Dust cloud lifetimes of Scallop-shell stars

Abstract We investigate the survival of dust trapped in magnetically confined cool gas clouds (or prominences) around rapidly rotating M-dwarfs exhibiting the “scallop-shell” light-curve morphology. Using a two-dimensional magnetohydrodynamic simulation, we extend previous coronal prominence models to include a passive tracer field to allow for a single injection of collisionally charged dust grains. The tracer evolution reveals how recurrent centrifugal breakouts–the slingshot process–remove dust and gas from the prominence while chromospheric evaporation replenishes gas from below. For our simulated star, which has R* = 0.6R⊙, M* = 0.3M⊙, and P* = 0.32 days, the resulting dust content decays exponentially with a minimum half-life of approximately 6 stellar rotations, representing a lower limit set by our assumption of fully coupled dust and gas dynamics. Synthetic velocity-phase diagnostics show a single, phase-locked feature that fades steadily, reproducing the behaviour of dips seen in TESS and K2 light curves. Comparison with observed river plots suggests a natural classification: (i) persistent, non-decaying features formed by quiescent prominences below co-rotation; (ii) gradually fading features produced by slingshot prominences near co-rotation; and (iii) abrupt disappearances linked to magnetic reconnection and flare-driven ejections. These results demonstrate that dust-bearing prominences–undergoing repeated slingshots–can persist for tens of rotations, linking the observed longevity of the scallop-shell photometric features with the dynamic cycle of prominence slingshot ejections.