Survey of Orion Disks with ALMA (SODA) IV: Millimeter variability of protoplanetary disks

Abstract

Context. Short-term millimeter-wavelength variability in Class II young stellar objects (YSOs) has been considered minimal, often dismissed as very rare in ALMA-based studies on short timescales. However, recent findings suggest that variability on sub-day timescales is significant. Aims. This study aims to quantify short timescale (~1 hour to ~3 days) millimeter flux variability in unresolved Class II YSO disks observed as part of the SODA survey, leveraging its large dataset to assess the statistical significance of such variability across the LDN 1641 and LDN 1647 regions in Orion. Methods. We analyze ALMA Band 6 observations of 873 YSOs obtained over multiple epochs using a custom flux extraction pipeline to separate individual execution blocks. By computing variability factors and statistical deviations from the mean flux, we identify sources exhibiting significant variability, accounting for noise and calibrator stability. Results. Even with conservative selection criteria, we identify 20 variable sources within the sample (~5.3% of all detections). The maximum of the measured variability factor is >=7 over about one hour. More liberal selection criteria yield up to 60 sources that could be called potentially variable (~15.9%). Conclusions. The number of variable sources detected in our sample indicates physical processes operating on time scales of hours, likely associated with magnetic reconnection and/or synchrotron radiation or strong cases of magnetic accretion changes. These detections show the need to reconsider typical stability assumptions in single-epoch observations and calls for follow-up studies in star-forming regions with ALMA. Masses derived from unresolved measurements could potentially be biased by this sort of variability.