constrain their onset and duration.Finally, we combine data of the entire Orion A and B clouds taken during the cyrogenic mission (two epochs in 2004) with warm mission data taken of the entire clouds with the OrionTFE program (two epochs in 2016/2017), the YSOVAR obser- vations of the Orion Nebula Cluster (high cadence data taken in to 2009 and 2010), and the Spitzer Beyond observations of known variables (2019). With these data, we identify every YSO that shows variations ≥ 1 mag in the 3.6 or 4.5 μm band. We study their light curves, color variations, and amplitude change vs time lag to classify these as bursts, fades, and fluctuations. Inside the Orion Nebula Cluster, we find that ~ 17 % of the protostars show variability, while only ~ 4 % stars with disks show variability. For the remainder of the clouds in Orion, we found that ~ 15% of the protostars and ~ 2 % stars with disk show variability over our time interval. We present percentages of variability for each YSO and variability class.From these data, we estimate that protostars undergo outbursts every 1000 years while pre-ms stars fed by accretion disks undergo bursts every 10,000 yrs. At this rate, protostars may accrete 10-30% of their mass with bursts. We also find that most large amplitude variables are not bursts, but instead are fluctuations, many of which appear result from rapidly evolving/moving structures in their environment.